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LTC3453 Synchronous Buck-Boost High Power White LED Driver
FEATURES

DESCRIPTIO
High Efficiency: 90% Typical Over Entire Li-Ion Battery Range Wide VIN Range: 2.7V to 5.5V Up to 500mA Continuous Output Current Internal Soft-Start Open/Shorted LED Protection LED Current Matching Typically <2% Constant Frequency 1MHz Operation Low Shutdown Current: 6A Overtemperature Protection Small Thermally Enhanced 16-Lead (4mm x 4mm) QFN Package
The LTC(R)3453 is a synchronous buck-boost DC/DC converter optimized for driving up to 4 white LEDs at a combined current of up to 500mA from a single Li-Ion battery input. The regulator operates in either synchronous buck, synchronous boost, or buck-boost mode, depending on input voltage and LED maximum forward voltage. Optimum efficiency is achieved using a proprietary architecture that determines which LED requires the largest forward voltage drop at its programmed current, and regulates the common output rail for lowest dropout. Efficiency of 90% can be achieved over the entire usable range of a Li-Ion battery (2.7V to 4.2V). LED current is programmable to one of four levels (including shutdown) with dual current setting resistors and dual enable pins. In shutdown, the supply current is only 6A. A high constant operating frequency of 1MHz allows the use of a small external inductor. The LTC3453 is offered in a low profile (0.75mm) thermally enhanced 16-lead (4mm x 4mm) QFN package.
APPLICATIO S

Cell Phones Digital Cameras PDAs Portable Devices
, LTC and LT are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners.
TYPICAL APPLICATIO
High Efficiency Torch/Flash LED Driver
L1 4.7H VIN 1-CELL Li-Ion 2.7V to 4.2V
LED POWER EFFICIENCY PLED/PIN (%)
2.2F
VIN
PVIN
SW1
SW2
VOUT
150mA/500mA
4.7F
D1 LED1 LED2 VC 0.1F EN1 (TORCH) EN2 (FLASH) 8.25k 1% EN1 EN2 ISET1 ISET2 3.48k 1% LTC3453 GND GND PGND
3453 TA01a
1MHz BUCK-BOOST
LED3 LED4 D1: LUMILEDS LXCL-PWF1 L1: VISHAY DALE IDCS-2512 EN1 0 1 0 1 EN2 0 0 1 1 ILED 0 (SHUTDOWN) 150mA 350mA 500mA
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Torch Mode Efficiency vs VIN
100 180 90 EFFICIENCY 80 140 160
INPUT CURRENT (mA)
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70 IIN 60 ILED = 150mA TA = 25C 3.1 3.5 3.9 4.3 VIN (V) 4.7 5.1 5.5
120
100
50 2.7
80
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LTC3453
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
VOUT SW1 SW2 PVIN
VIN, PVIN, SW1, SW2, VOUT Voltage ............ -0.3V to 6V LED1 to LED4 Voltage ...... -0.3V to (VOUT + 0.3V) or 6V VC, EN1, EN2, ISET1, ISET2 Voltage .......... -0.3V to (VIN + 0.3V) or 6V LED1 to LED4 Peak Current ................................ 250mA Storage Temperature Range .................. -65C to 125C Operating Temperature Range (Note 2) ... -40C to 85C Junction Temperature (Note 3) ............................. 125C
16 15 14 13 VIN 1 EN1 2 ISET1 3 LED1 4 5
GND
12 VC 17 11 EN2 10 ISET2 9 LED4 6
LED2
7
LED3
8
GND
UF PACKAGE 16-LEAD (4mm x 4mm) PLASTIC QFN TJMAX = 125C, JA = 40C/W, JC = 2.6C/W EXPOSED PAD (PIN 17) IS PGND, MUST BE SOLDERED TO PCB
ORDER PART NUMBER LTC3453EUF
UF PART MARKING 3453
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.
