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LTC3406B-2 2.25MHz, 600mA Synchronous Step-Down Regulator in ThinSOTTM
FEATURES

DESCRIPTIO
High Efficiency: Up to 96% 600mA Output Current at VIN = 3V 2.5V to 5.5V Input Voltage Range 2.25MHz Constant Frequency Operation No Schottky Diode Required Low Dropout Operation: 100% Duty Cycle Low Quiescent Current: 350A 0.6V Reference Allows Low Output Voltages Shutdown Mode Draws < 1A Supply Current Current Mode Operation for Excellent Line and Load Transient Response Overtemperature Protected Low Profile (1mm) SOT-23 Package
The LTC (R)3406B-2 is a high efficiency monolithic synchronous buck regulator using a constant frequency, current mode architecture. Supply current with no load is 350A, dropping to <1A in shutdown. The 2.5V to 5.5V input voltage range makes the LTC3406B-2 ideally suited for single Li-Ion battery-powered applications. 100% duty cycle capability provides low dropout operation, extending battery life in portable systems. PWM pulse skipping mode operation provides very low output ripple voltage for noise sensitive applications. The switching frequency is internally set at 2.25MHz, allowing the use of tiny surface mount inductors and capacitors. The internal synchronous switch increases efficiency and eliminates the need for an external Schottky diode. Low output voltages are easily supported with the 0.6V feedback reference voltage. The LTC3406B-2 is available in a low profile (1mm) SOT-23 package. Refer to LTC3406 for applications that require Burst Mode(R) operation.
, LTC and LT are registered trademarks of Linear Technology Corporation. Burst Mode is a registered trademark of Linear Technology Corporation. ThinSOT is a trademark of Linear Technology Corporation. Protected by U.S. Patents, including 6580258, 5481178.
APPLICATIO S

Cellular Telephones Personal Information Appliances Wireless and DSL Modems Digital Still Cameras MP3 Players Portable Instruments
TYPICAL APPLICATIO
High Efficiency Step-Down Converter
100
VIN 2.7V TO 5.5V
2.2H* VIN 4.7F CER SW 22pF 1M 10F CER
VOUT 1.8V 600mA
90 80
VOUT = 1.8V TA = 25C
LTC3406B-2 RUN GND VFB
EFFICIENCY (%)
70 60 50 40 30 20 10 0.1 VIN = 4.2V 1 100 10 OUTPUT CURRENT (mA) 1000
3406B TA01b
499k
3406B TA01a
U
Efficiency vs Load Current
VIN = 3.6V VIN = 2.7V
sn3406b2 3406b2fs
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1
LTC3406B-2
ABSOLUTE
(Note 1)
AXI U
RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW RUN 1 GND 2 SW 3 4 VIN 5 VFB
Input Supply Voltage .................................. - 0.3V to 6V RUN, VFB Voltages ..................................... - 0.3V to VIN SW Voltage (DC) ......................... - 0.3V to (VIN + 0.3V) P-Channel Switch Source Current (DC) ............. 800mA N-Channel Switch Sink Current (DC) ................. 800mA Peak SW Sink and Source Current ........................ 1.3A Operating Temperature Range (Note 2) .. - 40C to 85C Maximum Junction Temperature (Notes 3, 6) ..... 125C Storage Temperature Range ................ - 65C to 150C Lead Temperature (Soldering, 10 sec)................. 300C
ORDER PART NUMBER LTC3406B-2ES5 S5 PART MARKING LTAGH
S5 PACKAGE 5-LEAD PLASTIC TSOT-23
TJMAX = 125C, JA = 250C/ W, JC = 90C/ W
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25C. VIN = 3.6V unless otherwise specified.
