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FEATURES

LTC3550-1 Dual Input USB/AC Adapter Li-Ion Battery Charger with 600mA Buck Converter DESCRIPTIO
The LTC(R)3550-1 is a standalone linear charger with a 600mA monolithic synchronous buck converter. It is capable of charging a single-cell Li-Ion battery from both wall adapter and USB inputs. The charger automatically selects the appropriate power source for charging. Internal thermal feedback regulates the battery charge current to maintain a constant die temperature during high power operation or high ambient temperature conditions. The float voltage is fixed at 4.2V and the charge currents are programmed with external resistors. The LTC3550-1 terminates the charge cycle when the charge current drops below the programmed termination threshold after the final float voltage is reached. With power applied to both inputs, the LTC3550-1 can be put into shutdown mode reducing the DCIN supply current to 20A, the USBIN supply current to 10A, and the battery drain current to less than 2A. The synchronous buck converter generates a fixed output voltage of 1.875V. The switching frequency is internally set at 1.5MHz, allowing the use of small surface mount inductors and capacitors.
, LTC and LT are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. *Protected by U.S. patents, includng 6522118, 6700364, 6580258, 5481178, 6304066, 6127815, 6498466, 6611131

Charges Single-Cell Li-Ion Batteries from Wall Adapter and USB Inputs Automatic Input Power Detection and Selection Charge Current Programmable Up to 950mA from Wall Adapter Input High Efficiency 600mA Synchronous DC/DC Converter No External MOSFET, Sense Resistor or Blocking Diode Needed Thermal Regulation Maximizes Charge Rate Without Risk of Overheating* Preset Charge Voltage with 0.6% Accuracy Programmable Charge Current Termination 1.5MHz Constant Frequency Operation (Step-Down Converter) 18A USB Suspend Current in Shutdown Independent "Power Present" Status Outputs Charge Status Output Automatic Recharge Available in a Thermally Enhanced, Low Profile (0.75mm) 16-Lead (5mm x 3mm) DFN Package
APPLICATIO S
Cellular Telephones
TYPICAL APPLICATIO
Complete Charge Cycle (1100mA Battery)
1000 800 600 400 200 0 4.2 4.0 3.8 3.6 3.4 5.0 2.5 0 -2.5 0 0.5 1.0 2.0 1.5 TIME (HR) 2.5 3.0
35501f
3550-1 TA02
Dual Input Battery Charger and DC/DC Converter
LTC3550-1 SW RUN DCIN USBIN 1F 2k 1% 1F 1.24k 1% IUSB IDC VOUT VCC BAT ITERM GND 800mA (WALL) 500mA (USB) 4.7F 2k 1% 2.2H COUT 10F CER VOUT 1.875V 600mA
WALL ADAPTER USB PORT
BATTERY CHARGE VOLTAGE (V) CURRENT (mA)
+
4.2V SINGLE-CELL Li-Ion BATTERY
3550-1 TA01
DCIN VOLTAGE (V)
U
U
U
CONSTANT VOLTAGE USBIN = 5V TA = 25C RIDC = 1.25k RIUSB = 2k
1
LTC3550-1 ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW USBIN IUSB ITERM PWR CHRG VOUT VCC GND 1 2 3 4 5 6 7 8 17 16 DCIN 15 BAT 14 IDC 13 USBPWR 12 ENABLE 11 RUN 10 SW 9 GND
DCIN, USBIN .............................................. -0.3V to 10V ENABLE, CHRG, PWR, USBPWR ............... -0.3V to 10V BAT, IDC, IUSB, ITERM ................................ -0.3V to 7V VCC ............................................................... -0.3V to 6V RUN, VOUT ..................................................-0.3V to VCC SW (DC)........................................-0.3V to (VCC + 0.3V) DCIN Pin Current (Note 2) ..........................................1A USBIN Pin Current (Note 2) .................................700mA BAT Pin Current (Note 2) ............................................1A P-Channel SW Source Current (DC).....................800mA N-Channel SW Source Current (DC) ....................800mA Peak SW Sink and Source Current ...........................1.3A Operating Temperature Range (Note 3) ... -40C to 85C Maximum Junction Temperature .......................... 125C Storage Temperature Range................... -65C to 125C
DHC PACKAGE 16-LEAD (5mm x 3mm) PLASTIC DFN TJMAX = 125C, JA = 40C (Note 4) EXPOSED PAD IS GROUND (PIN 17) MUST BE SOLDERED TO PCB
ORDER PART NUMBER LTC3550EDHC-1
PART MARKING 35501
Order Options Tape and Reel: Add #TR Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF Lead Free Part Marking: http://www.linear.com/leadfree/ Consult LTC Marketing for parts specified with wider operating temperature ranges.
ELECTRICAL CHARACTERISTICS
SYMBOL VDCIN VUSBIN VCC VENABLE RENABLE VRUN IRUN VCHRG VPWR VUSBPWR VUVDC VUVUSB PARAMETER Wall Adapter Input Supply Voltage USB Port Input Supply Voltage Buck Regulator Input Supply Voltage ENABLE Input Threshold Voltage ENABLE Pulldown Resistance RUN Threshold Voltage RUN Leakage Current CHRG Output Low Voltage PWR Output Low Voltage USBPWR Output Low Voltage DCIN Undervoltage Lockout Voltage USBIN Undervoltage Lockout Voltage
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VDCIN = 5V, VUSBIN = 5V, VCC = 3.6V unless otherwise noted.
CONDITIONS

MIN 4.3 4.3 2.5 0.4 1.2

TYP
MAX 8 8 5.5
UNITS V V V V M V A V V V V mV V mV
35501f
0.7 2 1 0.01 0.35 0.35 0.35
1.0 5 1.5 1 0.6 0.6 0.6 4.3 4.1
0.3
ICHRG = 5mA IPWR = 5mA IUSBPWR = 200A From Low to High Hysteresis From Low to High Hysteresis 4.0 3.8
4.15 200 3.95 200
2
U
W
U
U
WW
W
LTC3550-1 ELECTRICAL CHARACTERISTICS
SYMBOL VASD-DC VASD-USB Battery Charger IDCIN DCIN Supply Current Charge Mode (Note 5) Standby Mode Shutdown Mode USBIN Supply Current Charge Mode (Note 6) Standby Mode Shutdown Mode Shutdown Mode Regulated Output (Float) Voltage BAT Pin Current Constant-Current Mode Constant-Current Mode Constant-Current Mode Standby Mode Shutdown Mode Sleep Mode IDC Pin Regulated Voltage IUSB Pin Regulated Voltage Charge Current Termination Threshold RIDC = 10k Charge Terminated ENABLE = 5V RIUSB = 10k, VDCIN = 0V Charge Terminated VDCIN = 0V, ENABLE = 0V VDCIN > VUSBIN IBAT = 1mA IBAT = 1mA, 0C < TA < 85C RIDC = 1.25k RIUSB = 2.1k RIDC = 10k or RIUSB = 10k Charge Terminated Charger Disabled DCIN = 0V, USBIN = 0V Constant-Current Mode Constant-Current Mode RITERM = 1k RITERM = 2k RITERM = 10k RITERM = 20k VFLOAT - VRECHRG, 0C < TA < 85C VBAT from High to Low IBAT Drops Below Termination Threshold IBAT = 10% to 90% Full-Scale

The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VDCIN = 5V, VUSBIN = 5V, VCC = 3.6V unless otherwise noted.
