LTC3406A 1.5MHz, 600mA Synchronous Step-Down Regulator in ThinSOT DESCRIPTION FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ High Efficiency: Up to 96% Very Low Quiescent Current: Only 20µA Low Output Ripple Voltage During Burst Mode® Operation 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 ±2% 0.6V Reference Shutdown Mode Draw ≤1µA 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 ■ ■ ■ ■ ■ ■ The LTC®3406A is a high efficiency monolithic synchronous buck regulator using a constant frequency, current mode architecture. Supply current during operation is only 20μA, dropping to ≤1μA in shutdown. The 2.5V to 5.5V input voltage range makes the LTC3406A ideally suited for single Li-Ion battery-powered applications. 100% duty cycle provides low dropout operation, extending battery runtime portable systems. Automatic Burst Mode operation increases efficiency at light loads, further extending battery runtime. 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 LTC3406A is available in a low profile (1mm) ThinSOT package. , LT, LTC and LTM are registered trademarks of Linear Technology Corporation. ThinSOT is a registered trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. Protected by U.S. Patents including 5481178, 6580258. Cellular Telephones Wireless and DSL Modems Digital Still Cameras Media Players Portable Instruments Point of Load Regulation TYPICAL APPLICATION Efficiency vs Load Current 100 2.2μH VIN 4.7μF CER SW 22pF 10μF CER LTC3406A RUN GND VFB 619k 309k 3406A TA01 90 80 EFFICIENCY (%) VIN VOUT 1.8V 600mA 70 60 50 40 30 20 10 VIN = 2.7V VIN = 3.6V VIN = 4.2V VOUT = 1.8V 0 0.1 10 100 1 OUTPUT CURRENT (mA) 1000 3406A TA01b 3406afa 1 LTC3406A ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) TOP VIEW 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) LTC3406AE ..............................................– 40°C to 85°C LTC3406AI .............................................– 40°C to 125°C Junction Temperature (Notes 3, 6)........................ 125°C Storage Temperature Range...................– 65°C to 150°C Lead Temperature (Soldering, 10 sec) .................. 300°C RUN 1 5 VFB GND 2 SW 3 4 VIN S5 PACKAGE 5-LEAD PLASTIC TSOT-23 TJMAX = 125°C, θJA = 250°C/W, θJC = 90°C/W ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LTC3406AES5#PBF LTC3406AES5#TRPBF LTCWJ 5-Lead Plastic TSOT-23 –40°C to 85°C LTC3406AIS5#PBF LTC3406AIS5#TRPBF LTCWJ 5-Lead Plastic TSOT-23 –40°C to 125°C Consult LTC Marketing for parts specified with wider operating temperature ranges. *Temperature grades are identified by a label on the shipping container. 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 The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 3.6V unless otherwise specified. SYMBOL PARAMETER CONDITIONS MIN IVFB Feedback Current VFB Regulated Feedback Voltage (Note 4) LTC3406AE (Note 4) LTC3406AI ● ● ∆VFB Reference Voltage Line Regulation VIN = 2.5V to 5.5V (Note 4) LTC3406AE VIN = 2.5V to 5.5V (Note 4) LTC3406AI ● ● IPK Peak Inductor Current VIN = 3V, VFB = 0.5V Duty Cycle < 35% VLOADREG Output Voltage Load Regulation VIN Input Voltage Range IS Input DC Bias Current Active Mode Sleep Mode Shutdown TYP MAX UNITS ±30 nA 0.6 0.6 0.6120 0.615 V V 0.04 0.04 0.4 0.6 %/V %/V 1 1.25 A ● 0.5880 0.585 0.75 0.5 ● 2.5 (Note 5) VFB = 0V VFB = 0.63V VRUN = 0V, VIN = 5.5V fOSC Oscillator Frequency VFB = 0.6V RPFET RDS(ON) of P-Channel FET ISW = 100mA 