SYMBOL IVFB VFB PARAMETER Feedback Current Regulated Feedback Voltage (Note 4) TA = 25C (Note 4) 0C TA 85C (Note 4) -40C TA 85C VIN = 2.5V to 5.5V (Note 4) VOVL = VOVL - VFB, LTC3406B VIN = 3V, VFB = 0.5V or VOUT = 90%, Duty Cycle < 35%
ELECTRICAL CHARACTERISTICS
CONDITIONS
MIN 0.5880 0.5865 0.5850 20 0.75
TYP 0.6 0.6 0.6 0.04 50 1 0.5
MAX 30 0.6120 0.6135 0.6150 0.4 80 1.25
UNITS nA V V V %/V mV A %

VFB VOVL IPK VLOADREG VIN IS
Reference Voltage Line Regulation Output Overvoltage Lockout Peak Inductor Current Output Voltage Load Regulation Input Voltage Range Input DC Bias Current Shutdown
2.5 350 0.1
5.5 500 1 2.7 0.5 0.45 1 1.5 1
(Note 5) VFB = 0.5V or VOUT = 90% VRUN = 0V, VIN = 4.2V VFB = 0.6V or VOUT = 100% VFB = 0V or VOUT = 0V ISW = 100mA ISW = -100mA VRUN = 0V, VSW = 0V or 5V, VIN = 5V

fOSC RPFET RNFET ILSW VRUN IRUN
Oscillator Frequency RDS(ON) of P-Channel FET RDS(ON) of N-Channel FET SW Leakage RUN Threshold RUN Leakage Current
1.8
2.25 310 0.4 0.35 0.01
0.3
1 0.01
Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: The LTC3406B-2ES5 is guaranteed to meet 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 3: TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formula: LTC3406B-2ES5: TJ = TA + (PD)(250C/W)
Note 4: The LTC3406B-2ES5 is tested in a proprietary test mode that connects VFB to the output of the error amplifier. Note 5: Dynamic supply current is higher due to the gate charge being delivered at the switching frequency. Note 6: This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed 125C when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature may impair device reliability.
sn3406b2 3406b2fs
2
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V A A MHz kHz A V A
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WW
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LTC3406B-2 TYPICAL PERFOR A CE CHARACTERISTICS
(From Figure 1a Except for the Resistive Divider Resistor Values) Efficiency vs Input Voltage
100 95 90
EFFICIENCY (%)
TA = 25C IOUT = 100mA
EFFICIENCY (%)
85 80
IOUT = 600mA
VIN = 3.6V VIN = 2.7V VIN = 4.2V
EFFICIENCY (%)
IOUT = 10mA 75 70 65 2 3 5 4 INPUT VOLTAGE (V) 6
3406B G01
Efficiency vs Output Current
100 90 80 VOUT = 2.5V TA = 25C
0.614
REFERENCE VOLTAGE (V)
EFFICIENCY (%)
70 60 50 40 30 20 10 0.1 1 100 10 OUTPUT CURRENT (mA) 1000
3406B G04
0.604 0.599 0.594 0.589 0.584 -50 -25
FREQUENCY (MHz)
VIN = 2.7V
VIN = 4.2V
VIN = 3.6V
Oscillator Frequency vs Supply Voltage
2.70
OSCILLATOR FREQUENCY (MHz)
TA = 25C
2.55
OUTPUT VOLTAGE (V)
2.40 2.25 2.10 1.95 1.80
1.814 1.804 1.794 1.784 1.774 0 100 200 300 400 500 600 700 800 900 LOAD CURRENT (mA)
3406B G08
RDS(ON) ()
2
3 4 5 SUPPLY VOLTAGE (V)
UW
6
3406B G07
Efficiency vs Output Current
100 90 80 70 60 50 40 30 20 10 0.1 1 100 10 OUTPUT CURRENT (mA) 1000
3406B G02
Efficiency vs Output Current
100 VOUT = 1.5V 90 TA = 25C 80 70 60 50 40 30 20 10 0.1 1 100 10 OUTPUT CURRENT (mA) 1000
3406B G03
VOUT = 1.2V TA = 25C
VIN = 3.6V VIN = 2.7V
VIN = 4.2V