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C, VIN = VOUT = 3.6V unless otherwise noted. (Note 2)
PARAMETER Input Supply Voltage Input DC Supply Current Normal Operation Shutdown UVLO Undervoltage Lockout Threshold EN1,2 DC Threshold for Normal Operation EN1,2 Input Current ISET1,2 Servo Voltage LED Output Current Ratio LED Output Current Matching LED Pin Drain Voltage Regulated Maximum VOUT PMOS Switch RON NMOS Switch RON Forward Current Limit Reverse Current Limit 2.7V VIN 5.5V, RISET1||RISET2 = 51.1k, ILEDx = 0 (Note 4) 2.7V VIN 5.5V; VEN1 = VEN2 = 0V VIN < UVLO Threshold VIN Rising VIN Falling 2.7V VIN 5.5V, VEN1,2 Rising VEN1,2 = 3.6V RISET1,2 = 4.12k, 0C TA 85C RISET1,2 = 4.12k, -40C TA 85C ILED/(ISET1 + ISET2), ILEDx = 75mA, VLEDx = 300mV, 2.7V VIN 5.5V (MAX - MIN)/[(MAX + MIN)/2] * 100%, ILEDx = 75mA VLEDx = 300mV ILEDx = 75mA VLEDx = 0V Switches A and D, @ 100mA Switches B and C, @ 100mA Switch A Switch D 1125

ELECTRICAL CHARACTERISTICS
CONDITIONS
MIN
TYP
MAX 5.5
UNITS V mA A A V V V V A mV mV mA/mA mA/mA % mV
2.7 0.6 6 3 1.6 2 1.9 0.65 0.2 -1 788 780 365 357 800 800 384 384 2 130 4.4 4.5 0.3 0.25 1612 200 0.63
1 18 5 2.3 1 1 812 812 403 403 6

EN1,2 DC Threshold for Shutdown (ILEDx = 0) 2.7V VIN 5.5V, VEN1,2 Falling
4.6
2100
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V mA mA
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LTC3453
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C, VIN = VOUT = 3.6V unless otherwise noted. (Note 2)
PARAMETER PMOS Switch Leakage NMOS Switch Leakage Oscillator Frequency Soft-Start Time 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: The LTC3453E is guaranteed to meet 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. CONDITIONS Switches A and D Switches B and C

ELECTRICAL CHARACTERISTICS
MIN
TYP
MAX 1 1
UNITS A A MHz ms
0.9
1 0.65
1.1
Note 3: TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formula: TJ = TA + (PD * JA C/W). Note 4: Dynamic supply current is higher due to the gate charge being delivered at the switching frequency.
TYPICAL PERFOR A CE CHARACTERISTICS
Input DC Supply Current in Shutdown vs Temperature
20 18 16 14 VIN = 5.5V 2.1 808 VIN RISING 2.0 804 800 796 1.8 792 788 -55 -35 -15 FRONT PAGE APPLICATION 2.2
IIN (A)
12 10 8 6 4 2 0 -55 -35 -15 5 25 45 65 85 105 125 TEMPERATURE (C)
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VIN = 3.6V VIN = 2.7V
1.9
VIN FALLING
1.7 -55 -35 -15
5 25 45 65 85 105 125 TEMPERATURE (C)
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VISET1,2 (mV)
VIN = 4.2V
UVLO THRESHOLD (V)
ISET1,2 Servo Voltage vs VIN
812 808 804 TA = 25C RISET1,2 = 8.25k 4.55
FREQUENCY (kHz)
VISET1,2 (mV)
VOUT (V)
800 796 792 788 2.7
3.1
3.5
3.9 4.3 VIN (V)
4.7
UW
5.1
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Undervoltage Lockout Threshold vs Temperature