PARAMETER CONDITIONS MIN 140 20 140 20 TYP 180 50 180 50 MAX 220 80 220 80 UNITS mV mV mV mV
VDCIN - VBAT Lockout Threshold Voltage VDCIN from Low to High, VBAT = 4.2V VDCIN from High to Low, VBAT = 4.2V VUSBIN - VBAT Lockout Threshold Voltage VUSBIN from Low to High, VBAT = 4.2V VUSBIN from High to Low, VBAT = 4.2V
250 50 20 250 50 18 10 4.175 4.158 760 450 93 4.2 4.2 800 476 100 -3 -1 1 1.0 1.0 100 50 10 5 100 6 1.5 250 400 550 105
800 100 40 800 100 36 20 4.225 4.242 840 500 107 -6 -2 2 1.05 1.05 110 55 11.5 6 135 9 2.2 325
A A A A A A A V V mA mA mA A A A V V mA mA mA mA mV ms ms s m m C
IUSBIN

VFLOAT IBAT
VIDC VIUSB ITERMINATE
0.95 0.95 90 45 8.5 4 65 3 0.8 175
VRECHRG tRECHRG tTERMINATE tSS RON-DC RON-USB TLIM Switching Regulator VOUT VOUT IPK VLOADREG IS
Recharge Battery Threshold Voltage Recharge Comparator Filter Time Termination Comparator Filter Time Soft-Start Time Power FET On-Resistance (Between DCIN and BAT) Power FET On-Resistance (Between USBIN and BAT) Junction Temperature in ConstantTemperature Mode Regulated Output Voltage Output Voltage Line Regulation Peak Inductor Current Output Voltage Load Regulation Input DC Bias Current Active Mode Sleep Mode Shutdown
IOUT = 100mA VCC = 3V, VOUT = 1.7V (Note 7) VOUT = 1.7V, ILOAD = 0A VOUT = 1.94V, ILOAD = 0A VRUN = 0V, VCC = 5.5V

1.819 0.75
1.875 0.04 1 0.5 300 20 0.1
1.931 0.4 1.25
V %/V A %
400 35 1
A A A
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3
LTC3550-1 ELECTRICAL CHARACTERISTICS
SYMBOL fOSC RPFET RNFET ILSW PARAMETER Oscillator Frequency RDS(ON) of P-Channel FET RDS(ON) of N-Channel FET SW Leakage Current
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VDCIN = 5V, VUSBIN = 5V, VCC = 3.6V unless otherwise noted.
CONDITIONS VOUT = 100% VOUT = 0V MIN 1.2 TYP 1.5 210 0.4 0.35 0.01 1 MAX 1.8 UNITS MHz kHz A
Note 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and lifetime. Note 2: Guaranteed by long term current density limitations. Note 3: The LTC3550E-1 is guaranteed to meet the performance specifications from 0C to 70C. Specifications over the -40C to 85C operating temperature range are assured by design, characterization and correlation with statistical process controls. Note 4: Failure to solder the exposed backside of the package to the PC board will result in a thermal resistance much higher than 40C/W. See Thermal Considerations.
Note 5: Supply Current includes IDC and ITERM pin current (approximately 100A each) but does not include any current delivered to the battery through the BAT pin (approximately 100mA). Note 6: Supply Current includes IUSB and ITERM pin current (approximately 100A each) but does not include any current delivered to the battery through the BAT pin (approximately 100mA). Note 7: Dynamic supply current is higher due to the gate charge being delivered at the switching frequency.
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LTC3550-1 TYPICAL PERFOR A CE CHARACTERISTICS
Regulated Charger Output (Float) Voltage vs Charge Current
4.26 4.24 4.22 VFLOAT (V) VFLOAT (V) 4.20 4.18 4.16 4.14 4.12 4.10 0 100 200 300 400 500 600 700 800 CHARGE CURRENT (mA)
3550-1 G01
VDCIN = VUSBIN = 5V 4.215 4.210 4.205 4.200 4.195 4.190 4.185 4.180 -50
VIDC (V)
RIDC = RIUSB = 2k
RIDC = 1.25k
IUSB Pin Voltage vs Temperature (Constant-Current Mode)
1.008 1.006 1.004 VIUSB (V) IBAT (mA) 1.002 VUSBIN = 8V 1.000 0.998 0.996 0.994 0.992 -50 -25 0 25 50 TEMPERATURE (C) 75 100 VUSBIN = 4.3V 900 800 700 600 500 400 300 200 100 0
IBAT (mA)
PWR Pin I-V Curve
35 VDCIN = VUSBIN = 5V 30 25 IPWR (mA) 20 15 10 5 0 0 1 2 4 3 VPWR (V) 5 6 7 TA = -40C TA = 25C ICHRG (mA) TA = 90C 35 30 25 20 15 10 5 0
IUSBPWR (mA)
UW
3550-1 G04
TA = 25C, unless otherwise noted. IDC Pin Voltage vs Temperature (Constant-Current Mode)
1.008 1.006 1.004 1.002 1.000 0.998 0.996 0.994 VDCIN = 4.3V VDCIN = 8V
Regulated Charger Output (Float) Voltage vs Temperature
4.220 VDCIN = VUSBIN = 5V
-25
0 25 50 TEMPERATURE (C)
75
100
0.992 -50
-25
0 25 50 TEMPERATURE (C)
75
100
3550-1 G02
3550-1 G03
Charge Current vs IDC Pin Voltage
VDCIN = 5V RIDC = 1.25k 900 800 700 600 RIDC = 2k 500 400 300 200 RIDC = 10k 100 0 0 0.2 0.4 0.6 0.8 VIDC (V) 1.0 1.2
Charge Current vs IUSB Pin Voltage
VUSBIN = 5V RIUSB = 1.25k
RIUSB = 2k
RIUSB = 10k
0
0.2
0.4
0.6 0.8 VIUSB (V)
1.0
1.2
3550-1 G05
3550-1 G06
CHRG Pin I-V Curve
VDCIN = VUSBIN = 5V 6 TA = -40C 5 TA = 25C TA = 90C 4 3 2 1 0
USBPWR Pin I-V Curve
VDCIN = 5V VUSBIN = 0V TA = - 40C TA = 25C TA = 90C
0
1
2
4 3 VCHRG (V)
5
6
7
0
1
2
4 3 5 VUSBPWR (V)
6
7
3550-1 G07
3550-1 G08
3550-1 G09
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LTC3550-1 TYPICAL PERFOR A CE CHARACTERISTICS
Charge Current vs Ambient Temperature
1000 ONSET OF THERMAL REGULATION 800 RIDC = 1.25k 700 IBAT (mA) IBAT (mA) 600 500 200 VDCIN = VUSBIN = 5V VBAT = 4V JA = 40C/W 50 25 75 0 TEMPERATURE (C) 100 125 400 RIDC = 1.25k VBAT = 4V JA = 35C/W 5.0 5.5 6.0 6.5 VDCIN (V) 7.0 7.5 8.0 200 VDCIN = VUSBIN = 5V JA = 40C/W RIDC = 1.25k 2.4 2.7 3.0 3.3 3.6 VBAT (V) 3.9 4.2 4.5 IBAT (mA) 600 RIDC = RIUSB = 2k 600 900 800
400
0 -50 -25
DCIN Power FET On-Resistance vs Temperature
550 500 450 RDS(ON) (m) 400 350 300 250 -50 -25 VBAT = 4V IBAT = 200mA 800 750 700 RDS(ON) (m)
600 550 500 450
VENABLE (mV)
50 25 75 0 TEMPERATURE (C)
DCIN Shutdown Current vs Temperature
50 45 40 35 IDCIN (A) 30 25 20 15 10 5 0 -50 -25 VDCIN = 5V VDCIN = 8V IUSBIN (A) 45 40 35
25 20 15 VUSBIN = 5V
RENABLE (M)
VDCIN = 4.3V
ENABLE = 5V 50 25 0 TEMPERATURE (C) 75 100
6
UW
100
3550-1 G13 3550-1 G16
TA = 25C, unless otherwise noted.
