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FEATURES

LTC3406AB 1.5MHz, 600mA Synchronous Step-Down Regulator in ThinSOT DESCRIPTION
The LTC(R)3406AB is a high efficiency monolithic synchronous buck regulator using a constant frequency, current mode architecture. Supply current with no load is 200A, dropping to <1A in shutdown. The 2.5V to 5.5V input voltage range makes the LTC3406AB ideally suited for single Li-Ion battery-powered applications. 100% duty cycle provides low dropout operation, extending battery run time in portable systems. PWM pulse skipping mode operation provides very low output ripple voltage for noise sensitive applications. Refer to LTC3406A for applications that require Burst Mode(R) operation. Switching frequency is internally set at 1.5MHz, allowing the use of small 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 LTC3406AB is available in a low profile (1mm) ThinSOT package.
, LT, LTC, LTM and Burst Mode are registered trademarks of Linear Technology Corporation. ThinSOT is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. Protected by U.S. Patents including 5481178, 6580258.

High Efficiency: Up to 96% 600mA Output Current 2.5V to 5.5V Input Voltage Range 1.5MHz Constant Frequency Operation No Schottky Diode Required Low Dropout Operation: 100% Duty Cycle Low Quiescent Current: 200A 2% 0.6V Reference Shutdown Mode Draws <1A Supply Current Internal Soft-Start Limits Inrush Current Current Mode Operation for Excellent Line and Load Transient Response Overtemperature Protected Low Profile (1mm) ThinSOTTM Package
APPLICATIONS

Cellular Telephones Satellite and GPS Receivers Wireless and DSL Modems Digital Still Cameras Media Players Portable Instruments
TYPICAL APPLICATION
Efficiency vs Load Current
100 2.2H VIN 4.7F CER VIN SW 22pF 10F CER 619k 309k
3406AB TA09
VOUT = 1.8V
1.8V, 600mA VOUT EFFICIENCY (%)
90 80 70 60 50 40 30 20 10 0.1 VIN = 2.7V VIN = 3.6V VIN = 4.2V 1 100 10 OUTPUT CURRENT (mA) 1000
3406B TA14
LTC3406AB RUN GND VFB
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LTC3406AB ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
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) (Note 7)................................................................800mA N-Channel Switch Sink Current (DC) (Note 7) .....800mA Peak SW Sink and Source Current (Note 7) .............1.3A Operating Temperature Range (Note 2) ...- 40C to 85C Junction Temperature (Notes 3, 6) ....................... 125C Storage Temperature Range...................- 65C to 150C Lead Temperature (Soldering, 10 sec) .................. 300C
S5 PACKAGE 5-LEAD PLASTIC TSOT-23 TJMAX = 125C, JA = 250C/W, JC = 90C/W
ORDER INFORMATION
LEAD FREE FINISH LTC3406ABES5#PBF TAPE AND REEL LTC3406ABES5#TRPBF PART MARKING LTCXZ PACKAGE DESCRIPTION 5-Lead Plastic TSOT-23 TEMPERATURE RANGE -40C to 85C
Consult LTC Marketing for parts specified with wider operating temperature ranges. Consult LTC Marketing for information on non-standard lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS
SYMBOL IVFB VFB VFB IPK VLOADREG VIN IS PARAMETER Feedback Current Regulated Feedback Voltage Reference Voltage Line Regulation Peak Inductor Current Output Voltage Load Regulation Input Voltage Range Input DC Bias Current Active Mode Shutdown Oscillator Frequency RDS(ON) of P-Channel FET RDS(ON) of N-Channel FET SW Leakage Soft-Start Time (Note 4)
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 3.6V unless otherwise specified.
CONDITIONS

MIN 0.5880 0.75
TYP 0.6 0.04 1 0.5
MAX 30 0.6120 0.4 1.25
UNITS nA V %/V A %
VIN = 2.5V to 5.5V (Note 4) VIN = 3V, VFB = 0.5V Duty Cycle < 35%
2.5 200 0.1
5.5 300 1 1.8 0.35 0.35 1 1.2
V A A MHz A ms
(Note 5) VFB = 0.63V VRUN = 0V, VIN = 5.5V VFB = 0.6V ISW = 100mA ISW = -100mA VRUN = 0V, VSW = 0V or 5V, VIN = 5V VFB from 10% to 90% Full-scale 0.6
fOSC RPFET RNFET ILSW tSOFTSTART
1.2
1.5 0.23 0.21 0.01 0.9
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LTC3406AB ELECTRICAL CHARACTERISTICS
SYMBOL VRUN IRUN PARAMETER RUN Threshold RUN Leakage Current
The denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25C. VIN = 3.6V unless otherwise specified.