812
ISET1,2 Servo Voltage vs Temperature
VIN = 3.6V RISET1,2 = 8.25k
5 25 45 65 85 105 125 TEMPERATURE (C)
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Regulated Maximum VOUT vs Temperature
VIN = 3.6V 4.54 ALL LED PINS OPEN 4.53 4.52 4.51 4.50 4.49 4.48 4.47 4.46 5.5 4.45 -55 -35 -15 5 25 45 65 85 105 125 TEMPERATURE (C)
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Oscillator Frequency vs Temperature
1050 1040 1030 1020 1010 1000 990 980 970 960 950 -55 -35 -15 5 25 45 65 85 105 125 TEMPERATURE (C)
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VOUT = 3V
VIN = 5.5V VIN = 4.2V
VIN = 3.6V
VIN = 2.7V
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LTC3453 TYPICAL PERFOR A CE CHARACTERISTICS
Efficiency vs LED Current
100 FRONT PAGE APPLICATION PLED/PIN, VIN = 3.6V, TA = 25C
CH1, VOUT 2V/DIV 20mV/DIV AC COUPLED 0V CH2, EN1 1V/DIV 0V
90
EFFICIENCY (%)
80
70
60
5s/DIV FRONT PAGE APPLICATION VIN = 3.6V ILED = 150mA
50 100 150 200 250 300 350 400 450 500 LED CURRENT (mA)
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PI FU CTIO S
VIN (Pin 1): Signal Voltage Input Supply Pin [2.7V VIN 5.5V]. Recommended bypass capacitor to GND is 2.2F ceramic or larger. Connect to PVIN (Pin 16). EN1 (Pin 2): Enable Input Pin for ISET1 Current. ISET1 (Pin 3): White Led Current Programming Pin. A resistor to ground programs each current source output to ILED = 384(0.8V/RISET1). This amount of current adds to any amount set by EN2/ISET2 if also used. LED1 to LED4 (Pins 4, 6, 7, 9): Individual Low Dropout Current Source Outputs for White LED Current Biasing. Connect each white LED between VOUT and an individual LEDx pin. Unused LEDx outputs should be connected to VOUT. GND (Pins 5 and 8): Signal Ground Pin. Connect to PGND (Exposed Pad). ISET2 (Pin 10): White Led Current Programming Pin. A resistor to ground programs each current source output to ILED = 384(0.8V/RISET2). This amount of current adds to any amount set by EN1/ISET1 if also used. EN2 (Pin 11): Enable Input Pin for ISET2 Current. VC (Pin 12): Compensation Point for the Internal Error Amplifier Output. Recommended compensation capacitor to GND is 0.1F ceramic or larger. VOUT (Pin 13): Buck-Boost Output Pin. Recommended bypass capacitor to GND is 4.7F ceramic. SW2 (Pin 14): Switching Node Pin. Connected to internal power switches C and D. External inductor connects between SW1 and SW2. Recommended value is 4.7H. SW1 (Pin 15): Switching Node Pin. Connected to internal power switches A and B. External inductor connects between SW1 and SW2. Recommended value is 4.7H. PVIN (Pin 16): Power Voltage Input Supply Pin. Connect to VIN (Pin 1). Exposed Pad (Pin 17): Power Ground Pin. Connect to GND (Pin 8) and solder to PCB ground for optimum thermal performance.