5.5 V 300 30 1 μA μA μA 1.5 1.8 MHz 0.23 0.35 Ω 200 16 0.1 ● 1.2 % 3406afa 2 LTC3406A ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 3.6V unless otherwise specified. SYMBOL PARAMETER CONDITIONS TYP MAX UNITS RNFET RDS(ON) of N-Channel FET ISW = –100mA 0.21 0.35 Ω ILSW SW Leakage VRUN = 0V, VSW = 0V or 5V, VIN = 5V ±0.01 ±1 μA tSOFT-START Soft-Start Time VFB from 10% to 90% Full-Scale 0.6 0.9 1.2 ms VRUN RUN Threshold ● 0.3 1 1.5 V RUN Leakage Current ● ±0.01 ±1 μA IRUN 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 LTC3406AE is guaranteed to meet performance specifications from 0°C to 85°C. Specifications over the –40°C to 85°C operating temperature range are assured by design, characterization and correlation with statistical process controls. The LTC3406AI is guaranteed to meet the specified performance over the full –40°C to 125°C operating temperature range. Note 3: TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formula: LTC3406A: TJ = TA + (PD)(250°C/W) MIN Note 4: The LTC3406A 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 125°C when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature may impair device reliability. Note 7: Limited by long term current density considerations. TYPICAL PERFORMANCE CHARACTERISTICS (From Front Page Figure Except for the Resistive Divider Resistor Values) Efficiency vs Load Current Efficiency vs Load Current 100 90 90 90 80 80 80 70 70 70 60 50 40 EFFICIENCY (%) 100 EFFICIENCY (%) EFFICIENCY (%) Efficiency vs Input Voltage 100 60 50 40 60 50 40 30 30 30 20 20 20 10 VOUT = 1.8V 0 2 3 IL = 10mA IL = 100mA IL = 600mA 4 5 INPUT VOLTAGE (V) 10 6 3406A G01 VIN = 2.7V VIN = 3.6V VIN = 4.2V VOUT = 1.2V 0 0.1 1 10 100 OUTPUT CURRENT (mA) 1000 3406A G02 10 VIN = 2.7V VIN = 3.6V VIN = 4.2V VOUT = 2.5V 0 0.1 1 10 100 OUTPUT CURRENT (mA) 1000 3406A G03 3406afa 3 LTC3406A TYPICAL PERFORMANCE CHARACTERISTICS (From Front Page Figure Except for the Resistive Divider Resistor Values) VOUT = 1.8V 1.812 0.615 0.610 80 EFFICIENCY (%) 1.808 1.804 1.800 1.796 70 60 50 40 1.792 30 1.788 20 1.784 10 1.780 2 600 400 200 OUTPUT CURRENT (mA) IL = 10mA IL = 100mA IL = 600mA VOUT = 2.5V 0 0 3 4 5 INPUT VOLTAGE (V) Oscillator Frequency vs Temperature 0.595 0.590 0.585 –50 –25 6 1.55 1.50 1.45 1.40 1.35 125 Burst Mode 1.55 SW 2V/DIV 1.50 1.45 VOUT 20mV/DIV AC COUPLED 1.40 IL 200mA/DIV 1.35 1.30 50 25 75 0 TEMPERATURE (°C) 100 1.20 2.0 125 2.5 3.0 3.5 4.0 4.5 5.0 INPUT VOLTAGE (V) 3406A G07 4μs/DIV VIN = 3.6V VOUT = 1.8V ILOAD = 10mA Burst Mode OPERATION 6.0 5.5 RDS(ON) vs Input Voltage RDS(ON) vs Temperature Dynamic Supply Current 0.40 10 9 0.35 0.35 0.30 RDS(ON) (Ω) 0.30 MAIN SWITCH 0.25 SYNCHRONOUS SWITCH 0.20 VIN = 2.7V VIN = 3.6V 0.25 VIN = 4.2V 0.20 0.15 0.10 0.15 0.05 0.10 4 3 5 2 INPUT VOLTAGE (V) 6 7 3406A G10 3406A G09 3406A G08 0.40 1 100 3406A G06 1.25 0 50 25 75 0 TEMPERATURE (°C) 1.60 OSCILLATOR FREQUENCY (MHz) OSCILLATOR FREQUENCY (MHz) 0.600 Oscillator Frequency vs Input Voltage VIN = 3.6V 1.30 –50 –25 RDS(0N) (Ω) 0.605 3406A G05 3406A G04 1.60 VIN = 3.6V 90 0 –50 –25 SYNCHRONOUS SWITCH MAIN SWITCH 75 50 25 TEMPERATURE (°C) 0 100 125 3406A G11 DYNAMIC SUPPLY CURRENT (μA) OUTPUT VOLTAGE (V) 100 VIN = 2.7V VIN = 3.6V VIN = 4.2V 1.816 REFERENCE VOLTAGE (V) 1.820 Reference Voltage vs Temperature Efficiency vs Input Voltage Output vs Load Current VOUT = 1.2V ILOAD = 0A 8 7 6 5 4 3 2 1 0 2.0 2.5 3.0 3.5 4.0 4.5 5.0 INPUT