Reference Voltage vs Temperature
2.55
VIN = 3.6V 0.609
Oscillator Frequency vs Temperature
VIN = 3.6V 2.40
2.25
2.10
50 25 75 0 TEMPERATURE (C)
100
125
1.95 -50 -25
50 25 75 0 TEMPERATURE (C)
100
125
3406B G05
3406B G06
Output Voltage vs Load Current
1.844 1.834 1.824 VIN = 3.6V TA = 25C
0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
RDS(ON) vs Input Voltage
TA = 25C
MAIN SWITCH
SYNCHRONOUS SWITCH
0
1
5 4 2 3 INPUT VOLTAGE (V)
6
7
3406B G09
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LTC3406B-2 TYPICAL PERFOR A CE CHARACTERISTICS
(From Figure 1a Except for the Resistive Divider Resistor Values) Dynamic Supply Current vs Supply Voltage
450
RDS(ON) vs Temperature
0.7 VIN = 2.7V VIN = 4.2V 0.5
RDS(ON) ()
VIN = 3.6V
DYNAMIC SUPPLY CURRENT (A)
DYNAMIC SUPPLY CURRENT (A)
0.6
0.4 0.3 0.2 0.1 0 -50 -25 MAIN SWITCH SYNCHRONOUS SWITCH 50 25 75 0 TEMPERATURE (C) 100 125
Switch Leakage vs Temperature
300 VIN = 5.5V RUN = 0V 250
SWITCH LEAKAGE (nA)
SWITCH LEAKAGE (pA)
200 150 100 50 0 -50 -25 MAIN SWITCH SYNCHRONOUS SWITCH 50 25 75 0 TEMPERATURE (C) 100 125
Start-Up from Shutdown
RUN 5V/DIV VOUT 1V/DIV AC COUPLED IL 500mA/DIV VOUT 100mV/DIV AC COUPLED IL 500mA/DIV ILOAD 500mA/DIV
VIN = 3.6V 40s/DIV VOUT = 1.8V ILOAD = 600mA (LOAD: 3 RESISTOR)
4
UW
3406B G10
3406B G13
Dynamic Supply Current vs Temperature
390 VIN = 3.6V VOUT = 1.8V 370 I LOAD = 0A 350 330 310 290 270 250 -50
VOUT = 1.8V 430 ILOAD = 0A TA = 25C 410 390 370 350 330 310 290 270 250 2 2.5 3 3.5 4 4.5 5 SUPPLY VOLTAGE (V) 5.5 6
50 0 TEMPERATURE (C)
100
125
3406B G11
3406B G12
Switch Leakage vs Input Voltage
120 100 80 60 40 20 0 RUN = 0V TA = 25C SYNCHRONOUS SWITCH
Discontinuous Operation
SW 2V/DIV VOUT 10mV/DIV AC COUPLED IL 100mA/DIV
MAIN SWITCH
VIN = 3.6V VOUT = 1.8V ILOAD = 50mA
1s/DIV
3406B G15
0
1
2 3 4 INPUT VOLTAGE (V)
5
6
3406B G14
Load Step
VOUT 100mV/DIV AC COUPLED IL 500mA/DIV ILOAD 500mA/DIV
Load Step
3406B G16
VIN = 3.6V 20s/DIV VOUT = 1.8V ILOAD = 0mA TO 600mA
3406B G17
VIN = 3.6V 20s/DIV VOUT = 1.8V ILOAD = 50mA TO 600mA
3406B G18
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LTC3406B-2 TYPICAL PERFOR A CE CHARACTERISTICS
(From Figure 1a Except for the Resistive Divider Resistor Values) Load Step
VOUT 100mV/DIV AC COUPLED IL 500mA/DIV ILOAD 500mA/DIV VOUT 100mV/DIV AC COUPLED IL 500mA/DIV ILOAD 500mA/DIV
VIN = 3.6V 20s/DIV VOUT = 1.8V ILOAD = 100mA TO 600mA
PI FU CTIO S
RUN (Pin 1): Run Control Input. Forcing this pin above 1.5V enables the part. Forcing this pin below 0.3V shuts down the device. In shutdown, all functions are disabled drawing <1A supply current. Do not leave RUN floating. GND (Pin 2): Ground Pin. SW (Pin 3): Switch Node Connection to Inductor. This pin connects to the drains of the internal main and synchronous power MOSFET switches. VIN (Pin 4): Main Supply Pin. Must be closely decoupled to GND, Pin 2, with a 2.2F or greater ceramic capacitor. VFB (Pin 5): Feedback Pin. Receives the feedback voltage from an external resistive divider across the output.