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UW
Output Voltage Ripple
Startup Transient
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1ms/DIV FRONT PAGE APPLICATION VIN = 3.6V ILED = 150mA
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BLOCK DIAGRA
VIN 2.7V TO 5.5V
16 1
PVIN VIN
UNDERVOLTAGE LOCKOUT
OVERTEMP PROTECTION
+
BANDGAP REFERENCE 1.23V 1612mA
- + -
AB PWM COMPARATOR UV 1MHz OSCILLATOR MAIN ERROR AMP SAFETY ERROR AMP CD PWM COMPARATOR OT LOGIC
12
VC
VBIAS VFB
- +
- +
1.23V LED CURRENT SETTING AMP 1 800mV
SOFT START CLAMP
+ -
ILED 384
6
3 RISET1
ISET1 LED CURRENT SETTING AMP 2 ILED 384 9 7
800mV
+ -
10 RISET2 2
ISET2
EN1 EN2 11 SHUTDOWN 5 GND 8 GND 17 EXPOSED PAD (PGND)
3453 BD
+
-
+
-
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OPTIONAL OPTIONAL 15 SWITCH A SW1 SW2 14 SWITCH D VOUT 13 VOUT SWITCH B UV GATE DRIVERS AND ANTICROSSCONDUCTION SWITCH C LED1 4 OT FORWARD CURRENT LIMIT REVERSE CURRENT LIMIT LED DETECT 200mA LED2 6 LED DETECT LED3 7 LED DETECT VOUT 1.23V 327k LED DETECT 123k OR 4 LED4 9
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LTC3453
OPERATIO
Buck-Boost DC-DC Converter The LTC3453 employs an LTC proprietary buck-boost DC/DC converter to generate the output voltage required to drive the LEDs. This architecture permits high-efficiency, low noise operation at input voltages above, below or equal to the output voltage by properly phasing four internal power switches. The error amp output voltage on the VC pin determines the duty cycle of the switches. Since the VC pin is a filtered signal, it provides rejection of frequencies well below the factory trimmed switching frequency of 1MHz. The low RDS(ON), low gate charge synchronous switches provide high frequency pulse width modulation control at high efficiency. Schottky diodes across synchronous rectifier switch B and synchronous rectifier switch D are not required, but if used do provide a lower voltage drop during the break-before-make time (typically 20ns), which improves peak efficiency by typically 1% to 2% at higher loads. Figure 1 shows a simplified diagram of how the four internal power switches are connected to the inductor, VIN, VOUT and GND. Figure 2 shows the regions of operation of the buck-boost as a function of the control voltage VC. The output switches are properly phased so transitions between regions of operation are continuous, filtered and transparent to the user. When VIN approaches VOUT, the buck-boost region is reached where the conduction time of the four switch region is typically 150ns. Referring to Figures 1 and 2, the various regions of operation encountered as VC increases will now be described.
PVIN 16 PMOS A SW1 15 NMOS B SW2 14
Figure 1. Simplified Diagram of Internal Power Switches
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Buck Mode (VIN > VOUT) In buck mode, switch D is always on and switch C is always off. Referring to Figure 2, when the control voltage VC is above voltage V1, switch A begins to turn on each cycle. During the off time of switch A, synchronous rectifier switch B turns on for the remainder of the cycle. Switches A and B will alternate conducting similar to a typical synchronous buck regulator. As the control voltage increases, the duty cycle of switch A increases until the maximum duty cycle of the converter in buck mode reaches DCBUCK|max given by: DCBUCK|max = 100% - DC4SW where DC4SW equals the duty cycle in % of the "four switch" range. DC4SW = (150ns * f) * 100% where f is the operating frequency in Hz. Beyond this point the "four switch" or buck-boost region is reached. Buck-Boost or Four-Switch Mode (VIN VOUT) Referring to Figure 2, when the control voltage VC is above voltage V2, switch pair AD continue to operate for duty cycle DCBUCK|max, and the switch pair AC begins to phase in. As switch pair AC phases in, switch pair BD phases out accordingly. When the VC voltage reaches the edge of the buck-boost range at voltage V3, switch pair AC completely phases out switch pair BD and the boost region begins at
75% DMAX BOOST V4 (2.1V) A ON, B OFF BOOST REGION PWM CD SWITCHES DMIN BOOST V3 (1.65V) FOUR SWITCH PWM BUCK/BOOST REGION V2 (1.55V) D ON, C OFF PWM AB SWITCHES BUCK REGION
VOUT 13 PMOS D
DMAX BUCK
NMOS C
0% DUTY CYCLE
V1 (0.9V) CONTROL VOLTAGE, VC
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Figure 2. Switch Control vs Control Voltage, VC
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LTC3453
OPERATIO
duty cycle DC4SW. The input voltage VIN where the four switch region begins is given by: VIN = VOUT/[1 - (150ns * f)] and the input voltage VIN where the four switch region ends is given by VIN = VOUT * (1 - DC4SW) = VOUT * [1 - (150ns * f)] Boost Mode (VIN < VOUT) In boost mode, switch A is always on and switch B is always off. Referring to Figure 2, when the control voltage VC is above voltage V3, switches C and D will alternate conducting similar to a typical synchronous boost regulator. The maximum duty cycle of the converter is limited to 88% typical and is reached when VC is above V4. Forward Current Limit If the current delivered from VIN through PMOS switch A exceeds 1612mA (typical), switch A is shut off immediately. Switches B and D are turned on for the remainder of the cycle in order to safely discharge the forward inductor current at the maximum rate possible. Reverse Current Limit If the current delivered from VOUT backwards through PMOS switch D exceeds 200mA (typical), switch D is shut off immediately. Switches A and C are turned on for the remainder of the cycle in order to safely discharge the reverse inductor current at the maximum rate possible. Undervoltage Lockout To prevent operation of the power switches at high RDS(ON), an undervoltage lockout is incorporated on the LTC3453. When the input supply voltage drops below approximately 1.9V, the four power switches and all control circuitry are turned off except for the undervoltage block, which draws only several microamperes. Overtemperature Protection If the junction temperature of the LTC3453 exceeds 130C for any reason, all four switches are shut off immediately. The overtemperature protection circuit has a typical hysteresis of 11C.