VOLTAGE (V) 5.5 6.0 3406A G12 3406afa 4 LTC3406A TYPICAL PERFORMANCE CHARACTERISTICS (From Front Page Figure Except for the Resistive Divider Resistor Values) Dynamic Supply Current vs Temperature Switch Leakage vs Temperature VIN = 3.6V VOUT = 1.2V ILOAD = 0A 1000 150 100 50 100 80 MAIN SWITCH 60 40 SYNCHRONOUS SWITCH 20 50 25 75 0 TEMPERATURE (°C) 100 125 700 600 500 300 0 50 25 75 0 TEMPERATURE (°C) 100 125 0 1 3 4 2 INPUT VOLTAGE (V) RUN 2V/DIV VOUT 200mV/DIV VOUT 1V/DIV IL 500mA/DIV ILOAD 500mA/DIV ILOAD 500mA/DIV Load Step 6 Load Step VOUT 200mV/DIV IL 500mA/DIV ILOAD 500mA/DIV VIN = 3.6V 20μs/DIV VOUT = 1.8V ILOAD = 0mA TO 600mA 3406A G16 5 3406A G15 Load Step 3406A G17 VIN = 3.6V 20μs/DIV VOUT = 1.8V ILOAD = 50mA TO 600mA Load Step 3406A G18 Discontinuous Operation VOUT 200mV/DIV VOUT 200mV/DIV SW 2V/DIV IL 500mA/DIV IL 500mA/DIV VOUT 20mV/DIV AC COUPLED ILOAD 500mA/DIV ILOAD 500mA/DIV 3406A G19 SYNCHRONOUS SWITCH 200 3406A G14 Start-Up from Shutdown VIN = 3.6V 20μs/DIV VOUT = 1.8V ILOAD = 100mA TO 600mA MAIN SWITCH 400 100 0 –50 –25 3406A G13 500μs/DIV VIN = 3.6V VOUT = 1.8V ILOAD = 600mA (3Ω RES) SWITCH LEAKAGE (pA) 800 200 0 –50 –25 RUN = 0V 900 120 SWITCH LEAKAGE (nA) DYNAMIC SUPPLY CURRENT (μA) 250 Switch Leakage vs Input Voltage 140 300 IL 200mA/DIV VIN = 3.6V 20μs/DIV VOUT = 1.8V ILOAD = 200mA TO 600mA 3406A G20 VIN = 3.6V VOUT = 1.8V ILOAD = 50mA 500ns/DIV 3406A G21 3406afa 5 LTC3406A 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 <1μA supply current. Do not leave RUN floating. VIN (Pin 4): Main Supply Pin. Must be closely decoupled to GND, Pin 2, with a 2.2μF or greater ceramic capacitor. VFB (Pin 5): Feedback Pin. Receives the feedback voltage from an external resistive divider across the output. 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. FUNCTIONAL DIAGRAM SLOPE COMP 0.65V OSC OSC 4 VIN FREQ SHIFT – VFB 5 0.6V + – EA + – 0.4V SLEEP – + S Q R Q RS LATCH VIN RUN SWITCHING LOGIC AND BLANKING CIRCUIT ANTISHOOTTHRU 3 SW 0.6V REF SHUTDOWN + 1 5Ω + ICOMP BURST IRCMP 2 GND – 3406A BD 3406afa 6 LTC3406A OPERATION (Refer to Functional Diagram) Main Control Loop The LTC3406A 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. off, reducing the quiescent current to 20μA. In this sleep state, the load current is being supplied solely from the output capacitor. As the output voltage droops, the EA amplifier’s output rises above the sleep threshold signaling the BURST comparator to trip and turn the top MOSFET on. This process repeats at a rate that is dependent on the load demand. Dropout Operation As the input supply voltage decreases to a value approaching the output voltage, the duty cycle increases toward the maximum on-time. Further reduction of the supply voltage forces the main switch to remain on for more than one cycle until it reaches 100% duty cycle. The output voltage will then be determined by the input voltage minus the voltage drop across the P-channel MOSFET and the inductor. 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. 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 LTC3406A is used at 100% duty cycle with low input voltage (See Thermal Considerations in the Applications Information section). Burst Mode Operation Slope Compensation and Inductor Peak Current The LTC3406A is capable of Burst Mode operation in which the internal power MOSFETs operate intermittently based on load demand. 