CONDITIONS

MIN 0.3
TYP 1 0.01
MAX 1.5 1
UNITS V 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: The LTC3406ABE is guaranteed to meet performance specifications from 0C to 85C. 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: LTC3406AB: TJ = TA + (PD)(250C/W)
Note 4: The LTC3406AB 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. Note7: Limited by long term current density considerations.
TYPICAL PERFORMANCE CHARACTERISTICS
(From Front Page Figure Except for the Resistive Divider Resistor Values) Efficiency vs Input Voltage
100 90 80 EFFICIENCY (%) 70 EFFICIENCY (%) 60 50 40 30 20 10 0 2 VOUT = 1.8V 3 IL = 100mA IL = 600mA IL = 10mA 5 4 INPUT VOLTAGE (V) 6
3406B G01
Efficiency vs Load Current
100 90 80 70 60 50 40 30 20 10 0 0.1 VIN = 2.7V VIN = 3.6V VIN = 4.2V 1 100 10 OUTPUT CURRENT (mA) 1000
3406B G02
VOUT = 1.2V
Efficiency vs Load Current
100 90 80 70 EFFICIENCY (%) 60 50 40 30 20 10 0 0.1 VIN = 2.7V VIN = 3.6V VIN = 4.2V 1 100 10 OUTPUT CURRENT (mA) 1000
3406B G03
Reference Voltage vs Temperature
0.615 0.610 REFERENCE VOLTAGE (V) 0.605 0.600 0.595 0.590 0.585 -50 -25 VIN = 3.6V
VOUT = 2.5V
50 25 75 0 TEMPERATURE (C)
100
125
3406AB G21
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LTC3406AB TYPICAL PERFORMANCE CHARACTERISTICS
(From Front Page Figure Except for the Resistive Divider Resistor Values) Oscillator Frequency vs Temperature
1.60 OSCILLATOR FREQUENCY (MHz) 1.55 1.50 1.45 1.40 1.35 1.30 -50 -25 VIN = 3.6V OSCILLATOR FREQUENCY (MHz) 1.60 1.55 OUTPUT VOLTAGE (V) 1.50 1.45 1.40 1.35 1.30 1.25 50 25 0 75 TEMPERATURE (C) 100 125 1.20 2.0 2.5 3.0 3.5 4.0 4.5 5.0 INPUT VOLTAGE (V) 5.5 6.0
Oscillator Frequency vs Supply Voltage
1.820 1.816 1.812 1.808 1.804 1.800 1.796 1.792 1.788 1.784 1.780
Output vs Load Current
VOUT = 1.8V
VIN = 2.7V VIN = 3.6V VIN = 4.2V 0 200 400 600
3406B G24
OUTPUT CURRENT (mA)
3406B G07
3406B G22
RDS(ON) vs Input Voltage
0.40 0.35 0.30 RDS(ON) () RDS(0N) () 0.25 0.20 0.15 0.10 0 1 4 3 5 2 INPUT VOLTAGE (V) 6 7 MAIN SWITCH SYNCHRONOUS SWITCH 0.40 0.35 0.30 0.25 0.20 0.15 0.10 0.05
RDS(ON) vs Input Voltage
300 VIN = 2.7V VIN = 3.6V DYNAMIC SUPPLY CURRENT (A) 250 200 150 100 50 0
Dynamic Supply Current
VOUT = 1.2V ILOAD = 0A
VIN = 4.2V
MAIN SWITCH SYNCHRONOUS SWITCH 0 50 75 25 TEMPERATURE (C) 100 125
0 -50 -25
2
2.5
3
4.5 5 3.5 4 INPUT VOLTAGE (V)
5.5
6
3406B G25
3406B G26
3406B G27
Dynamic Supply Current vs Temperature
300 DYNAMIC SUPPLY CURRENT (A) 250 200 150 100 50 0 -50 -25 VIN = 3.6V VOUT = 1.2V ILOAD = 0A SWITCH LEAKAGE (nA) 140 120 100 80 60 40 20
Switch Leakage vs Temperature
1000 MAIN SWITCH SYNCHRONOUS SWITCH SWITCH LEAKAGE (pA) 900 800 700 600 500 400 300 200 100 0 50 25 75 0 TEMPERATURE (C) 100 125
Switch Leakage vs Input Voltage
MAIN SWITCH SYNCHRONOUS SWITCH RUN = 0V
50 25 75 0 TEMPERATURE (C)
100
125
0 -50 -25
0
1
3 4 2 INPUT VOLTAGE (V)
5
6
3406B G30
3406B G28
3406B G29
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LTC3406AB TYPICAL PERFORMANCE CHARACTERISTICS
(From Front Page Figure Except for the Resistive Divider Resistor Values) Start-Up from Shutdown
VOUT 200mV/DIV
Load Step
VOUT 200mV/DIV
Load Step
RUN 2V/DIV VOUT 1V/DIV ILOAD 500mA/DIV IL 500mA/DIV IL 500mA/DIV
ILOAD 500mA/DIV VIN = 3.6V 400s/DIV VOUT = 1.8V ILOAD = 600mA (3 RES)
3406B G31
ILOAD 500mA/DIV VIN = 3.6V 20s/DIV VOUT = 1.8V ILOAD = 0mA TO 600mA