Charge Current vs Supply Voltage
ONSET OF THERMAL REGULATION 1000
Charge Current vs Battery Voltage
800
400
300 4.0 4.5
0
3550-1 G10
3550-1 G11
3550-1 G12
USBIN Power On-Resistance vs Temperature
VBAT = 4V IBAT = 200mA 900
ENABLE Pin Threshold Voltage (On-to-Off) vs Temperature
VDCIN = VUSBIN = 5V 850 800 750 700 650
650
400 125 350 -50 -25 50 25 75 0 TEMPERATURE (C) 100 125 600 -50 -25 50 25 0 TEMPERATURE (C) 75 100
3550-1 G14
3550-1 G15
USBIN Shutdown Current vs Temperature
2.8 2.6 VUSBIN = 8V 2.4 2.2 2.0 1.8 ENABLE = 0V -25 25 0 TEMPERATURE (C) 50 75 100
ENABLE Pin Pulldown Resistance vs Temperature
30
10 5
VUSBIN = 4.3V
0 -50
1.6 -50
-25
50 25 0 TEMPERATURE (C)
75
100
3550-1 G17
3550-1 G18
35501f
LTC3550-1 TYPICAL PERFOR A CE CHARACTERISTICS
Undervoltage Lockout Threshold vs Temperature
4.25 4.20 DCIN UVLO 4.15 VRECHRG (V) IBAT (A) 4.10 VUV (V) 4.05 4.00 USBIN UVLO 3.95 3.90 3.85 -50 -25 0 25 50 TEMPERATURE (C) 75 100 4.06 4.04 -50 0 -1 -50 4.12 4.10 4.08 VDCIN = VUSBIN = 4.3V VDCIN = VUSBIN = 8V 3 2 1 4.16 4.14
Charge Current During Turn-On and Turn-Off
100 95 90 85 80 75 70 VDCIN = 5V RIDC = 1.25k 100s/DIV
3550-1 G22
IBAT 500mA/DIV EFFICIENCY (%)
IOUT = 10mA
EFFICIENCY (%)
ENABLE 5V/DIV
Buck Regulator Output Voltage vs Temperature
1.91 1.90 1.89 VOUT (V) 1.88 1.87 1.86 1.85 1.84 -50 -25 VCC = 3.6V IOUT = 100mA 1.70 1.65
FREQUENCY (MHz)
FREQUENCY (MHz)
50 25 75 0 TEMPERATURE (C)
UW
100
TA = 25C, unless otherwise noted. Battery Drain Current vs Temperature
5 4 VBAT = 4.2V VDCIN, VUSBIN (NOT CONNECTED)
Recharge Threshold Voltage vs Temperature
-25
0 25 50 TEMPERATURE (C)
75
100
-25
0 25 50 TEMPERATURE (C)
75
100
3550-1 G19
3550-1 G20
3550-1 G21
Buck Regulator Efficiency vs VCC
100
Buck Regulator Efficiency vs Output Current
VCC = 2.7V
IOUT = 100mA
90 VCC = 4.2V 80
70
VCC = 3.6V
IOUT = 600mA
60
2
3
4 VCC (V)
5
6
35501 G23
50 0.1
1
10 ILOAD (mA)
100
1000
35501 G24
Oscillator Frequency vs Temperature
VCC = 3.6V 1.60 1.55 1.50 1.45 1.40 1.35 125 1.30 -50 -25 50 25 75 0 TEMPERATURE (C) 100 125 1.3 1.2 1.6 1.5 1.4 1.8 1.7
Oscillator Frequency vs VCC
2
3
4 VCC (V)
5
6
3550-1 G27
3550-1 G25
3550-1 G26
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LTC3550-1 TYPICAL PERFOR A CE CHARACTERISTICS
Buck Regulator Output Voltage vs Load Current
1.90 VCC = 3.6V 0.7 0.6 1.89 0.5 RDS(ON) () VOUT (V) 1.88 0.4 0.3 0.2 1.86 0.1 0 0 100 200 300 400 500 600 700 800 900 ILOAD (mA)
35501 G28
RDS(ON) ()
1.87
1.85
Buck Regulator Supply Current vs VCC
50 45 40 SUPPLY CURRENT (A) SUPPLY CURRENT (A) 35 30 25 20 15 10 5 0 2 3 4 VCC (V) 5 6
3550-1 G31
VOUT = 1.875V ILOAD = 0A
35 30 25 20 15 10 5 0 -50 -25 50 25 0 75 TEMPERATURE (C) 100 125
SWITCH LEAKAGE (nA)
Switch Leakage Current vs VCC
120 100 SWITCH LEAKAGE (pA) 80 60 40 20 0 SYNCHRONOUS SWITCH RUN = 0V SW 5V/DIV VOUT 20mV/DIV MAIN SWITCH
0
1
8
UW
2
TA = 25C, unless otherwise noted. Buck Regulator Switches RDS(ON) vs Temperature
0.7 VCC = 2.7V 0.6 VCC = 4.2V 0.5 0.4 0.3 0.2 0.1 MAIN SWITCH SYNCHRONOUS SWITCH 0 50 -50 -25 25 75 0 TEMPERATURE (C) VCC = 3.6V
RDS(ON) vs VCC
TA = 25C
MAIN SWITCH
SYNCHRONOUS SWITCH
0
1
2
4 3 VCC (V)
5
6
7
100
125
3550-1 G29
3550-1 G30
Buck Regulator Supply Current vs Temperature
50 45 40 VCC = 3.6V VOUT = 1.875V ILOAD = 0A 300
Switch Leakage Current vs Temperature
VCC = 5.5V RUN = 0V 250 200 150 100 50 0 -50 -25 MAIN SWITCH SYNCHRONOUS SWITCH 50 25 75 0 TEMPERATURE (C) 100 125
3550-1 G32
3550-1 G33
Burst Mode Operation
IL 200mA/DIV VCC = 3.6V ILOAD = 10mA 4s/DIV
35501 G35
3 VCC (V)
4
5
6
3550-1 G34
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LTC3550-1 TYPICAL PERFOR A CE CHARACTERISTICS
Start-Up from Shutdown
RUN 2V/DIV VOUT 1V/DIV VOUT 100mV/DIV AC COUPLED
IL 500mA/DIV VCC = 3.6V ILOAD = 600mA 40s/DIV
35501 G36
Load Step
VOUT 100mV/DIV AC COUPLED VOUT 100mV/DIV AC COUPLED
IL 500mA/DIV VCC = 3.6V 20s/DIV ILOAD = 100mA TO 600mA
35501 G39
UW
TA = 25C, unless otherwise noted.
