LTC3405A-1.5/LTC3405A-1.8 1.5V, 1.8V, 1.5MHz, 300mA Synchronous Step-Down Regulators in ThinSOT U FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO The LTC ®3405A-1.5/LTC3405A-1.8 are high efficiency monolithic synchronous buck regulators using a constant frequency, current mode architecture. Supply current during operation is only 20µA and drops to <1µA in shutdown. The 2.5V to 5.5V input voltage range makes the LTC3405A-1.5/LTC3405A-1.8 ideally suited for single Li-Ion battery-powered applications. 100% duty cycle provides low dropout operation, extending battery life in portable systems. High Efficiency: Up to 93% Very Low Quiescent Current: Only 20µA During Operation 300mA Output Current at VIN = 3V 2.5V to 5.5V Input Voltage Range 1.5MHz Constant Frequency Operation No Schottky Diode Required Low Dropout Operation: 100% Duty Cycle Stable with Ceramic Capacitors Shutdown Mode Draws < 1µA Supply Current ±3% Output Voltage Accuracy Current Mode Operation for Excellent Line and Load Transient Response Overtemperature Protected Low Profile (1mm) ThinSOTTM Package Switching frequency is internally set at 1.5MHz, allowing the use of small surface mount inductors and capacitors. The LTC3405A-1.5/LTC3405A-1.8 are specifically designed to work well with ceramic output capacitors, achieving very low output voltage ripple and a small PCB footprint. U APPLICATIO S ■ ■ ■ ■ ■ Cellular Telephones Personal Information Appliances Wireless and DSL Modems Digital Still Cameras MP3 Players Portable Instruments For adjustable output voltage, refer to the LTC3405A data sheet. , LTC and LT are registered trademarks of Linear Technology Corporation. ThinSOT is a trademark of Linear Technology Corporation. U ■ The internal synchronous switch increases efficiency and eliminates the need for an external Schottky diode. The LTC3405A-1.5/LTC3405A-1.8 are available in a low profile (1mm) ThinSOT package. TYPICAL APPLICATIO 100 VIN = 2.7V VIN 2.7V TO 5.5V 4 CIN** 4.7µF CER SW VIN 3 4.7µH* LTC3405A-1.8 1 6 RUN MODE VOUT GND 5 VOUT 1.8V 300mA COUT** 4.7µF CER EFFICIENCY (%) 90 80 VIN = 3.6V VIN = 4.2V 70 VIN = 5.5V 60 3405A1518 F01 2 *MURATA LQH3C4R7M34 **TAIYO YUDEN JMK212BJ475MG 50 40 0.1 1 100 10 OUTPUT CURRENT (mA) 1000 3405A1518 F01b Figure 1a. High Efficiency Step-Down Converter Figure 1b. Efficiency vs Load Current sn3405a1518 3405a1518fs 1 LTC3405A-1.5/LTC3405A-1.8 W W W AXI U U ABSOLUTE RATI GS U U W PACKAGE/ORDER I FOR ATIO (Note 1) Input Supply Voltage .................................. – 0.3V to 6V MODE, RUN, VOUT Voltages....................... – 0.3V to VIN SW Voltage .................................. – 0.3V to (VIN + 0.3V) P-Channel Switch Source Current (DC) ............. 400mA N-Channel Switch Sink Current (DC) ................. 400mA Peak SW Sink and Source Current .................... 630mA Operating Temperature Range (Note 2) .. – 40°C to 85°C Junction Temperature (Note 3) ............................ 125°C Storage Temperature Range ................ – 65°C to 150°C Lead Temperature (Soldering, 10 sec)................. 