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 LTC3406A uses a patented scheme that counteracts this compensating ramp, which allows the maximum inductor peak current to remain unaffected throughout all duty cycles. In Burst Mode operation, the peak current of the inductor is set to approximately 100mA regardless of the output load. Each burst event can last from a few cycles at light loads to almost continuously cycling with short sleep intervals at moderate loads. In between these burst events, the power MOSFETs and any unneeded circuitry are turned 3406afa 7 LTC3406A APPLICATIONS INFORMATION The basic LTC3406A 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. Table 1. Representative Surface Mount Inductors PART NUMBER VALUE (μH) DCR (Ω MAX) MAX DC CURRENT (A) SIZE W × L × H (mm3) Sumida CDRH3D16 1.5 2.2 3.3 4.7 0.043 0.075 0.110 0.162 1.55 1.20 1.10 0.90 3.8 × 3.8 × 1.8 Sumida CMD4D06 2.2 3.3 4.7 0.116 0.174 0.216 0.950 0.770 0.750 3.5 × 4.3 × 0.8 Panasonic ELT5KT 3.3 4.7 0.17 0.20 1.00 0.95 4.5 × 5.4 × 1.2 Murata LQH32CN 1.0 2.2 4.7 0.060 0.097 0.150 1.00 0.79 0.65 2.5 × 3.2 × 2.0 Inductor Selection For most applications, the value of the inductor will fall in the range of 1μH to 4.7μH. 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) 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. The inductor value also has an effect on Burst Mode operation. The transition to low current operation begins when the inductor current peaks fall to approximately 100mA. Lower inductor values (higher ∆IL) will cause this to occur at lower load currents, which can cause a dip in efficiency in the upper range of low current operation. In Burst Mode operation, lower inductance values will cause the burst frequency to increase. Inductor Core Selection Different core materials and shapes will change the size/current and price/current relationship of an inductor. Toroid or shielded pot cores in ferrite or permalloy materials are small and don’t radiate much energy, but generally cost more than powdered iron core inductors with similar electrical characteristics. The choice of which style inductor to use often depends more on the price vs size requirements and any radiated field/EMI requirements than on what the LTC3406A requires to operate. Table 1 shows some typical surface mount inductors that work well in LTC3406A 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 1/2 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. 3406afa 8 LTC3406A APPLICATIONS INFORMATION 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 + 8fCOUT 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. 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 = 0.6V 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 LTC3406A R1 GND 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 LTC3406A’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 3406A F01 Figure 1. Setting the LTC3406A 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. 