Load Step
VOUT 100mV/DIV AC COUPLED
Load Step
IL 500mA/DIV VCC = 3.6V 20s/DIV ILOAD = 0mA TO 600mA
35501 G37
IL 500mA/DIV VCC = 3.6V 20s/DIV ILOAD = 50mA TO 600mA
35501 G38
Load Step
IL 500mA/DIV VCC = 3.6V 20s/DIV ILOAD = 200mA TO 600mA
35501 G40
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LTC3550-1 PI FU CTIO S
USBIN (Pin 1): USB Input Supply Pin. Provides power to the battery charger. The maximum supply current is 650mA. This should be bypassed with a 1F capacitor. IUSB (Pin 2): USB Charge Current Program and Monitor Pin. The charge current can be set by connecting a resistor, RIUSB, to ground. When charging in constant-current mode, this pin servos to 1V. The voltage on this pin can be used to measure the charge current delivered from the USB input using the following formula: V IBAT = IUSB * 1000 RIUSB ITERM (Pin 3): Termination Current Threshold Program Pin. The current termination threshold, ITERMINATE, can be set by connecting a resistor, RITERM, to ground. ITERMINATE is set by the following formula: 100 V ITERMINATE = RITERM When the charge current, IBAT, falls below the termination threshold, charging stops and the CHRG output becomes high impedance. This pin is internally clamped to approximately 1.5V. Driving this pin to voltages beyond the clamp voltage should be avoided. PWR (Pin 4): Open-Drain Power Supply Status Output. When the DCIN or USBIN pin voltage is sufficient to begin charging (i.e., when the supply is greater than the undervoltage lockout threshold and at least 180mV above the battery terminal), the PWR pin is pulled low by an internal N-channel MOSFET. Otherwise, PWR is high impedance. The output is capable of sinking up to 10mA, making it suitable for driving an LED. CHRG (Pin 5): Open-Drain Charge Status Output. When the LTC3550-1 is charging, the CHRG pin is pulled low by an internal N-channel MOSFET. When the charge cycle is completed, CHRG becomes high impedance. This output is capable of sinking up to 10mA, making it suitable for driving an LED. VOUT (Pin 6): Output Voltage Feedback Pin. Receives the feedback voltage from the buck regulator output. VCC (Pin 7): Buck Regulator Input Supply Pin. Must be closely decoupled to GND (Pins 8, 9) with a 2.2F or greater ceramic capacitor. GND (Pins 8, 9): Ground. SW (Pin 10): Switch Node Connection to Inductor. This pin connects to the drains of the internal main (top) and synchronous (bottom) power MOSFET switches. RUN (Pin 11): Buck Regulator Run Control Input. Forcing this pin above 1.5V enables the regulator. Forcing this pin below 0.3V shuts it down. In shutdown, all buck regulator functions are disabled drawing <1A supply current from VCC. Do not leave RUN floating. ENABLE (Pin 12): Charger Enable Input. When the LTC3550-1 is charging from the DCIN source, a logic low on this pin enables the charger. When the LTC3550-1 is charging from the USBIN source, a logic high on this pin enables the charger. If this input is left floating, an internal 2M pulldown resistor defaults the LTC3550-1 to charge when a wall adapter is applied and to shut down if only the USB source is applied. USBPWR (Pin 13): Open-Drain USB Power Status Output. When the voltage on the USBIN pin is sufficient to begin charging and there is insufficient power at DCIN, the USBPWR pin is high impedance. In all other cases, this pin is pulled low by an internal N-channel MOSFET, provided that there is power present at DCIN, USBIN, or BAT inputs. This output is capable of sinking up to 1mA, making it suitable for driving high impedance logic inputs. IDC (Pin 14): Wall Adapter Charge Current Program and Monitor Pin. The charge current is set by connecting a resistor, RIDC, to ground. When charging in constantcurrent mode, this pin servos to 1V. The voltage on this pin can be used to measure the charge current using the following formula: V IBAT = IDC * 1000 RIDC BAT (Pin 15): Charger Output. This pin provides charge current to the battery and regulates the final float voltage to 4.2V.
10
U
U
U
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LTC3550-1 PI FU CTIO S
DCIN (Pin 16): Wall Adapter Input Supply Pin. Provides power to the battery charger. The maximum supply current is 950mA. This should be bypassed with a 1F capacitor. Exposed Pad (Pin 17): GND. The exposed backside of the package is ground and must be soldered to the PCB ground for electrical connection and maximum heat transfer.
BLOCK DIAGRA
DCIN 16
USBPWR 13
1mA MAX
+
4.15V
PWR
4
10mA MAX
DCIN UVLO
USBIN UVLO
BURST CLAMP
+
BAT CHRG 5 10mA MAX
+
7 VCC
-
-
BAT
-
ICOMP
+
+
RECHRG LOGIC RECHARGE
4.1V
S R
Q Q ANTISHOOTTHRU
-
BAT
RS LATCH SWITCHING LOGIC AND BLANKING CIRCUIT
DC_ENABLE TERM
USB_ENABLE
+
THERMAL REGULATION
TDIE
CHARGER CONTROL ENABLE 12 RENABLE
-
105C
+
TERMINATION
100mV
IBAT /1000
IBAT /1000
IBAT /1000
-
3 14 2 11
3550-1 BD
ITERM RITERM
IDC RIDC
IUSB RIUSB
RUN
-
IRCMP
-
-
DC SOFTSTART
USB SOFTSTART
-
ITH 3.95V
EA
+
+
W
U
U
U
BAT 15
USBIN 1
CC/CV REGULATOR
CC/CV REGULATOR OSC SLOPE COMP FREQ SHIFT R1 0.6V 6 VOUT
+
R2
5
10 SW
8, 9, 17 GND
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LTC3550-1 OPERATIO
The LTC3550-1 consists of two main blocks: a lithium-ion battery charger and a high-efficiency buck converter that can be powered from the battery. The charger is designed to efficiently manage charging of a single-cell lithium-ion battery from two separate power sources: a wall adapter and USB power bus. The internal P-channel MOSFETs can supply up to 950mA from the wall adapter source and 500mA from the USB power source. The final float voltage accuracy is 0.6%. The buck converter uses a constant frequency, current mode step-down architecture. Both the main (P-channel MOSFET) and synchronous (N-channel MOSFET) switches for the buck converter are internal. The LTC3550-1 requires no external diodes or sense resistors. Lithium-Ion Battery Charger A charge cycle begins when the voltage at either the DCIN pin or USBIN pin rises above the UVLO threshold level and the charger is enabled through the ENABLE pin. The "on" state of this pin depends on which source is supplying power. When the wall adapter input (DCIN) is supplying power, logic low enables the charger and logic high disables it (a 2M pulldown defaults the charger to the charging state). The opposite is true when the USB
BAT < 4.1V
ENABLE DRIVEN LOW
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input (USBIN) is supplying power; logic low disables the charger and logic high enables it (the default is the shutdown state). The DCIN input draws 20A when the charger is in shutdown. The USBIN input draws 18A during shutdown if no power is applied to DCIN, but draws only 10A when VDCIN > VUSBIN. Once the charger is enabled, it enters constant-current mode, where the programmed charge current is supplied to the battery. When the BAT pin approaches the final float voltage (4.2V), the charger enters constant-voltage mode and the charge current begins to decrease. Once the charge current drops below the programmed termination threshold (set by the external resistor RITERM), the internal P-channel MOSFET is shut off and the charger enters standby mode. In standby mode, the charger sits idle and monitors the battery voltage using a comparator with a 6ms filter time (tRECHRG). A charge cycle automatically restarts when the battery voltage falls below 4.1V (which corresponds to approximately 80% to 90% battery capacity). This ensures that the battery is kept near a fully charged condition and eliminates the need for periodic charge cycle initiations. Figure 1 uses a state diagram to describe the behavior of the LTC3550-1 battery charger.