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Soft-Start The LTC3453 includes an internally fixed soft-start which is active when powering up or coming out of shutdown. The soft-start works by clamping the voltage on the VC node and gradually releasing it such that it requires 0.65ms to linearly slew from 0.9V to 2.1V. This has the effect of limiting the rate of duty cycle change as VC transitions from the buck region through the buck-boost region into the boost region. Once the soft-start times out, it can only be reset by entering shutdown, or by an undervoltage or overtemperature condition. Main Error Amp The main error amplifier is a transconductance amplifier with source and sink capability. The output of the main error amplifier drives a capacitor to GND at the VC pin. This capacitor sets the dominant pole for the regulation loop. (See the Applications Information section for selecting the capacitor value.) The error amp gets its feedback signal from a proprietary circuit which monitors all 4 LED current sources to determine which LED to close the regulation loop on. Safety Error Amp The safety error amplifier is a transconductance amplifier with sink only capability. In normal operation, it has no effect on the loop regulation. However, if any of the LED pins open-circuits, the output voltage will keep rising, and safety error amp will eventually take over control of the regulation loop to prevent VOUT runaway. The VOUT threshold at which this occurs is approximately 4.5V. LED Current Setting Amplifiers and Enable Circuit The LTC3453 includes two LED current setting amplifiers that work in conjunction with dual external current setting resistors and dual enable pins to program LED current to one of four levels (including shutdown). All four LED current source outputs are programmed to the same level. When both enable inputs are logic low, the LTC3453 is in shutdown, the buck-boost is disabled and all LED currents are zero. In shutdown, the input supply current is typically 6A. If either enable input is logic high, the buck-boost will regulate the output voltage such that the LEDs are biased
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LTC3453
OPERATIO
at the current programmed by resistors RISET1 and/or RISET2. Individually enabled, each LED current setting amplifier programs the output LED current to ILED = 384 (0.8V/RISET1,2) If both enable inputs are logic high, the setting currents are summed internally and the output LED current will be given by ILED = 384 [0.8V/(RISET1 || RISET2) ] Thus three different (nonzero) current levels are programmable, optimal for low current LED torch and high current LED camera flash applications. LED Current Sources Each LED pin is driven by a current source specifically designed for low dropout. The LTC3453 employs a propri-
APPLICATIO S I FOR ATIO
Component Selection Inductor Selection
The high frequency operation of the LTC3453 allows the use of small surface mount inductors. The inductor current ripple is typically set to 20% to 40% of the maximum inductor current. For a given ripple the inductance terms are given as follows:
L> L>
VIN(MIN)2 * VOUT - VIN(MIN) * 100% f * IOUT(MAX) * %Ripple * VOUT2 VOUT * VIN(MAX) - VOUT * 100% f * IOUT(MAX) * %Ripple * VIN(MAX)
(
)
,
(
)
where f = operating frequency, Hz %Ripple = allowable inductor current ripple, % VIN(MIN) = minimum input voltage, V VIN(MAX) = maximum input voltage, V VOUT = output voltage, V IOUT(MAX) = maximum output load current