3406afa 9 LTC3406A APPLICATIONS INFORMATION Although all dissipative elements in the circuit produce losses, two main sources usually account for most of the losses in LTC3406A 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 0.1 POWER LOSS (W) 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. VIN = 3.6V 0.01 Other losses including CIN and COUT ESR dissipative losses and inductor core losses generally account for less than 2% total additional loss. 0.001 0.0001 0.1 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: VOUT = 1.2V VOUT = 1.8V VOUT = 2.5V 10 100 1 OUTPUT CURRENT (mA) 1000 3406A F02 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. Thermal Considerations In most applications the LTC3406A does not dissipate much heat due to its high efficiency. But, in applications where the LTC3406A 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 150°C, both power switches will be turned off and the SW node will become high impedance. To avoid the LTC3406A 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)(θJA) where PD is the power dissipated by the regulator and θJA is the thermal resistance from the junction of the die to the ambient temperature. 3406afa 10 LTC3406A APPLICATIONS INFORMATION The junction temperature, TJ, is given by: TJ = TA + TR where TA is the ambient temperature. As an example, consider the LTC3406A in dropout at an input voltage of 2.7V, a load current of 600mA and an ambient temperature of 70°C. From the typical performance graph of switch resistance, the RDS(ON) of the P-channel switch at 70°C is approximately 0.27Ω. Therefore, power dissipated by the part is: PD = ILOAD2 • RDS(ON) = 97.2mW For the SOT-23 package, the θJA is 250°C/ W. Thus, the junction temperature of the regulator is: TJ = 70°C + (0.0972)(250) = 94.3°C which is below the maximum junction temperature of 125°C. 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 (>1μF) 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 10μF capacitor charging to 3.3V would require a 250μs 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 LTC3406A. 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 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, and the IC ground, as close as possible. 3406afa 11 LTC3406A APPLICATIONS INFORMATION 1 RUN VFB 5 LTC3406A 2 R2 – COUT VOUT 3 + R1 GND SW L1 VIN CFWD 4 CIN VIN BOLD LINES INDICATE HIGH CURRENT PATHS 3406A F03 Figure 3. LTC3406A Layout Diagram VIA TO VIN CFWD LTC3406A L1 VIA TO VOUT R2 PIN 1 VOUT VIN R1 SW COUT CIN GND 3406A F04 Figure 4. LTC3406A Suggested Layout Design Example As a design example, assume the LTC3406A 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 ) (3) Substituting VOUT = 2.5V, VIN = 4.2V, ∆IL = 240mA and f = 1.5MHz in Equation (3) gives: L= 2.5V 2.5V 1 = 2.81μH 1.5MHz(240mA) 4.2V A 2.2μH inductor works well for this application. For best efficiency choose a 720mA or greater inductor with less than 0.2Ω 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 0.25Ω. In most cases, a ceramic capacitor will satisfy this requirement. 3406afa 12 LTC3406A APPLICATIONS INFORMATION Figure 5 shows the complete circuit along with its efficiency curve. 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 (4) 4 VIN 2.7V TO 4.2V † CIN 4.7μF CER VIN SW 3 2.2μH* VOUT 2.5V 600mA COUT** 10μF CER 22pF LTC3406A 1 VFB RUN 5 GND 2 1M 316k 3406A F05a * MURATA LQH32CN2R2M33 ** TAIYO YUDEN JMK316BJ106ML † TAIYO YUDEN LMK212BJ475MG 100 90 EFFICIENCY (%) 80 VOUT 100mV/DIV 70 60 IL 500mA/DIV 50 40 ILOAD 500mA/DIV 30 20 10 0 0.1 VIN = 2.7V VIN = 3.6V VIN = 4.2V 1 10 100 OUTPUT CURRENT (mA) 20μs/DIV VIN = 3.6V VOUT = 2.5V ILOAD = 300mA TO 600mA 1000 3406A F05d 3406A F05b Figure 5. 