STARTUP DCIN POWER APPLIED POWER SELECTION CHARGE MODE FULL CURRENT CHRG STATE: PULLDOWN IBAT < ITERMINATE IN VOLTAGE MODE STANDBY MODE NO CHARGE CURRENT CHRG STATE: Hi-Z ENABLE DRIVEN HIGH ENABLE DRIVEN LOW DCIN POWER REMOVED USBIN POWER REMOVED OR DCIN POWER APPLIED CHARGE MODE FULL CURRENT CHRG STATE: PULLDOWN IBAT < ITERMINATE IN VOLTAGE MODE STANDBY MODE NO CHARGE CURRENT CHRG STATE: Hi-Z BAT < 4.1V ONLY USB POWER APPLIED SHUTDOWN MODE IDCIN DROPS TO 20A CHRG STATE: Hi-Z DCIN POWER REMOVED USBIN POWER REMOVED OR DCIN POWER APPLIED SHUTDOWN MODE IUSBIN DROPS TO 18A CHRG STATE: Hi-Z
3550-1 F01
ENABLE DRIVEN HIGH
Figure 1. LTC3550-1 State Diagram of a Charge Cycle
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LTC3550-1 OPERATIO
600mA Step-Down Regulator The LTC3550-1 regulator uses a constant frequency, current mode step-down architecture. Both the top (P-channel MOSFET) and bottom (N-channel MOSFET) switches are internal. During normal operation, the internal top power MOSFET is turned on each cycle when the oscillator sets the RS latch, and is turned off when the current comparator, ICOMP, resets the RS latch. The peak inductor current at which ICOMP resets the RS latch, is controlled by the output of error amplifier EA. When the load current increases, it causes a slight decrease in the output voltage (VOUT), relative to the internal reference, which in turn causes the EA amplifier's output voltage to increase until the average inductor current matches the new load current. While the top MOSFET is off, the bottom MOSFET is turned on until either the inductor current starts to reverse, as indicated by the current reversal comparator IRCMP, or the beginning of the next clock cycle. Burst Mode Operation The LTC3550-1 buck regulator is capable of Burst Mode operation in which the internal power MOSFETs operate intermittently based on load current demand. In Burst Mode operation, the peak current of the inductor is set to approximately 200mA regardless of the output load. Each burst event can last from a few cycles at light loads to almost continuously cycling with short sleep intervals at moderate loads. In between these burst events, the power MOSFETs and any unneeded circuitry are turned off, reducing the quiescent current to 20A. In this sleep state, the load current is being supplied solely from the output capacitor. As the output voltage droops, the EA amplifier's output rises above the sleep threshold signaling the BURST comparator to trip and turn the top MOSFET on. This process repeats at a rate that is dependent on the load demand. Dropout Operation As the input supply voltage decreases to a value approaching the output voltage, the duty cycle increases toward the maximum on-time. Further reduction of the supply voltage forces the main switch to remain on for more than one cycle until it reaches 100% duty cycle. The output voltage will
Burst Mode is a registered trademark of Linear Technology Corporation.
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then be determined by the input voltage minus the voltage drop across the P-channel MOSFET and the inductor. An important detail to remember is that at low input supply voltages, the RDS(ON) of the P-channel switch increases (see Typical Performance Characteristics). Therefore, the user should calculate the power dissipation when the LTC3550-1 is used at 100% duty cycle with low input voltage (See Thermal Considerations in the Applications Information section). Battery Charger Power Source Selection The LTC3550-1 can charge a battery from either the wall adapter input or the USB port input. The charger automatically senses the presence of voltage at each input. If both power sources are present, the charger defaults to the wall adapter source provided sufficient power is present at the DCIN input. "Sufficient power" is defined as: * Supply voltage is greater than the UVLO threshold. * Supply voltage is greater than the battery voltage by 50mV (180mV rising, 50mV falling). The open drain power status outputs (PWR and USBPWR) indicate which power source has been selected. Table 1 describes the behavior of these status outputs.
Table 1. Power Source Selection
VUSBIN > 3.95V and VUSBIN > BAT + 50mV VDCIN > 4.15V and VDCIN > BAT + 50mV Device Powered from Wall Adapter Source; USBIN Current < 25A PWR: LOW USBPWR: LOW Device Powered from USB Source; PWR: LOW USBPWR: Hi-Z VUSBIN < 3.95V or VUSBIN < BAT + 50mV Device Powered from Wall Adapter Source PWR: LOW USBPWR: LOW No Charging PWR: Hi-Z USBPWR: LOW VDCIN < 4.15V or VDCIN < BAT + 50mV
Status Indicators The charge status output (CHRG) has two states: pulldown and high impedance. The pull-down state indicates that the LTC3550-1 is in a charge cycle. Once the charge cycle has terminated or the LTC3550-1 is disabled, the pin state becomes high impedance. The pull-down state is strong enough to drive an LED and is capable of sinking up to 10mA.
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LTC3550-1 OPERATIO
The power supply status output (PWR) has two states: pulldown and high impedance. The pull-down state indicates that power is present at either DCIN or USBIN. If no power is applied at either pin, the PWR pin is high impedance, indicating that the LTC3550-1 lacks sufficient power to charge the battery. The pull-down state is strong enough to drive an LED and is capable of sinking up to 10mA. The USB power status output (USBPWR) has two states: pull-down and high impedance. The high impedance state indicates that the LTC3550-1 is being powered from the USBIN input. The pull-down state indicates that the charger is either powered from DCIN or is in a UVLO condition (see Table 1). The pull-down state is capable of sinking up to 1mA. Thermal Limiting An internal thermal feedback loop reduces the programmed charge current if the die temperature attempts to rise above a preset value of approximately 105C. This feature protects the LTC3550-1 from excessive temperature and allows the user to push the limits of the power handling capability of a given circuit board without risk of damaging the device. The charge current can be set according to typical (not worst-case) ambient temperature with the Short-Circuit Protection When the regulator output (VOUT) is shorted to ground, the frequency of the oscillator is reduced to about 210kHz, one seventh the nominal frequency. This frequency foldback ensures that the inductor current has more time to decay, thereby preventing runaway. The oscillator's frequency will progressively increase to 1.5MHz when VOUT rises above 0V.
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assurance that the charger will automatically reduce the current in worst case conditions. DFN package power considerations are discussed further in the Applications Information section. Charge Current Soft-Start and Soft-Stop The battery charger includes a soft-start circuit to minimize the inrush current at the start of a charge cycle. When a charge cycle is initiated, the charge current ramps from zero to full-scale current over a period of 250s. Likewise, internal circuitry slowly ramps the charge current from full-scale to zero in approximately 30s when the charger shuts down or self terminates. This minimizes the transient current load on the power supply during start-up and shut-off.
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LTC3550-1 APPLICATIO S I FOR ATIO
Figure 2 shows the basic LTC3550-1 application circuit. External component selection is driven by the charging requirements and the buck regulator load requirements.