UW
Load Step
3406B G19
VIN = 3.6V 20s/DIV VOUT = 1.8V ILOAD = 200mA TO 600mA
3406B G20
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LTC3406B-2
FU CTIO AL DIAGRA W
OSC 4 VIN
VFB 5 VIN RUN 1
0.6V
OVDET 0.65V 0.6V REF
SHUTDOWN
OPERATIO
(Refer to Functional Diagram)
Main Control Loop The LTC3406B-2 uses a constant frequency, current mode step-down architecture. Both the main (P-channel MOSFET) and synchronous (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 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 feedback voltage, FB, relative to the 0.6V 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.
The comparator OVDET guards against transient overshoots >6.25% by turning the main switch off and keeping it off until the fault is removed. Pulse Skipping Mode Operation At light loads, the inductor current may reach zero or reverse on each pulse. The bottom MOSFET is turned off by the current reversal comparator, IRCMP, and the switch voltage will ring. This is discontinuous mode operation, and is normal behavior for the switching regulator. At very light loads, the LTC3406B-2 will automatically skip pulses in pulse skipping mode operation to maintain output regulation. Refer to LTC3406 data sheet if Burst Mode operation is preferred. Short-Circuit Protection When the output is shorted to ground, the frequency of the oscillator is reduced to about 310kHz, 1/7 the nominal
6
-
IRCMP
+
-
+
-
+
U
U
U
SLOPE COMP OSC
FREQ SHIFT
- +
S EA R Q Q SWITCHING LOGIC AND BLANKING CIRCUIT
-
ICOMP
+
5
RS LATCH
OV
ANTISHOOTTHRU
3 SW
2 GND
3406B BD
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LTC3406B-2
OPERATIO
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 2.25MHz when VFB rises above 0V. 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 then be determined by the input voltage minus the voltage drop across the P-channel MOSFET and the inductor.
VIN 2.7V TO 5.5V 2.2H* CIN** 4.7F CER VIN SW 22pF 1M COUT 10F CER
*MURATA LQH32CN2R2M33 **TAIYO YUDEN JMK212BJ475MG TAIYO YUDEN JMK316BJ106ML
Figure 1a. High Efficiency Step-Down Converter
100 90 80
EFFICIENCY (%)
MAXIMUM OUTPUT CURRENT (mA)
VOUT = 1.8V TA = 25C
70 60 50 40 30 20 10 0.1 VIN = 4.2V 1 100 10 OUTPUT CURRENT (mA) 1000
3406B F01b
VIN = 3.6V VIN = 2.7V
Figure 1b. Efficiency vs Load Current
U
RUN
(Refer to Functional Diagram)
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 LTC3406B-2 is used at 100% duty cycle with low input voltage (See Thermal Considerations in the Applications Information section). Low Supply Operation The LTC3406B-2 will operate with input supply voltages as low as 2.5V, but the maximum allowable output current is reduced at this low voltage. Figure 2 shows the reduction in the maximum output current as a function of input voltage for various output voltages. Slope Compensation and Inductor Peak Current Slope compensation provides stability in constant frequency architectures by preventing subharmonic oscillations at high duty cycles. It is accomplished internally by adding a compensating ramp to the inductor current signal at duty cycles in excess of 40%. Normally, this results in a reduction of maximum inductor peak current for duty cycles > 40%. However, the LTC3406B-2 uses a patent-pending scheme that counteracts this compensating ramp, which allows the maximum inductor peak current to remain unaffected throughout all duty cycles.