3406B G32
VIN = 3.6V 20s/DIV VOUT = 1.8V ILOAD = 50mA TO 600mA
3406B G33
Load Step
VOUT 200mV/DIV VOUT 200mV/DIV
Load Step
Discontinuous Operation
SW (2V/DIV)
IL 500mA/DIV
IL 500mA/DIV
VOUT 20mV/DIV AC COUPLED
ILOAD 500mA/DIV
ILOAD 500mA/DIV
IL 200mA/DIV VIN = 3.6V 20s/DIV VOUT = 1.8V ILOAD = 200mA TO 600mA
3406B G35
VIN = 3.6V 20s/DIV VOUT = 1.8V ILOAD = 100mA TO 600mA
3406B G34
VIN = 3.6V VOUT = 1.8V ILOAD = 25mA
500ns/DIV
3406B G36
PIN FUNCTIONS
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.
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LTC3406AB FUNCTIONAL DIAGRAM
SLOPE COMP OSC OSC
4 VIN FREQ SHIFT VFB 5 0.6V S EA R Q Q SWITCHING LOGIC AND BLANKING CIRCUIT ANTISHOOTTHRU
- + + -
ICOMP
+
5
VIN RUN 1 0.6V REF
SHUTDOWN
OPERATION
Main Control Loop
(Refer to Functional Diagram)
The LTC3406AB 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 main control loop is shut down by grounding RUN, resetting the internal soft-start. Re-enabling the main control loop by pulling RUN high activates the internal soft-start, which slowly ramps the output voltage over approximately 0.9ms until it reaches regulation. 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 LTC3406AB will automatically skip pulses in pulse skipping mode operation to maintain output regulation. Refer to the LTC3406A data sheet if Burst Mode operation is preferred.
6
-
IRCMP
+
-
RS LATCH
3 SW
2 GND
3406AB BD
3406abfa
LTC3406AB OPERATION
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. 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 LTC3406AB is used at 100% duty cycle with low input voltage (See Thermal Considerations in the Applications Information section).
(Refer to Functional Diagram)
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 LTC3406AB uses a patented scheme that counteracts this compensating ramp, which allows the maximum inductor peak current to remain unaffected throughout all duty cycles.
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LTC3406AB APPLICATIONS INFORMATION
The basic LTC3406AB application circuit is shown on the front page. 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 1 VOUT 1- OUT VIN ( f )(L ) (1)
Sumida CMD4D06 Panasonic ELT5KT Murata LQH32CN
Table 1. 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 (mm3) 3.8 x 3.8 x 1.8
3.5 x 4.3 x 0.8
4.5 x 5.4 x 1.2 2.5 x 3.2 x 2.0
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
1/2 2
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 LTC3406AB requires to operate. Table 1 shows some typical surface mount inductors that work well in LTC3406AB applications.
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).
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LTC3406AB APPLICATIONS INFORMATION
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 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. 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 LTC3406AB'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 In the adjustable version, the output voltage is set by a resistive divider according to the following formula: R2 VOUT = 0.6 V 1+ R1
(2)
The external resistive divider is connected to the output, allowing remote voltage sensing as shown in Figure 1.
0.6V VOUT 5.5V R2 VFB LTC3406AB GND
3406AB F03
R1
Figure 1. Setting the LTC3406AB 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.