300°C ORDER PART NUMBER TOP VIEW RUN 1 6 MODE GND 2 5 VOUT 4 VIN SW 3 LTC3405AES6-1.5 LTC3405AES6-1.8 S6 PACKAGE 6-LEAD PLASTIC SOT-23 S6 PART MARKING TJMAX = 125°C, θJA = 250°C/ W LTZQ LTZP Consult LTC Marketing for parts specified with wider operating temperature ranges. ELECTRICAL CHARACTERISTICS The ● denotes specifications which apply over the full operating temperature range, otherwise specifications are TA = 25°C. VIN = 3.6V unless otherwise specified. SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS IPK Peak Inductor Current VIN = 3V, VOUT = 90%, Duty Cycle < 35% 375 500 625 mA VOUT Regulated Output Voltage LTC3405A-1.5, MODE = 3.6V LTC3405A-1.8, MODE = 3.6V 1.455 1.746 1.500 1.800 1.545 1.854 V V ∆VOVL ∆Output Overvoltage Lockout ∆VOVL = VOVL – VOUT 2.5 7.8 13 % ∆VOUT Output Voltage Line Regulation VIN = 2.5V to 5.5V 0.04 0.4 %/V VLOADREG Output Voltage Load Regulation VIN Input Voltage Range IS Input DC Bias Current Pulse Skipping Mode Burst Mode® Operation Shutdown (Note 4) VOUT = 90%, MODE = 3.6V, ILOAD = 0A VOUT = 103%, MODE = 0V, ILOAD = 0A VRUN = 0V, VIN = 4.2V fOSC Oscillator Frequency VOUT = 100% VOUT = 0V RPFET RDS(ON) of P-Channel FET ISW = 100mA RNFET RDS(ON) of N-Channel FET ISW = –100mA ILSW SW Leakage VRUN = 0V, VSW = 0V or 5V, VIN = 5V VRUN RUN Threshold ● IRUN RUN Leakage Current ● VMODE MODE Threshold ● IMODE MODE Leakage Current ● ● ● ● 0.5 ● Burst Mode is a registered trademark of Linear Technology Corporation. Note 1: Absolute Maximum Ratings are those values beyond which the life of a device may be impaired. Note 2: The LTC3405AE is guaranteed to meet performance specifications from 0°C to 70°C. Specifications over the –40°C to 85°C operating temperature range are assured by design, characterization and correlation with statistical process controls. ● 2.5 1.2 0.3 0.3 % 5.5 V 300 20 0.1 400 35 1 µA µA µA 1.5 170 1.8 MHz kHz 0.7 0.85 Ω 0.6 0.90 Ω ±0.01 ±1 µA 1 1.5 V ±0.01 ±1 µA 1.5 2 V ±0.01 ±1 µA Note 3: TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formula: LTC3405A: TJ = TA + (PD)(250°C/W) Note 4: Dynamic supply current is higher due to the gate charge being delivered at the switching frequency. sn3405a1518 3405a1518fs 2 LTC3405A-1.5/LTC3405A-1.8 U W TYPICAL PERFOR A CE CHARACTERISTICS (From Figure1a) Efficiency vs Input Voltage IOUT = 100mA 80 IOUT = 10mA IOUT = 1mA 80 VIN = 4.2V 70 75 70 65 60 VIN = 3.6V 50 VIN = 4.2V 40 30 IOUT = 0.1mA 60 PULSE SKIPPING MODE Burst Mode OPERATION 20 Burst Mode OPERATION VOUT = 1.8V 55 50 2.5 3.0 3.5 4.0 4.5 INPUT VOLTAGE (V) 90 10 5.0 EFFICIENCY (%) IOUT = 250mA VIN = 2.7V 90 V = 3.6V IN EFFICIENCY (%) 85 EFFICIENCY (%) 100 100 90 5.5 VOUT = 1.8V Oscillator Frequency vs Supply Voltage 1.8 VIN = 3.6V OSCILLATOR FREQUENCY (MHz) 1.65 1.60 FREQUENCY (MHz) VIN = 3.6V 80 VIN = 4.2V 70 60 1.55 1.50 1.45 1.40 50 1.35 10 100 1 OUTPUT CURRENT (mA) 1000 1.30 –50 –25 50 25 75 0 TEMPERATURE (°C) 100 1.7 1.6 1.5 1.4 1.3 1.2 125 2 3 4 5 SUPPLY VOLTAGE (V) 3405A1518 G07 RDS(ON) vs Input Voltage Output Voltage vs Load Current 1.834 RDS(ON) vs Temperature 1.2 1.2 1.1 VIN = 4.2V 1.0 V = 2.7V IN 1.0 0.9 MAIN SWITCH 1.804 1.794 0.7 0.6 SYNCHRONOUS SWITCH 0.5 0.6 0.4 0.4 0.3 PULSE SKIPPING MODE 1.784 0.2 0.2 SYNCHRONOUS SWITCH MAIN SWITCH 0.1 1.774 0 100 200 300 400 LOAD CURRENT (mA) VIN = 3.6V 0.8 RDS(ON) (Ω) 0.8 RDS(0N) (Ω) OUTPUT VOLTAGE (V) 1.824 Burst Mode OPERATION 6 3405A1518 G08 3405A1518 G05 1.814 1000 1 100 10 OUTPUT CURRENT (mA) 3405A1518 G04 1.70 VIN = 2.7V 40 0.1 VIN = 5.5V 60 Oscillator Frequency vs Temperature VOUT = 1.5V 90 70 3405A1518 G03 Efficiency vs Output Current 100 VIN = 4.2V 40 0.1 1000 1 100 10 OUTPUT CURRENT (mA) VIN = 3.6V 80 50 VOUT = 1.8V 0 0.1 3405A1518 G02 EFFICIENCY (%) Efficiency vs Output Current Efficiency