1200 1000 800 600 400 200 0 VOUT = 1.8V VOUT = 2.5V VOUT = 1.5V
VOUT 1.8V 600mA
LTC3406B-2 VFB GND
499k
3406B F01a
2.5
3.0
3.5 4.0 4.5 SUPPLY VOLTAGE (V)
5.0
5.5
3406B F02
Figure 2. Maximum Output Current vs Input Voltage
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LTC3406B-2
APPLICATIO S I FOR ATIO
The basic LTC3406B-2 application circuit is shown in Figure 1. External component selection is driven by the load requirement and begins with the selection of L followed by CIN and COUT. 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 ripple current. Large value inductors lower ripple current and small value inductors result in higher ripple currents. Higher VIN 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).
IL =
( )( )
V VOUT 1 - OUT VIN fL 1
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 better efficiency, choose a low DC-resistance inductor. 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 LTC3406B-2 requires to operate. Table 1 shows some typical surface mount inductors that work well in LTC3406B-2 applications. CIN and COUT Selection In continuous mode, the source current of the top MOSFET is a square wave of duty cycle VOUT/VIN. 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 VIN - VOUT VIN
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Table 1. Representative Surface Mount Inductors
Part Number Sumida CDRH2D11 Sumida CDRH2D18/LD Sumida CMD4D06 Murata LQH32C Taiyo Yuden LQLBC2518 Toko D412F Value (H) 1.5 2.2 3.3 2.2 3.3 4.7 2.2 3.3 4.7 1.0 2.2 4.7 1.0 1.5 2.2 2.2 3.3 4.7 DCR (MAX) 0.068 0.098 0.123 0.041 0.054 0.078 0.116 0.174 0.216 0.060 0.097 0.150 0.080 0.110 0.130 0.14 0.20 0.22 MAX DC Current (A) 0.90 0.78 0.60 0.85 0.75 0.63 0.95 0.77 0.75 1.00 0.79 0.65 0.78 0.66 0.60 1.14 0.90 0.80 Size WxLxH (mm3) 3.2 x 3.2 x 1.2 3.2 x 3.2 x 2.0 3.5 x 4.1 x 0.8 2.5 x 3.2 x 2.0 1.8 x 2.5 x 1.8 (1) 4.6 x 4.6 x 1.2
W
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This formula has a maximum at VIN = 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
)]
1/ 2
where f = operating frequency, COUT = output capacitance and IL = ripple current in the inductor. For a fixed output voltage, the output ripple is highest at maximum input voltage since IL increases with input voltage.
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LTC3406B-2
APPLICATIO S I FOR ATIO
Aluminum electrolytic and dry 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. Using Ceramic Input and Output Capacitors Higher 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 LTC3406B-2'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. However, care must be taken when ceramic capacitors are used at the input and the output. When a ceramic capacitor is used at the input and the power is supplied by a wall adapter through long wires, a load step at the output can induce ringing at the input, VIN. At best, this ringing can couple to the output and be mistaken as loop instability. At worst, a sudden inrush of current through the long wires can potentially cause a voltage spike at VIN, large enough to damage the part. 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. Output Voltage Programming The output voltage is set by a resistive divider according to the following formula: R2 VOUT = 0.6 1 + R1
(2)
POWER LOSS (W)
The external resistive divider is connected to the output, allowing remote voltage sensing as shown in Figure 3.
U
0.6V VOUT 5.5V R2 VFB LTC3406B-2 GND
3406B F03
W
UU
R1
Figure 3. Setting the LTC3406B-2 Output Voltage
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 LTC3406B-2 circuits: VIN quiescent current and I2R losses. The VIN 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 4.