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LTC3406AB APPLICATIONS INFORMATION
Although all dissipative elements in the circuit produce losses, two main sources usually account for most of the losses in LTC3406AB 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 2.
1 VIN = 3.6V
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. Thermal Considerations In most applications the LTC3406AB does not dissipate much heat due to its high efficiency. But, in applications where the LTC3406AB 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. To avoid the LTC3406AB 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.
0.1 POWER LOSS (W)
0.01
0.001
VOUT = 1.2V VOUT = 1.8V VOUT = 2.5V 10.0 100.0 1.0 OUTPUT CURRENT (mA) 1000.0
3406B F08
0.0001 0.1
Figure 2. Power Lost vs Load Current
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.
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LTC3406AB APPLICATIONS INFORMATION
The junction temperature, TJ, is given by: TJ = TA + TR where TA is the ambient temperature. As an example, consider the LTC3406AB 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 0.27. Therefore, power dissipated by the part is: PD = ILOAD2 * RDS(ON) = 97.2mW For the SOT-23 package, the JA is 250C/ W. Thus, the junction temperature of the regulator is: TJ = 70C + (0.0972)(250) = 94.3C 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 steady-state 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. 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 LTC3406AB. These items are also illustrated graphically in Figures 3 and 4. Check the following in your layout: 1. The power traces, consisting of the GND trace, the SW trace, the VOUT 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 CIN connect to VIN as closely aspossible? 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 and the IC ground, as close as possible.
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LTC3406AB APPLICATIONS INFORMATION
1 RUN VFB LTC3406AB GND 4 CFWD 5 R2 R1 2
-
VOUT COUT
+
3 L1
SW
VIN CIN
+
VIN
-
BOLD LINES INDICATE HIGH CURRENT PATHS
3406AB F05a
Figure 3. LTC3406AB Layout Diagram
VIA TO VIN
R1 R2 LTC3406AB CFWD
VIN VIA TO VOUT
PIN 1 VOUT L1 SW
COUT GND
CIN
3406AB F06a
Figure 4. LTC3406AB Suggested Layout
Design Example As a design example, assume the LTC3406AB 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), L= V 1 VOUT 1- OUT VIN ( f )( IL )
Substituting VOUT = 2.5V, VIN = 4.2V, IL = 240mA and f = 1.5MHz in Equation (3) gives: L= 2.5V 2.5V 1- = 2.81H 1.5MHz(240mA) 4.2V
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.
(3)
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LTC3406AB APPLICATIONS INFORMATION
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
2.2H* 22pF
Figure 5 shows the complete circuit along with its efficiency curve.
(4)
2.5V, 600mA VOUT COUT** 10F CER
100 90
VOUT = 2.5V
VIN
4 CIN 4.7F CER 1
VIN
SW
3
80 70 EFFICIENCY (%) 60 50 40 30 20 10 0 0.1 VIN = 2.7V VIN = 3.6V VIN = 4.2V 1 100 10 OUTPUT CURRENT (mA) 1000
3406B G03
LTC3406AB RUN GND 2 VFB 5 1M 316k
*MURATA LQH32CN2R2M33 ** TAIYO YUDEN JHK316BJ106ML 3406AB TA09a TAIYO YUDEN JMK212BJ475MG
Load Step
VOUT 200mV/DIV VOUT 100mV/DIV
Load Step
IL 500mA/DIV
IL 500mA/DIV
ILOAD 500mA/DIV VIN = 3.6V 20s/DIV VOUT = 2.5V ILOAD = 200mA TO 450mA
3406B F10
ILOAD 500mA/DIV
VIN = 3.6V 20s/DIV VOUT = 2.5V ILOAD = 300mA TO 600mA
3406B F16
Figure 5
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13
LTC3406AB TYPICAL APPLICATIONS
Single Li-Ion 1.2V/600mA Regulator for High Efficiency and Small Footprint
3 2.2H* 22pF 100 1.2V, 600mA VOUT COUT** 10F CER 301k 301k *MURATA LQH32CN2R2M33 ** TAIYO YUDEN JHK316BJ106ML TAIYO YUDEN JMK212BJ475MG 3406AB TA09b 90 80 70 EFFICIENCY (%) 60 50 40 30 20 10 0 0.1 VIN = 2.7V VIN = 3.6V VIN = 4.2V 1 100 10 OUTPUT CURRENT (mA) 1000
3406B G02
Efficiency vs Load Current
VOUT = 1.2V
VIN
CIN 4.7F CER 1
VIN
SW
LTC3406AB RUN GND 2 VFB 5
Load Step
VOUT 200mV/DIV VOUT 100mV/DIV
Load Step
IL 500mA/DIV
IL 500mA/DIV
ILOAD 500mA/DIV VIN = 3.6V 20s/DIV VOUT = 1.2V ILOAD = 200mA TO 500mA
3406B F12
ILOAD 500mA/DIV
VIN = 3.6V 20s/DIV VOUT = 1.2V ILOAD = 300mA TO 600mA
3406B F14
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14
LTC3406AB PACKAGE DESCRIPTION
S5 Package 5-Lead Plastic SOT-23
(Reference LTC DWG # 05-08-1633 Rev B)
0.62 MAX 0.95 REF
2.90 BSC (NOTE 4)
1.22 REF
3.85 MAX 2.62 REF
1.4 MIN
2.80 BSC
1.50 - 1.75 (NOTE 4)
PIN ONE RECOMMENDED SOLDER PAD LAYOUT PER IPC CALCULATOR 0.30 - 0.45 TYP 5 PLCS (NOTE 3)
0.95 BSC
0.80 - 0.90 0.20 BSC 1.00 MAX DATUM `A' 0.01 - 0.10
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
1.90 BSC
S5 TSOT-23 0302 REV B
3406abfa
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.