vs Output Current 95 500 600 3405A1518 G09 0 0 1 3 2 5 4 INPUT VOLTAGE (V) 6 7 3405A1518 G10 0 –50 –25 50 25 75 0 TEMPERATURE (°C) 100 125 3405F G11 sn3405a1518 3405a1518fs 3 LTC3405A-1.5/LTC3405A-1.8 U W TYPICAL PERFOR A CE CHARACTERISTICS (From Figure 1a) Dynamic Supply Current vs Supply Voltage VOUT = 1.8V ILOAD = 0A 1400 SUPPLY CURRENT (µA) SUPPLY CURRENT (µA) 1600 1200 1000 800 600 400 PULSE SKIPPING MODE 0 3 3.5 4.5 4 SUPPLY VOLTAGE (V) 5.5 5 VIN = 5.5V 140 RUN = 0V PULSE SKIPPING MODE 220 180 140 100 BURST MODE OPERATION 20 0 50 –50 –25 25 75 0 TEMPERATURE (°C) BURST MODE OPERATION 2.5 VIN = 3.6V 300 VOUT = 1.8V ILOAD = 0A 260 60 200 2 Switch Leakage vs Temperature 160 340 SWITCH LEAKAGE (nA) 1800 Dynamic Supply Current vs Temperature 3405A1518 G12 120 100 80 60 SYNCHRONOUS SWITCH 40 MAIN SWITCH 20 100 0 –50 –25 125 50 25 75 0 TEMPERATURE (°C) 125 3405F G14 3405A1518 G13 Switch Leakage vs Input Voltage 100 Pulse Skipping Mode Operation Burst Mode Operation 60 RUN = 0V SWITCH LEAKAGE (pA) 50 40 SW 5V/DIV SW 5V/DIV SYNCHRONOUS SWITCH VOUT 100mV/DIV AC COUPLED 30 VOUT 10mV/DIV AC COUPLED 20 IL 100mA/DIV MAIN SWITCH 10 0 0 1 2 3 4 INPUT VOLTAGE (V) 5 6 3405F G15 IL 100mA/DIV VIN = 3.6V VOUT = 1.8V ILOAD = 20mA 5µs/DIV 3405A1518 G16 Start-Up from Shutdown VIN = 3.6V VOUT = 1.8V ILOAD = 20mA 500ns/DIV 3405A1518 G17 Load Step VOUT 100mV/DIV AC COUPLED RUN 2V/DIV VOUT 1V/DIV IL 200mA/DIV IL 200mA/DIV ILOAD 200mA/DIV VIN = 3.6V VOUT = 1.8V ILOAD = 250mA 100µs/DIV 3405A1518 G18 VIN = 3.6V 40µs/DIV VOUT = 1.8V ILOAD = 0mA TO 250mA PULSE SKIPPING MODE 3405A1518 G19 sn3405a1518 3405a1518fs 4 LTC3405A-1.5/LTC3405A-1.8 U W TYPICAL PERFOR A CE CHARACTERISTICS (From Figure 1a) Load Step Load Step Load Step VOUT 100mV/DIV AC COUPLED VOUT 100mV/DIV AC COUPLED IL 200mA/DIV IL 200mA/DIV IL 200mA/DIV ILOAD 200mA/DIV ILOAD 200mA/DIV ILOAD 200mA/DIV VIN = 3.6V 40µs/DIV VOUT = 1.8V ILOAD = 20mA TO 250mA PULSE SKIPPING MODE 3405A1518 G20 VOUT 100mV/DIV AC COUPLED VIN = 3.6V 40µs/DIV VOUT = 1.8V ILOAD = 20mA TO 250mA Burst Mode OPERATION 3405A1518 G21 VIN = 3.6V 40µs/DIV VOUT = 1.8V ILOAD = 0mA TO 250mA Burst Mode OPERATION 3405A1518 G22 U U U 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 <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. GND (Pin 2): Ground Pin. VOUT (Pin 5): Output Voltage Feedback Pin. An internal resistive divider divides the output voltage down for comparison to the internal 1.2V reference voltage. SW (Pin 3): Switch Node Connection to Inductor. This pin connects to the drains of the internal main and synchronous power MOSFET switches. MODE (Pin 6): Mode Select Input. To select pulse skipping mode, tie to VIN. Grounding this pin selects Burst Mode operation. Do not leave this pin floating. sn3405a1518 3405a1518fs 5 LTC3405A-1.5/LTC3405A-1.8 W FU CTIO AL DIAGRA U U MODE 6 SLOPE COMP 0.65V OSC OSC 4 VIN FREQ SHIFT VOUT – 5 + LTC3405A-1.5 R1 = 110k R2 = 440k R1 LTC3405A-1.8 R1 = 180k R2 = 360k R2 – + 1.2V 0.4V – EA SLEEP S Q R Q RS LATCH RUN – OVDET 1.294V ICOMP SWITCHING LOGIC AND BLANKING CIRCUIT ANTISHOOTTHRU 3 SW OV + + 1.2V REF 5Ω + – + BURST VFB VIN 1 EN SHUTDOWN IRCMP 2 GND – 3405A1518 BD U OPERATIO (Refer to Functional Diagram) Main Control Loop The LTC3405A Fixed output voltage series parts use a constant frequency, current mode step-down architecture. Their 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, the output voltage decreases