1 VIN = 3.6V
0.1
0.01
VOUT = 2.5V VOUT = 1.8V VOUT = 1.2V
0.001 VOUT = 1.5V
0.0001 0.1
1
10 100 LOAD CURRENT (mA)
1000
3406B F04
Figure 4. Power Lost vs Load Current
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LTC3406B-2
APPLICATIO S I FOR ATIO
1. The VIN 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 VIN to ground. The resulting dQ/dt is the current out of VIN that is typically larger 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 VIN 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 Charateristics 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. Thermal Considerations In most applications the LTC3406B-2 does not dissipate much heat due to its high efficiency. But, in applications where the LTC3406B-2 is running at high ambient temperature with low supply voltage and high duty cycles, such as in dropout, the heat dissipated may exceed the maximum junction temperature of the part. If the junction temperature reaches approximately 150C, both power switches will be turned off and the SW node will become high impedance.
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To avoid the LTC3406B-2 from exceeding the maximum junction temperature, the user will need to do some thermal analysis. The goal of the thermal analysis is to determine whether the power dissipated exceeds the maximum junction temperature of the part. The temperature rise is given by: TR = (PD)(JA) where PD is the power dissipated by the regulator and JA is the thermal resistance from the junction of the die to the ambient temperature. The junction temperature, TJ, is given by: TJ = TA + TR where TA is the ambient temperature. As an example, consider the LTC3406B-2 in dropout at an input voltage of 2.7V, a load current of 600mA and an ambient temperature of 70C. From the typical performance graph of switch resistance, the RDS(ON) of the P-channel switch at 70C is approximately 0.52. Therefore, power dissipated by the part is: PD = ILOAD2 * RDS(ON) = 187.2mW For the SOT-23 package, the JA is 250C/ W. Thus, the junction temperature of the regulator is: TJ = 70C + (0.1872)(250) = 116.8C which is below the maximum junction temperature of 125C. Note that at higher supply voltages, the junction temperature is lower due to reduced switch resistance (RDS(ON)). Checking 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 steadystate value. During this recovery time VOUT can be monitored for overshoot or ringing that would indicate a stability problem. For a detailed explanation of switching control loop theory, see Application Note 76.
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LTC3406B-2
APPLICATIO S I FOR ATIO
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. PC Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the
1
-
VOUT COUT
+
BOLD LINES INDICATE HIGH CURRENT PATHS
Figure 5. LTC3406B-2 Layout Diagram
VIA TO GND
PIN 1 VOUT L1 SW LTC3406B-2
Figure 6. LTC3406B-2 Suggested Layout
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LTC3406B-2. 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 VIN trace should be kept short, direct and wide. 2. Does the VFB pin connect directly to the feedback resistors? The resistive divider R1/R2 must be connected between the (+) plate of COUT and ground. 3. Does the (+) plate of CIN connect to VIN as closely as possible? This capacitor provides the AC current to the internal power MOSFETs. 4. Keep the switching node, SW, away from the sensitive VFB node. 5. Keep the (-) plates of CIN and COUT as close as possible.
RUN VFB 5 R2 R1 LTC3406B-2 2 GND 4 CFWD 3 L1 SW VIN CIN
W
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+
VIN
-
3406B F05
R1 VIA TO VIN R2 CFWD VIN VIA TO VOUT
COUT GND
CIN
3406B F06
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11
LTC3406B-2
APPLICATIO S I FOR ATIO
Design Example
As a design example, assume the LTC3406B-2 is used in a single lithium-ion battery-powered cellular phone application. The VIN will be operating from a maximum of 4.2V down to about 2.7V. The load current requirement is a maximum of 0.6A but most of the time it will be in standby mode, requiring only 2mA. Efficiency at both low and high load currents is important. Output voltage is 2.5V. With this information we can calculate L using equation (1),
V L= VOUT 1 - OUT VIN f IL 1
( )( )
Substituting VOUT = 2.5V, VIN = 4.2V, IL = 240mA and f = 2.25MHz in equation (3) gives:
L= 2.5V 2.5V 1 - = 1.87H 1.5MHz(240mA) 4.2V
VIN 2.7V TO 4.2V 4 CIN 4.7F CER 1 VIN
100 90 80
EFFICIENCY (%)
VOUT = 2.5V TA = 25C VIN = 2.7V VIN = 4.2V
70 60 50 40 30 20 10 0.1 1 100 10 OUTPUT CURRENT (mA) 1000
3406B G04
12
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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. For the feedback resistors, choose R1 = 316k. R2 can then be calculated from equation (2) to be:
V R2 = OUT - 1 R1 = 1000k 0.6
(3)
W
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Figure 7 shows the complete circuit along with its efficiency curve.