15
LTC3406AB RELATED PARTS
PART NUMBER LTC3406/LTC3406B LTC3407/LTC3407-2 LTC3410/LTC3410B LTC3411 LTC3412 LTC3440 LTC3530 DESCRIPTION 600mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converters Dual 600mA/800mA (IOUT), 1.5MHz/2.25MHz, Synchronous Step-Down DC/DC Converters 300mA (IOUT), 2.25MHz, Synchronous Step-Down DC/DC Converters 1.25A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 2.5A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 600mA (IOUT), 2MHz, Synchronous Buck-Boost DC/DC Converter 600mA (IOUT), 2MHz, Synchronous Buck-Boost DC/DC Converter COMMENTS 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, ISD <1A, MS10E, DFN Packages 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 26A, ISD <1A, SC70 Package 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 60mA, ISD <1A, MS10, DFN Packages 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 60A, ISD <1A, TSSOP-16E Package 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 2.5V to 5.5V, IQ = 25A, ISD <1A, MS10, DFN Packages 95% Efficiency, VIN: 1.8V to 5.5V, VOUT(MIN) = 1.8V to 5.25V, IQ = 40A, ISD <1A, MS10, DFN Packages 95% Efficiency, VIN: 1.8V to 5.5V, VOUT(MIN) = 2V to 5V, IQ = 16A, ISD <1A, ThinSOT, DFN Packages 95% Efficiency, VIN: 2.4V to 5.5V, VOUT(MIN) = 2.4V to 5.25V, IQ = 35A, ISD <1A, MS10, DFN Packages 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 26A, ISD <1A, 2mm x 2mm DFN Package 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 70A, ISD <1A, 3mm x 3mm QFN Package 96% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 40A, ISD <1A, 2mm x 3mm DFN Package 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 40A, ISD <1A, MS10E, DFN Packages 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 16A, ISD <1A, ThinSOT Package 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 240A, ISD <1A, DFN Package
LTC3531/LTC3531-3/ 200mA (IOUT), 1.5MHz, Synchronous Buck-Boost DC/DC Converters LTC3531-3.3 LTC3532 LTC3542 LTC3544/LTC3544B LTC3547/LTC3547B 500mA (IOUT), 2MHz, Synchronous Buck-Boost DC/DC Converter 500mA (IOUT), 2.25MHz, Synchronous Step-Down DC/DC Converter Quad 300mA + 2 x 200mA + 100mA 2.25MHz, Synchronous Step-Down DC/DC Converters Dual 300mA 2.25MHz, Synchronous Step-Down DC/DC Converters
LTC3548/LTC3548-1/ Dual 400mA and 800mA (IOUT), 2.25MHz, Synchronous Step-Down DC/DC Converters LTC3548-2 LTC3560 LTC3561 800mA (IOUT), 2.25MHz, Synchronous Step-Down DC/DC Converter 1.25A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter
3406abfa
16 Linear Technology Corporation
(408) 432-1900 FAX: (408) 434-0507
LT 0907 REV A * PRINTED IN USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.linear.com
(c) LINEAR TECHNOLOGY CORPORATION 2007

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