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etary architecture that determines which of the four LEDs requires the largest forward voltage drop at its programmed current, and then generates a feedback voltage based on this one for closing the buck-boost regulation loop. This results in the lowest output voltage required for regulating all of the LEDs and thus the highest LED power efficiency. The voltage present at the LED pin of the "controlling LED" will be typically 130mV at 75mA of current. LED Detect Circuit If fewer than four LED outputs are required, unused ones should be connected to VOUT. Each LED pin has an internal LED detect circuit that disables the output current source to save power if an output is not needed. A small 30A current is employed to detect the presence of an LED at startup. For high efficiency, choose an inductor with a high frequency core material, such as ferrite, to reduce core loses. The inductor should have low ESR (equivalent series resistance) to reduce the I2R losses, and must be able to handle the peak inductor current without saturating. Molded chokes or chip inductors usually do not have enough core to support peak inductor currents >1A. To minimize radiated noise, use a toroid, pot core or shielded bobbin inductor. For the white LED application, a 4.7H inductor value is recommended. See Table 1 for a list of component suppliers.
Table 1. Inductor Vendor Information
SUPPLIER Coilcraft Cooper/Coiltronics Murata Sumida Vishay-Dale WEB SITE www.coilcraft.com www.cooperet.com www.murata.com www.japanlink.com/sumida www.vishay.com
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APPLICATIO S I FOR ATIO
Input Capacitor Selection
Since the VIN pin is the supply voltage for the IC it is recommended to place at least a 2.2F, low ESR bypass capacitor to ground. See Table 2 for a list of component suppliers.
Table 2. Capacitor Vendor Information
SUPPLIER AVX Sanyo Taiyo Yuden TDK WEB SITE www.avxcorp.com www.sanyovideo.com www.t-yuden.com www.component.tdk.com
Output Capacitor Selection The bulk value of the capacitor is set to reduce the ripple due to charge into the capacitor each cycle. The steady state ripple due to charge is given by:
%Ripple _ Boost = %Ripple _ Buck = IOUT(MAX ) * VOUT - VIN(MIN) * 100
(
8 * VIN(MAX ) * f 2 * L * COUT
( VIN(MAX) - VOUT ) * 100 %
COUT * VOUT * f
2
where COUT = output filter capacitor, F The output capacitance is usually many times larger in order to handle the transient response of the converter. For a rule of thumb, the ratio of the operating frequency to the unity-gain bandwidth of the converter is the amount the output capacitance will have to increase from the above calculations in order to maintain the desired transient response. The other component of ripple is due to the ESR (equivalent series resistance) of the output capacitor. Low ESR capacitors should be used to minimize output voltage ripple. For surface mount applications, Taiyo Yuden, TDK, AVX ceramic capacitors, AVX TPS series tantalum capacitors or Sanyo POSCAP are recommended. For the white LED application, a 4.7F capacitor value is recommended. See Table 2 for a list of component suppliers.
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Optional Schottky Diodes Schottky diodes across the synchronous switches B and D are not required, but provide a lower drop during the break-before-make time (typically 20ns) of the NMOS to PMOS transition, improving efficiency. Use a Schottky diode such as an MBRM120T3 or equivalent. Do not use ordinary rectifier diodes, since the slow recovery times will compromise efficiency. Closing the Feedback Loop The LTC3453 incorporates voltage mode PWM control. The control to output gain varies with operation region (Buck, Boost, Buck/Boost), but is usually no greater than 15. The output filter exhibits a double pole response given by:
fFILTER _ POLE = 1 Hz 2 * * L * COUT
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where COUT is the output filter capacitor. The output filter zero is given by:
fFILTER _ ZERO = 1 2 * * RESR * COUT Hz
where RESR is the capacitor equivalent series resistance. A troublesome feature in Boost mode is the right-half plane zero (RHP), and is given by:
fRHPZ
VIN = Hz 2 * * IOUT * L * VOUT
2
The loop gain is typically rolled off before the RHP zero frequency. A simple Type I compensation network can be incorporated to stabilize the loop but at a cost of reduced bandwidth and slower transient response. To ensure proper phase margin, the loop requires to be crossed over a decade before the LC double pole.