LTC3550-1 SW RUN DCIN USBIN C2 RIUSB C1 RIDC IUSB IDC VOUT VCC BAT ITERM GND CIN RITERM L1 COUT VOUT 1.875V 600mA
WALL ADAPTER USB POWER
+
3550-1 F02
Figure 2. LTC3550-1 Basic Circuit
Programming and Monitoring Charge Current The charge current delivered to the battery from the wall adapter supply is programmed using a single resistor from the IDC pin to ground. Likewise, the charge current from the USB supply is programmed using a single resistor from the IUSB pin to ground. The program resistor and the charge current (ICHRG) are calculated using the following equations: RIDC = 1000 V ICHRG(DC) , ICHRG(DC) = 1000 V RIDC 1000 V RIUSB
RIUSB =
1000 V ICHRG(USB)
, ICHRG(USB) =
Charge current out of the BAT pin can be determined at any time by monitoring the IDC or IUSB pin voltage and using the following equations: IBAT = IBAT VIDC * 1000 (ch arg ing from wall adapter ) RIDC V = IUSB * 1000 (ch arg ing from USB sup ply ) RIUSB
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Programming Charge Termination The charge cycle terminates when the charge current falls below the programmed termination threshold during constant-voltage mode. This threshold is set by connecting an external resistor, RITERM, from the ITERM pin to ground. The charge termination current threshold (ITERMINATE) is set by the following equation: 100 V 100 V RITERM = , ITERMINATE = ITERMINATE RITERM The termination condition is detected by using an internal filtered comparator to monitor the ITERM pin. When the ITERM pin voltage drops below 100mV* for longer than tTERMINATE (typically 1.5ms), charging is terminated. The charge current is latched off and the LTC3550-1 enters standby mode. When charging, transient loads on the BAT pin can cause the ITERM pin to fall below 100mV for short periods of time before the DC charge current has dropped below the programmed termination current. The 1.5ms filter time (tTERMINATE) on the termination comparator ensures that transient loads of this nature do not result in premature charge cycle termination. Once the average charge current drops below the programmed termination threshold, the LTC3550-1 terminates the charge cycle and stops providing any current out of the BAT pin. In this state, any load on the BAT pin must be supplied by the battery. Buck Regulator Inductor Selection For most applications, the value of the inductor will fall in the range of 1H to 4.7H. Its value is chosen based on the desired inductor ripple current. Large value inductors lower ripple current and small value inductors result in higher ripple currents. Higher VCC or VOUT also increases the ripple current as shown in Equation 1. A reasonable starting point for setting ripple current is IL = 240mA (40% of 600mA). V V IL = OUT * 1 - OUT fO * L VCC (1)
* Any external sources that hold the ITERM pin above 100mV will prevent the LTC3550-1 from terminating a charge cycle. 4.2V SINGLE CELL Li-Ion BATTERY
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LTC3550-1 APPLICATIO S I FOR ATIO
The DC current rating of the inductor should be at least equal to the maximum load current plus half the ripple current to prevent core saturation. Thus, a 720mA rated inductor should be enough for most applications (600mA + 120mA). For best efficiency, choose a low DC-resistance inductor. The inductor value also has an effect on Burst Mode operation. The transition to low current operation begins when the inductor current peaks fall to approximately 200mA. Lower inductor values (higher IL) will cause this to occur at lower load currents, which can cause a dip in efficiency in the upper range of low current operation. In Burst Mode operation, lower inductance values will cause the burst frequency to increase. Inductor Core Selection Different core materials and shapes will change the size/current and price/current relationship of an inductor. Toroid or shielded pot cores in ferrite or permalloy materials are small and don't radiate much energy, but generally cost more than powdered iron core inductors with similar electrical characteristics. The choice of which style inductor to use often depends more on the price vs size requirements and any radiated field/EMI requirements than on what the LTC3550-1 requires to operate. Table 2 shows some typical surface mount inductors that work well in LTC3550-1 applications.
Table 2. Representative Surface Mount Inductors
PART NUMBER Sumida CDRH3D16 VALUE (H) 1.5 2.2 3.3 4.7 2.2 3.3 4.7 3.3 4.7 1.0 2.2 4.7 DCR ( MAX) 0.043 0.075 0.110 0.162 0.116 0.174 0.216 0.17 0.20 0.060 0.097 0.150 MAX DC CURRENT (A) 1.55 1.20 1.10 0.90 0.950 0.770 0.750 1.00 0.95 1.00 0.79 0.65 SIZE W x L x H (mm) 3.8 x 3.8 x 1.8
Sumida CMD4D06 Panasonic ELT5KT Murata LQH32CN
3.5 x 4.3 x 0.8
4.5 x 5.4 x 1.2 2.5 x 3.2 x 2.0
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CIN and COUT Selection In continuous mode, the source current of the top MOSFET is a square wave of duty cycle VOUT/VCC. To prevent large voltage transients, a low ESR input capacitor sized for the maximum RMS current must be used. The maximum RMS capacitor current is given by: CIN required IRMS IOMAX VOUT ( VCC - VOUT ) VCC (2) This formula has a maximum at VCC = 2VOUT, where IRMS = IOUT/2. This simple worst-case condition is commonly used for design because even significant deviations do not offer much relief. Note that the capacitor manufacturer's ripple current ratings are often based on 2000 hours of life. This makes it advisable to further derate the capacitor, or choose a capacitor rated at a higher temperature than required. Always consult the manufacturer if there is any question. The selection of COUT is driven by the required effective series resistance (ESR). Typically, once the ESR requirement for COUT has been met, the RMS current rating generally far exceeds the IRIPPLE(P-P) requirement. The output ripple VOUT is determined by: 1 VOUT IL ESR + 8 fCOUT (3) where f = operating frequency, COUT = output capacitance and IL = ripple current in the inductor. For a fixed output voltage, the output ripple voltage is highest at maximum input voltage since IL increases with input voltage. Aluminum electrolytic and solid tantalum capacitors are both available in surface mount configurations. In the case of tantalum, it is critical that the capacitors are surge tested for use in switching power supplies. An excellent choice is the AVX TPS series of surface mount tantalum. These are specially constructed and tested for low ESR so they give the lowest ESR for a given volume. Other capacitor types include Sanyo POSCAP, Kemet T510 and T495 series, and Sprague 593D and 595D series. Consult the manufacturer for other specific recommendations.
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LTC3550-1 APPLICATIO S I FOR ATIO
Using Ceramic Input and Output Capacitors Higher capacitance values, lower cost ceramic capacitors are now becoming available in smaller case sizes. Their high ripple current, high voltage rating and low ESR make them ideal for switching regulator applications. Because the LTC3550-1's control loop does not depend on the output capacitor's ESR for stable operation, ceramic capacitors can be used freely to achieve very low output ripple and small circuit size. When choosing the input and output ceramic capacitors, choose the X5R or X7R dielectric formulations. These dielectrics have the best temperature and voltage characteristics of all the ceramics for a given value and size. Efficiency Considerations The efficiency of a switching regulator is equal to the output power divided by the input power times 100%. It is often useful to analyze individual losses to determine what is limiting the efficiency and which change would produce the most improvement. Efficiency can be expressed as: Efficiency = 100% - (L1 + L2 + L3 + ...) where L1, L2, etc. are the individual losses as a percentage of input power. Although all dissipative elements in the circuit produce losses, two main sources usually account for most of the losses in LTC3550-1 circuits: VCC quiescent current and I2R losses. The VCC quiescent current loss dominates the efficiency loss at very low load currents whereas the I2R loss dominates the efficiency loss at medium to high load currents. In a typical efficiency plot, the efficiency curve at very low load currents can be misleading since the actual power lost is of no consequence as illustrated in Figure 3. 1. The VCC quiescent current is due to two components: the DC bias current as given in the Electrical Characteristics and the internal main switch and synchronous switch gate charge currents. The gate charge current results from switching the gate capacitance of the internal power MOSFET switches. Each time the gate is switched from high to low to high again, a packet of charge, dQ, moves from VCC to ground. The resulting dQ/dt is the current out of VCC that is typically larger
POWER LOSS (W)
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than the DC bias current. In continuous mode, IGATECHG = f(QT + QB) where QT and QB are the gate charges of the internal top and bottom switches. Both the DC bias and gate charge losses are proportional to VCC and thus their effects will be more pronounced at higher supply voltages. 2. I2R losses are calculated from the resistances of the internal switches, RSW, and external inductor RL. In continuous mode, the average output current flowing through inductor L is "chopped" between the main switch and the synchronous switch. Thus, the series resistance looking into the SW pin is a function of both top and bottom MOSFET RDS(ON) and the duty cycle (DC) as follows: RSW = (RDS(ON)TOP)(DC) + (RDS(ON)BOT)(1 - DC) The RDS(ON) for both the top and bottom MOSFETs can be obtained from the Typical Performance Characteristics curves. Thus, to obtain I2R losses, simply add RSW to RL and multiply the result by the square of the average output current. Other losses including CIN and COUT ESR dissipative losses and inductor core losses generally account for less than 2% total additional loss.