which causes a slight decrease in VFB relative to the 1.2V 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. Comparator OVDET guards against transient overshoots > 7.8% by turning the main switch off and keeping it off until the fault is removed. Burst Mode Operation The LTC3405A series parts are capable of Burst Mode operation in which the internal power MOSFETs operate intermittently based on load demand. To enable Burst Mode operation, simply connect the MODE pin to GND. To disable Burst Mode operation and enable PWM pulse skipping mode, connect the MODE pin to VIN or drive it with a logic high (VMODE > 1.5V). In this mode, the efficiency is lower at light loads, but becomes comparable to Burst Mode operation when the output load exceeds 25mA. The advantage of pulse skipping mode is lower output ripple and less interference to audio circuitry. sn3405a1518 3405a1518fs 6 LTC3405A-1.5/LTC3405A-1.8 U OPERATIO (Refer to Functional Diagram) 600 VOUT = 1.5V MAXIMUM OUTPUT CURRENT (mA) When the converter is 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 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. 500 VOUT = 1.8V 400 300 200 100 0 2.5 3.0 3.5 4.0 4.5 SUPPLY VOLTAGE (V) 5.0 5.5 3405A1518 F02 Figure 2. Maximum Output Current vs Input Voltage Short-Circuit Protection When the output is shorted to ground, the frequency of the oscillator is reduced to about 210kHz, 1/7 the nominal frequency. This frequency foldback ensures that the inductor current has more time to decay, thereby preventing runaway. The oscillator’s frequency will progressively increase to 1.5MHz when VOUT rises above 0V. Low Supply Operation The LTC3405A series parts 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 both fixed 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 LTC3405A series parts use a patent-pending scheme that counteracts this compensating ramp, which allows the maximum inductor peak current to remain unaffected throughout all duty cycles. sn3405a1518 3405a1518fs 7 LTC3405A-1.5/LTC3405A-1.8 U W U U APPLICATIO S I FOR ATIO The basic LTC3405A series parts 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 inductor value will fall in the range of 2.2µH to 10µH. Its value is determined by 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 = 120mA (40% of 300mA). V 1 ∆IL = VOUT 1 − OUT ( f)(L) VIN (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 360mA rated inductor should be enough for most applications (300mA + 60mA). 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 LTC3405A series parts require to operate. Table 1 shows some typical surface mount inductors that work well in LTC3405A series parts applications. Table 1. Representative Surface Mount Inductors MANUFACTURER PART NUMBER Taiyo Yuden MAX DC VALUE CURRENT DCR HEIGHT LB2016T2R2M LB2012T2R2M LB2016T3R3M 2.2µH 2.2µH 3.3µH 315mA 240mA 280mA 0.13Ω 1.6mm 0.23Ω 1.25mm 0.2Ω 1.6mm Panasonic ELT5KT4R7M 4.7µH 950mA 0.2Ω 1.2mm Murata LQH32CN2R2M33 4.7µH 450mA 0.2Ω Taiyo Yuden LB2016T4R7M 4.7µH 210mA 0.25Ω 1.6mm Panasonic ELT5KT6R8M 6.8µH 760mA 0.3Ω 1.2mm Panasonic ELT5KT100M 10µH 680mA 0.36Ω 1.2mm Sumida CMD4D116R8MC 6.8µH 620mA 0.23Ω 1.2mm 2mm 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: 1/ 2 VOUT ( VIN − VOUT )] [ CIN required IRMS ≅ IOMAX VIN 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 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. sn3405a1518 3405a1518fs 8 LTC3405A-1.5/LTC3405A-1.8 U W U U APPLICATIO S I FOR ATIO 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 LTC3405A series’ 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. 