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LTC3453
APPLICATIO S I FOR ATIO
The unity-gain frequency of the error amplifier with the Type I compensation is given by:
fUG =
gm 2 * * CVC
where gm is the error amp transconductance (typically 1/5.2k) and CVC is the external capacitor to GND at the VC pin. For the white LED application, a 0.1F or greater capacitor value is recommended. Paralleling LED Outputs for Higher Current Two or more LED output pins can be connected together in parallel to achieve higher output current in fewer than 4 LEDs. For a very high power LED such as a LumiLED, all four outputs can be connected in parallel for maximum total output current, as shown in the cover page application of this datasheet. Maximum LED Current As described in the Operation section, the output LED current with both enable pins logic high is equal to ILED = 384 [0.8V/(RISET1 || RISET2)]
VIN VOUT
ENx ISETx RSET RMIN VOLTAGE DAC VDAC
LED1 LTC3453 LED4 ILED = 384 0.8V - VDAC RSET CURRENT DAC
(a)
VIN VOUT VIN
ENx ISETx RMIN RPOT
LED1 LTC3453 LED4 ILED = 384 0.8V RMIN + RPOT RSET 100
(c)
Figure 3. Brightness Control Methods: (a) Using Voltage DAC, (b) Using Current DAC, (c) Using Potentiometer, (d) Using PWM Input
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Since the maximum continuous output current is limited to 500mA, this sets a minimum limit on the parallel combination of RISET1 and RISET2 equal to RMIN = (RISET1 || RISET2)|MIN = 4(384[0.8V/500mA]) = 2458 Although the LTC3453 can safely provide this current continuously, the external LED(s) may not be rated for this high a level of continuous current. Higher current levels are generally reserved for pulsed applications, such as LED camera flash. This is accomplished by programming a high current with one of the RISET resistors and pulsing the appropriate enable pin. Varying LED Brightness Continuously variable LED brightness control can be achieved by interfacing directly to one or both of the ISET pins. Figure 3 shows four such methods employing a voltage DAC, a current DAC, a simple potentiometer or a PWM input. It is not recommended to control brightness by PWMing the enable pins directly as this will toggle the LTC3453 in and out of shutdown and result in erratic operation.
VIN VOUT ENx ISETx IDAC 0.8V RMIN LED1 LTC3453 LED4 ILED = 384 * IDAC
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VOUT
ENx ISETx RSET RMIN VPWM 1F
LED1 LTC3453 LED4 ILED = 384 = 384 DVCC fPWM 5kHz 0.8V - VPWM RSET 0.8V - (DC% * VDVCC) RSET
(d)
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LTC3453
APPLICATIO S I FOR ATIO
Unused Outputs
If fewer than 4 LED pins are to be used, unused LEDx pins should be connected to VOUT. The LTC3453 senses which current source outputs are not being used and shuts off the corresponding output currents to save power. A small trickle current (~30A) is still applied to unused outputs to detect if a white LED is later switched in and also to distinguish unused outputs from used outputs during startup. LED Failure Modes If an individual LED fails as a short circuit, the current source biasing it is shut off to save power. This is the same
PACKAGE DESCRIPTIO
UF Package 16-Lead Plastic QFN (4mm x 4mm)
(Reference LTC DWG # 05-08-1692)
4.35 0.05 2.15 0.05 2.90 0.05 (4 SIDES)
0.30 0.05 0.65 BSC RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS BOTTOM VIEW--EXPOSED PAD 4.00 0.10 (4 SIDES) PIN 1 TOP MARK (NOTE 6) 2.15 0.10 (4-SIDES) 0.75 0.05 R = 0.115 TYP PIN 1 NOTCH R = 0.20 TYP OR 0.35 x 45 CHAMFER
NOTE: 1. DRAWING CONFORMS TO JEDEC PACKAGE OUTLINE MO-220 VARIATION (WGGC) 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
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.