3406afa 13 LTC3406A TYPICAL APPLICATIONS Single Li-Ion 1.2V/600mA Regulator for High Efficiency and Small Footprint 4 VIN † CIN 4.7μF CER VIN SW 3 2.2μH* 22pF LTC3406A 1 VFB RUN GND 2 5 VOUT 1.2V 600mA COUT** 10μF CER 301k * MURATA LQH32CN2R2M33 ** TAIYO YUDEN JMK316BJ106ML † TAIYO YUDEN JMK212BJ475MG 3406A TA02 301k Efficiency vs Load Current Load Step 100 90 VOUT 100mV/DIV EFFICIENCY (%) 80 70 60 IL 500mA/DIV 50 ILOAD 500mA/DIV 40 30 20 10 VOUT = 1.2V 0 0.1 VIN = 2.7V VIN = 3.6V VIN = 4.2V 1 10 100 OUTPUT CURRENT (mA) 1000 VIN = 3.6V 20μs/DIV VOUT = 1.2V ILOAD = 300mA TO 600mA 3406A TA05 3406A TA03 3406afa 14 LTC3406A PACKAGE DESCRIPTION S5 Package 5-Lead Plastic TSOT-23 (Reference LTC DWG # 05-08-1635) 0.62 MAX 0.95 REF 2.90 BSC (NOTE 4) 1.22 REF 1.4 MIN 3.85 MAX 2.62 REF 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 0.01 – 0.10 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 1.90 BSC S5 TSOT-23 0302 REV B 3406afa 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 LTC3406A RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LTC3406/LTC3406B 600mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converters 96% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 20μA, ISD <1μA, ThinSOT Package LTC3407/LTC3407-2 Dual 600mA/800mA (IOUT), 1.5MHz/2.25MHz, Synchronous Step-Down DC/DC Converters 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 40μA, ISD <1μA, MS10E, DFN Packages LTC3410/LTC3410B 300mA (IOUT), 2.25MHz, Synchronous Step-Down DC/DC Converters 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 26μA, ISD <1μA, SC70 Package LTC3411 1.25A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 60μA, ISD <1μA, MS10, DFN Packages LTC3412 2.5A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 60μA, ISD <1μA, TSSOP-16E Package LTC3440 600mA (IOUT), 2MHz, Synchronous Buck-Boost DC/DC Converter 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN): 2.5V to 5.5V, IQ = 25μA, ISD <1μA, MS10, DFN Packages LTC3530 600mA (IOUT), 2MHz, Synchronous Buck-Boost DC/DC Converter 95% Efficiency, VIN: 1.8V to 5.5V, VOUT(MIN): 1.8V to 5.25V, IQ = 40μA, ISD <1μA, MS10, DFN Packages LTC3531/LTC3531-3/ LTC3531-3.3 200mA (IOUT), 1.5MHz, Synchronous Buck-Boost DC/DC Converters 95% Efficiency, VIN: 1.8V to 5.5V, VOUT(MIN): 2V to 5V, IQ = 16μA, ISD <1μA, ThinSOT, DFN Packages LTC3532 500mA (IOUT), 2MHz, Synchronous Buck-Boost DC/DC Converter 95% Efficiency, VIN: 2.4V to 5.5V, VOUT(MIN): 2.4V to 5.25V, IQ = 35μA, ISD <1μA, MS10, DFN Packages LTC3542 500mA (IOUT), 2.25MHz, Synchronous Step-Down DC/DC Converter 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 26μA, ISD <1μA, 2mm × 2mm DFN Package LTC3544/LTC3544B Quad 300mA + 2 × 200mA + 100mA, 2.25MHz, Synchronous Step-Down DC/DC Converters 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 70μA, ISD <1μA, 3mm × 3mm QFN Package LTC3547/LTC3547B Dual 300mA, 2.25MHz, Synchronous Step-Down DC/DC Converters 96% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 40μA, ISD <1μA, 2mm × 3mm DFN Package LTC3548/LTC3548-1/ LTC3548-2 Dual 400mA/800mA (IOUT), 2.25MHz, Synchronous Step-Down DC/DC Converters 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 40μA, ISD <1μA, MS10E, DFN Packages LTC3560 800mA (IOUT), 2.25MHz, Synchronous Step-Down DC/DC Converter 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.6V, IQ = 16μA, ISD <1μA, ThinSOT Package LTC3561 1.25A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 95% Efficiency, VIN: 2.5V to 5.5V, VOUT(MIN) = 0.8V, IQ = 240μA, ISD <1μA, DFN Package 3406afa 16 Linear Technology Corporation LT 1207 REV A • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 2007