1 0.1 0.01 0.001 0.0001 0.00001 0.1 1 10 100 LOAD CURRENT (mA) 1000
3550-1 F03
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Figure 3. Power Lost vs Load Current
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LTC3550-1 APPLICATIO S I FOR ATIO
Thermal Considerations The battery charger's thermal regulation feature and the buck regulator's high efficiency make it unlikely that enough power is dissipated to exceed the LTC3550-1 maximum junction temperature. Nevertheless, it is a good idea to do some thermal analysis for worst-case conditions. The junction temperature, TJ, is given by: TJ = TA + TRISE where TA is the ambient temperature. The temperature rise is given by: TRISE = PD * JA where PD is the power dissipated and JA is the thermal resistance from the junction of the die to the ambient temperature. In most applications the buck regulator does not dissipate much heat due to its high efficiency. The majority of the LTC3550-1 power dissipation occurs when charging a battery. Fortunately, the LTC3550-1 automatically reduces the charge current during high power conditions using a patented thermal regulation circuit. Thus, there is no need to design for worst-case power dissipation scenarios because the LTC3550-1 ensures that the battery charger power dissipation never raises the junction temperature above a preset value of 105C. In the unlikely case that the junction temperature is forced above 105C (due to abnormally high ambient temperatures or excessive buck regulator power dissipation), the battery charge current will be reduced to zero and thus dissipate no heat. As an added measure of protection, even if the junction temperature reaches approximately 150C, the buck regulator's power switches will be turned off and the SW node will become high impedance. The conditions that cause the LTC3550-1 to reduce charge current through thermal feedback can be approximated by considering the power dissipated in the IC. The approximate ambient temperature at which the thermal feedback begins to protect the IC is: TA = 105C - TRISE TA = 105C - (PD * JA) TA = 105C - (PD(CHARGER) + PD(BUCK)) * JA (4)
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Most of the charger's power dissipation is generated from the internal charger MOSFET. Thus, the power dissipation is calculated to be: PD(CHARGER) = (VIN - VBAT) * IBAT (5) VIN is the charger supply voltage (either DCIN or USBIN), VBAT is the battery voltage and IBAT is the charge current. Example: An LTC3550-1 operating from a 5V wall adapter (on the DCIN input) is programmed to supply 650mA full-scale current to a discharged Li-Ion battery with a voltage of 2.7V. The charger power dissipation is calculated to be: PD(CHARGER) = (5V - 2.7V) * 650mA = 1.495W For simplicity, assume the buck regulator is disabled and dissipates no power (PD(BUCK) = 0). For a properly soldered DHC16 package, the thermal resistance (JA) is 40C/W. Thus, the ambient temperature at which the LTC3550-1 charger will begin to reduce the charge current is: TA = 105C - 1.495W * 40C/W TA = 105C - 59.8C TA = 45.2C The LTC3550-1 can be used above 45.2C ambient, but the charge current will be reduced from 650mA. Assuming no power dissipation from the buck converter, the approximate current at a given ambient temperature can be approximated by: IBAT = 105 C - TA ( VIN - VBAT ) * JA (6) Using the previous example with an ambient temperature of 60C, the charge current will be reduced to approximately: IBAT = 105 C - 60 C 45 C = (5V - 2 . 7 V) * 40 C/W 92 C/A IBAT = 489mA Because the regulator typically dissipates significantly less heat than the charger (even in worst-case situations), the calculations here should work well as an approximation.
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LTC3550-1 APPLICATIO S I FOR ATIO
However, the user may wish to repeat the previous analysis to take the buck regulator's power dissipation into account. Equation (6) can be modified to take into account the temperature rise due to the buck regulator: IBAT = 105 C - TA - (PD(BUCK ) * JA ) ( VIN - VBAT ) * JA (7)
For optimum performance, it is critical that the exposed metal pad on the backside of the LTC3550-1 package is properly soldered to the PC board ground. When correctly soldered to a 2500mm2 double sided 1oz copper board, the LTC3550-1 has a thermal resistance of approximately 40C/W. Failure to make thermal contact between the exposed pad on the backside of the package and the copper board will result in thermal resistances far greater than 40C/W. As an example, a correctly soldered LTC3550-1 can deliver over 800mA to a battery from a 5V supply at room temperature. Without a good backside thermal connection, this number would drop to much less than 500mA. Battery Charger Stability Considerations The constant-voltage mode feedback loop is stable without any compensation provided a battery is connected to the charger output. When the charger is in constant-current mode, the charge current program pin (IDC or IUSB) is in the feedback loop, not the battery. The constant-current mode stability is affected by the impedance at the charge current program pin. With no additional capacitance on this pin, the charger is stable with program resistor values as high as 20k (ICHG = 50mA); however, additional capacitance on these nodes reduces the maximum allowed program resistor value. Checking Regulator Transient Response The regulator loop response can be checked by looking at the load transient response. Switching regulators take several cycles to respond to a step in load current. When a load step occurs, VOUT immediately shifts by an amount equal to (ILOAD * ESR), where ESR is the effective series resistance of COUT. ILOAD also begins to charge or discharge COUT, which generates a feedback error signal. The regulator loop then acts to return VOUT to its steady state value. During this recovery time VOUT can be monitored
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for overshoot or ringing that would indicate a stability problem. For a detailed explanation of switching control loop theory, see Application Note 76. A second, more severe transient is caused by switching in loads with large (>1F) supply bypass capacitors. The discharged bypass capacitors are effectively put in parallel with COUT, causing a rapid drop in VOUT. No regulator can deliver enough current to prevent this problem if the load switch resistance is low and it is driven quickly. The only solution is to limit the rise time of the switch drive so that the load rise time is limited to approximately (25 * CLOAD). Thus, a 10F capacitor charging to 3.3V would require a 250s rise time, limiting the charging current to about 130mA. Protecting the USB Pin and Wall Adapter Input from Overvoltage Transients Caution must be exercised when using ceramic capacitors to bypass the USBIN pin or the wall adapter inputs. High voltage transients can be generated when the USB or wall adapter is hot-plugged. When power is supplied via the USB bus or wall adapter, the cable inductance along with the self resonant and high Q characteristics of ceramic capacitors can cause substantial ringing which could exceed the maximum voltage ratings and damage the LTC3550-1. Refer to Linear Technology Application Note 88, entitled "Ceramic Input Capacitors Can Cause Overvoltage Transients" for a detailed discussion of this problem. The long cable lengths of most wall adapters and USB cables makes them especially susceptible to this problem. To bypass the USB and the wall adapter inputs, add a 1 resistor in series with a ceramic capacitor to lower the effective Q of the network and greatly reduce the ringing. A tantalum, OS-CON, or electrolytic capacitor can be used in place of the ceramic and resistor, as their higher ESR reduces the Q, thus reducing the voltage ringing. The oscilloscope photograph in Figure 4 shows how serious the overvoltage transient can be for the USB and wall adapter inputs. For both traces, a 5V supply is hot-plugged using a three foot long cable. For the top trace, only a 4.7F ceramic X5R capacitor (without the recommended 1 series resistor) is used to locally bypass the input. This trace shows excessive ringing when the 5V cable is inserted, with the overvoltage spike reaching 10V. For
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LTC3550-1 APPLICATIO S I FOR ATIO
the bottom trace, a 1 resistor is added in series with the 4.7F capacitor to locally bypass the 5V input. This trace shows the clean response resulting from the addition of the 1 resistor.