SW
3
2.2H* 22pF
VOUT 2.5V COUT** 10F CER
LTC3406B-2 RUN GND 2 VFB 5 1M 316k
3406B F07a
*MURATA LQH32CN2R2M33 ** TAIYO YUDEN JHK316BJ106ML TAIYO YUDEN JMK212BJ475MG
Figure 7a
VIN = 3.6V
Figure 7b
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LTC3406B-2
TYPICAL APPLICATIO S
Single Li-Ion 1.2V/600mA Regulator for High Efficiency and Small Footprint
VIN 2.7V TO 4.2V 4 CIN 4.7F CER 1 VIN SW 3 2.2H* 22pF VOUT 1.2V COUT** 10F CER 301k 301k
3406B TA09
LTC3406B-2 RUN GND 2 VFB 5
*MURATA LQH32CN2R2M33 ** TAIYO YUDEN JHK316BJ106ML TAIYO YUDEN JMK212BJ475MG
Efficiency vs Output Current
100 90 80 VOUT = 1.2V TA = 25C
EFFICIENCY (%)
70 60 50 40 30 20 10 0.1 VIN = 2.7V VIN = 4.2V
VIN = 3.6V
1 100 10 OUTPUT CURRENT (mA)
U
Load Step
VOUT 100mV/DIV AC COUPLED IL 500mA/DIV ILOAD 500mA/DIV VIN = 3.6V 20s/DIV VOUT = 1.2V ILOAD = 0mA TO 600mA
3406B TA11
Load Step
VOUT 100mV/DIV AC COUPLED IL 500mA/DIV ILOAD 500mA/DIV
VIN = 3.6V 20s/DIV VOUT = 1.2V ILOAD = 100mA TO 600mA
3406B TA12
1000
3406B G02
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13
LTC3406B-2
TYPICAL APPLICATIO S
5V Input to 3.3V/0.6A Regulator
VIN 5V 4 CIN 4.7F CER 1
VIN
SW
3
LTC3406B-2 RUN GND 2 VFB 5 1M 221k
3406B TA13
*MURATA LQH32CN2R2M33 ** TAIYO YUDEN JHK316BJ106ML TAIYO YUDEN JMK212BJ475MG
Efficiency vs Output Current
100 90 80 VOUT = 3.3V VIN = 5V
EFFICIENCY (%)
70 60 50 40 30 20 10 0.1 1 100 10 OUTPUT CURRENT (mA) 1000
3406B TA14
14
U
Load Step
VOUT 3.3V COUT** 10F CER
2.2H* 22pF
VOUT 100mV/DIV AC COUPLED IL 500mA/DIV ILOAD 500mA/DIV VIN = 3.6V 20s/DIV VOUT = 3.3V ILOAD = 0mA TO 600mA
3406B TA15
Load Step
VOUT 100mV/DIV AC COUPLED IL 500mA/DIV ILOAD 500mA/DIV
VIN = 3.6V 20s/DIV VOUT = 3.3V ILOAD = 100mA TO 600mA
3406B TA16
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LTC3406B-2
PACKAGE DESCRIPTIO
0.62 MAX
0.95 REF
3.85 MAX 2.62 REF
RECOMMENDED SOLDER PAD LAYOUT PER IPC CALCULATOR
0.20 BSC 1.00 MAX DATUM `A'
0.30 - 0.50 REF 0.09 - 0.20 (NOTE 3) NOTE: 1. DIMENSIONS ARE IN MILLIMETERS 2. DRAWING NOT TO SCALE 3. DIMENSIONS ARE INCLUSIVE OF PLATING 4. DIMENSIONS ARE EXCLUSIVE OF MOLD FLASH AND METAL BURR 5. MOLD FLASH SHALL NOT EXCEED 0.254mm 6. JEDEC PACKAGE REFERENCE IS MO-193
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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S5 Package 5-Lead Plastic TSOT-23
(Reference LTC DWG # 05-08-1635)
2.90 BSC (NOTE 4) 1.22 REF 1.4 MIN 2.80 BSC 1.50 - 1.75 (NOTE 4) PIN ONE 0.30 - 0.45 TYP 5 PLCS (NOTE 3) 0.95 BSC 0.80 - 0.90 0.01 - 0.10 1.90 BSC
S5 TSOT-23 0302
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LTC3406B-2 RELATED PARTS