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operation as described previously (if the output were initially designated unused at power-up by connecting its LEDx pin to VOUT). Efficiency is not materially affected. If an individual LED fails as an open circuit, the control loop will initially attempt to regulate off of its current source feedback signal, since it will appear to be the one requiring the largest forward voltage drop to run at its programmed current. This will drive VOUT higher. As the open circuited LED will never accept its programmed current, VOUT must be voltage-limited by means of a secondary control loop. The LTC3453 limits VOUT to 4.5V in this failure mode. The other LEDs will still remain biased at the correct programmed current but the overall circuit efficiency will decrease.
0.72 0.05 PACKAGE OUTLINE 15 16 0.55 0.20 1 2
(UF16) QFN 1004
U
W
U
U
0.200 REF 0.00 - 0.05
0.30 0.05 0.65 BSC
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LTC3453
TYPICAL APPLICATIO U
High Efficiency 4 White LED Driver
4.7H VIN 1-CELL Li-Ion 2.2F VIN PVIN SW1 SW2 VOUT 4.7F 30mA 30mA 30mA 30mA D1 LED1 LED2 VC 0.1F EN EN1 EN2 ISET1 10.2k ISET2 GND GND LTC3453 PGND
3453 TA02
D2
D3
D4
1MHz BUCK-BOOST
LED3 LED4 D1 TO D4: NICHIA NSCW100 L1: VISHAY DALE IDCS-2512
RELATED PARTS
PART NUMBER LT1618 DESCRIPTION Constant Current, Constant Voltage 1.4MHz, High Efficiency Boost Regulator COMMENTS VIN: 1.6V to 18V, VOUT(MAX) = 34V, IQ = 1.8mA, ISD = <1A, MS10 Package/EDD Package VIN: 2.6V to 16V, VOUT(MAX) = 34V, IQ = 4.2mA/5.5mA, ISD = <1A, ThinSOT Package VIN: 1V to 10V, VOUT(MAX) = 34V, IQ = 1.2mA, ISD = <1A, ThinSOT Package VIN: 2.5V to 10V, VOUT(MAX) = 34V, IQ = 1.9mA, ISD = <1A, ThinSOT Package/SC70 Package VIN: 2.8V to 4.5V, VOUT(MAX) = 6V, IQ = 50A, ISD = <1A, QFN-24 Package VIN: 2.9V to 4.4V, VOUT(MAX) = 5.5V, IQ = 300A, ISD = <2.5A, DFN Package VIN: 2.4V to 5.5V, VOUT(MAX) = 5.25V, IQ = 25A/50A, ISD = <1A, MS-10 Package/DFN Package VIN: 2.4V to 5.5V, VOUT(MAX) = 5.25V, IQ = 28A, ISD = <1A, DFN Package VIN: 2.7V to 5.5V, 1MHz, ISD < 6A, DFN Package
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LT3465/LT3465A Constant Current, 1.2MHz/2.7MHz, High Efficiency White LED VIN: 2.7V to 16V, VOUT(MAX) = 34V, IQ = 1.9mA, ISD = <1A, Boost Regulator with Integrated Schottky Diode ThinSOT Package LT3466 LT3479 Dual Constant Current, 2MHz, High Efficiency White LED Boost Regulator with Integrated Schottky Diode 3A, Full Featured DC/DC Converter with Soft-Start and Inrush Current Protection VIN: 2.7V to 24V, VOUT(MAX) = 40V, IQ = 5mA, ISD = <16A, DFN Package VIN: 2.5V to 24V, VOUT(MAX) = 40V, IQ = 6.5mA, ISD = <1A, DFN Package/TSOPP Package
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Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507
LT 0206 REV A * PRINTED IN USA
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2005

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