4.7F ONLY 2V/DIV
4.7F + 1 2V/DIV 20s/DIV
3550-1 F04
Figure 4. Waveforms Resulting from Hot-Plugging a 5V Input Supply When Using Ceramic Bypass Capacitors
Even with the additional 1 resistor, bad design techniques and poor board layout can often make the overvoltage problem even worse. System designers often add extra inductance in series with input lines in an attempt to minimize the noise fed back to those inputs by the application. In reality, adding these extra inductances only makes the overvoltage transients worse. Since cable inductance is one of the fundamental causes of the excessive ringing, adding a series ferrite bead or inductor increases the effective cable inductance, making the problem even worse. For this reason, do not add additional inductance (ferrite beads or inductors) in series with the USB or wall adapter inputs. For the most robust solution, 6V transorbs or zener diodes may also be added to further protect the USB and wall adapter inputs. Two possible protection devices are the SM2T from STMicroelectronics and the EDZ series devices from ROHM. Always use an oscilloscope to check the voltage waveforms at the USBIN and DCIN pins during USB and wall adapter hot-plug events to ensure that overvoltage transients have been adequately removed.
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PC Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC3550-1. These items are also illustrated graphically in Figures 5 and 6. Check the following in your layout: 1. The power traces, consisting of the GND trace, the SW trace and the VCC trace should be kept short, direct and wide. 2. Does the VOUT pin connect directly to the output? 3. Does the (+) plate of CIN connect to VCC as closely as possible? This capacitor provides the AC current to the internal power MOSFETs. 4. Keep the (-) plates of CIN and COUT as close as possible. 5. Solder the exposed pad on the backside of the package to PC board ground for optimum thermal performance. The thermal resistance of the package can be further enhanced by increasing the area of the copper used for PC board ground. Design Example As a design example, assume the LTC3550-1 is used in a single lithium-ion battery-powered cellular phone application. The battery is charged by either plugging a wall adapter into the phone or putting the phone in a USB cradle. The optimum charge current for this particular lithium-ion battery is determined to be 800mA. Starting with the charger, choosing RIDC to be 1.24k programs the charger for 806mA. Choosing RIUSB to be 2.1k programs the charger for 475mA when charging from the USB cradle, ensuring that the charger never exceeds the 500mA maximum current supplied by the USB port. A good rule of thumb for ITERMINATE is onetenth the full charge current, so RITERM is picked to be 1.24k (ITERMINATE = 80mA). Moving on to the step-down converter, VCC will be powered from the battery which can range from a maximum of 4.2V down to about 2.7V. The load current requirement is a maximum of 600mA but most of the time it will be in standby mode, requiring only 2mA. Efficiency at both low
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LTC3550-1 APPLICATIO S I FOR ATIO
VOUT fO * L V * 1 - OUT VCC and high load currents is important. With this information we can calculate L using Equation (1), IL =
Substituting VOUT = 1.875V, VCC = 4.2V, IL = 240mA and fO = 1.5MHz in Equation (3) gives: L= 1 . 875V 1 . 875V * 1- = 2 . 88H 1 . 5MHz * (240mA) 4 . 2V
VCC
+ -
CIN
Figure 5. DC-DC Converter Layout Diagram
VIA TO VOUT
CIN L1 VCC GND COUT VOUT
Figure 6. DC-DC Converter Suggested Layout
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A 2.2H inductor works well for this application. For best efficiency choose a 720mA or greater inductor with less than 0.2 series resistance. CIN will require an RMS current rating of at least 0.3A = ILOAD(MAX)/2 at temperature and COUT will require an ESR of less than 0.25. In most cases, a ceramic capacitor will satisfy this requirement. Figure 7 shows the complete circuit along with its efficiency curve.
LTC3550-1 6 7 VOUT VCC SW GND 10 BOLD LINES INDICATE HIGH CURRENT PATHS 8 GND 9 L1
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-
17 COUT
+
VOUT
3550-1 F05
VIA TO VCC
SW
3550-1 F06
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WALL ADAPTER USB POWER 1F 2.1k 1% 1F 1.24k 1% * MURATA LQH32CN2R2M33 ** TAIYO YUDEN JMK316BJ106ML TAIYO YUDEN LMK212BJ475MG
Figure 7a. Design Example Circuit
100 VCC = 2.7V 90 EFFICIENCY (%) VCC = 4.2V 80
70
60
50 0.1
Figure 7b. Buck Regulator Efficiency vs Output Current
TYPICAL APPLICATIO S
Full Featured Dual Input Charger Plus Step-Down Converter
LTC3550-1 BAT DCIN USBIN 1F 1F 1k PWR CHRG IUSB IDC 2.1k 1% 1.24k 1% 1k 1% ITERM GND VCC 2.2H SW VOUT 10F CER VOUT 1.875V 600mA 1k 800mA (WALL) 475mA (USB) 4.7F
WALL ADAPTER USB POWER
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LTC3550-1 SW RUN DCIN USBIN IUSB IDC VOUT VCC BAT ITERM GND 4.7F 1.24k 1% 800mA (WALL) 475mA (USB) 2.2H* VOUT 1.875V 600mA 10F** CER
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+
4.2V SINGLECELL Li-Ion BATTERY
3550-1 F07a
VCC = 3.6V
1
10 ILOAD (mA)
100
1000
35501 F07b
+
4.2V SINGLE-CELL Li-Ion BATTERY
3550-1 TA03
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LTC3550-1 PACKAGE DESCRIPTIO U
DHC Package 16-Lead Plastic DFN (5mm x 3mm)
(Reference LTC DWG # 05-08-1706)
0.65 0.05 3.50 0.05 PACKAGE OUTLINE 0.25 0.05 0.50 BSC 4.40 0.05 (2 SIDES) RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS 5.00 0.10 (2 SIDES) R = 0.20 TYP R = 0.115 TYP 9 16 0.40 0.10 3.00 0.10 (2 SIDES) PIN 1 TOP MARK (SEE NOTE 6) 8 0.200 REF 0.75 0.05 4.40 0.10 (2 SIDES) BOTTOM VIEW--EXPOSED PAD NOTE: 1. DRAWING PROPOSED TO BE MADE VARIATION OF VERSION (WJED-1) IN JEDEC PACKAGE OUTLINE MO-229 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE 1 0.25 0.05 0.50 BSC 1.65 0.10 (2 SIDES) PIN 1 NOTCH
(DHC16) DFN 1103
1.65 0.05 2.20 0.05 (2 SIDES)
0.00 - 0.05
35501f
Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights.
23
LTC3550-1 TYPICAL APPLICATIO S
Dual Input Charger Plus Step-Down Converter with Wall Adapter PowerPathTM
USB POWER
1F
2.1k 1%
1.24k 1%
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PowerPath and ThinSOT are trademarks of Linear Technology Corporation.
35501f
24 Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2005
U
LTC3550-1 DCIN 1F USBIN VCC 4.7F 1k
WALL ADAPTER
800mA (WALL) 475mA (USB) IUSB IDC ITERM 1k 1% GND BAT 2.2H SW VOUT 10F CER VOUT 1.875V 600mA
+
4.2V SINGLE-CELL Li-Ion BATTERY
3550-1 TA04
LT 1205 * PRINTED IN USA

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