PART NUMBER LT1616 LT1676 LTC1877 LTC1878 LTC1879 LTC3403 LTC3404 LTC3405/LTC3405A LTC3406/LTC3406B LTC3407 LTC3408 LTC3411 LTC3412 LTC3414 LTC3440 LTC3441 DESCRIPTION 500mA (IOUT), 1.4MHz, High Efficiency Step-Down DC/DC Converter 450mA (IOUT), 100kHz, High Efficiency Step-Down DC/DC Converter 600mA (IOUT), 550kHz, Synchronous Step-Down DC/DC Converter 600mA (IOUT), 550kHz, Synchronous Step-Down DC/DC Converter 1.2A (IOUT), 550kHz, Synchronous Step-Down DC/DC Converter 600mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converter with Bypass Transistor 600mA (IOUT), 1.4MHz, Synchronous Step-Down DC/DC Converter 300mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converter 600mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converter Dual Output (600mA x 2) 1.5MHz Synchronous Step-Down DC/DC Converter 600mA (IOUT), 1.5MHz Synchronous Step-Down DC/DC Converter with 0.08 Bypass Transistor 1.25A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 2.5A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 4A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 600mA (IOUT), 2MHz, Synchronous Buck-Boost DC/DC Converter 1A (IOUT), 1MHz, Synchronous Buck-Boost DC/DC Converter COMMENTS 90% Efficiency, VIN: 3.6V to 25V, VOUT(MIN) = 1.25V, IQ = 1.9mA, ISD < 1A, ThinSOT Package 90% Efficiency, VIN: 7.4V to 60V, VOUT(MIN) = 1.24V, IQ = 3.2mA, ISD = 2.5A, S8 Package 95% Efficiency, VIN: 2.7V to 10V, VOUT(MIN) = 0.8V, IQ = 10A, ISD < 1A, MS8 Package 95% Efficiency, VIN: 2.7V to 6V, VOUT(MIN) = 0.8V, IQ = 10A, ISD < 1A, MS8 Package 95% Efficiency, VIN: 2.7V to 10V, VOUT(MIN) = 0.8V, IQ = 15A, ISD < 1A, TSSOP-16 Package 96% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = Dynamically Adjustable, IQ = 20A, ISD < 1A, DFN Package 95% Efficiency, VIN: 2.7V to 6V, VOUT(MIN) = 0.8V, IQ = 10A, ISD < 1A, MS8 Package 96% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 20A, ISD < 1A, ThinSOT Package 96% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 20A, ISD < 1A, ThinSOT Package 95% Efficiency VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 40A, MS10E Package 96% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = Dynamically Adjustable, IQ = 1.5mA, ISD < 1A, DFN Package 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 60A, ISD < 1A, MS Package 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 60A, ISD < 1A, TSSOP-16E Package 95% Efficiency, VIN: 2.25V to 5.5V, VOUT(MIN) = 0.8V, IQ = 64A, TSSOP-20E Package 95% Efficiency, VIN: 2.5V to 5.5V, VOUT: 2.5V to 5.5V, IQ = 25A, ISD < 1A, MS Package 95% Efficiency, VIN: 2.5V to 5.5V, VOUT: 2.4V to 5.25V, IQ = 25A, DFN Package
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16
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 FAX: (408) 434-0507
LT/TP 0204 1K * PRINTED IN USA
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2003

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