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. 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 LTC3405A series parts 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 3. 1 VIN = 3.6V 0.1 POWER LOST (W) 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. 0.01 0.001 0.0001 0.1 VOUT = 1.8V VOUT = 1.5V 1 100 10 LOAD CURRENT (mA) 1000 3405A1518 F03 Figure 3. 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. sn3405a1518 3405a1518fs 9 LTC3405A-1.5/LTC3405A-1.8 U W U U APPLICATIO S I FOR ATIO 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 LTC3405A series parts do not dissipate much heat due to their high efficiency. But, in applications where they run at high ambient temperature with low supply voltage, 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 keep the LTC3405A series parts 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. The junction temperature, TJ, is given by: TJ = TA + TR where TA is the ambient temperature. As an example, consider the LTC3405A-1.8 with an input voltage of 2.7V, a load current of 300mA 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 0.94Ω and the RDS(ON) of the N-channel synchronous switch is approximately 0.75Ω. The series resistance looking into the SW pin is: RSW = 0.95Ω (0.67) + 0.75Ω (0.33) = 0.88Ω Therefore, power dissipated by the part is: PD = ILOAD2 • RSW = 79.2mW For the SOT-23 package, the θJA is 250°C/ W. Thus, the junction temperature of the regulator is: TJ = 70°C + (0.0792)(250) = 89.8°C which is well 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 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. sn3405a1518 3405a1518fs 10 LTC3405A-1.5/LTC3405A-1.8 U W U U APPLICATIO S I FOR ATIO PC Board Layout Checklist When laying out the printed circuit board, the following checklist should be used to ensure proper operation of the LTC3405A series parts. These items are also illustrated graphically in Figures 4 and 5. 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 (+) plate of CIN connect to VIN as closely as possible? This capacitor provides the AC current to the internal power MOSFETs. 3. Keep the (–) plates of CIN and COUT as close as possible. Design Example As a design example, assume the LTC3405A-1.8 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 0.25A but most of the time it will be in 1 RUN MODE standby mode, requiring only 2mA. Efficiency at both low and high load currents is important. Output voltage is 1.8V. With this information we can calculate L using equation (1), L= V 1 VOUT 1 − OUT ( f)(∆IL ) VIN (3) Substituting VOUT = 1.8V, VIN = 4.2V, ∆IL = 100mA and f = 1.5MHz in equation (3) gives: L= 1.8 V 1.8 V 1 − ≅ 6.8µH 1.5MHz(100mA) 4.2V For best efficiency choose a 300mA or greater inductor with less than 0.3Ω series resistance. CIN will require an RMS current rating of at least 0.125A ≅ ILOAD(MAX)/2 at temperature and COUT will require an ESR of less than 0.5Ω. In most cases, a tantalum capacitor will satisfy this requirement. Figure 6 shows the complete circuit along with its efficiency curve. VIA TO VIN 6 VIN VOUT LTC3405A-1.8 2 – GND VOUT PIN 1 5 COUT VOUT + 3 L1 SW VIN LTC3405A-1.8 4 L1 CIN SW VIN 3405A1518 F04 BOLD LINES INDICATE HIGH CURRENT PATHS COUT CIN GND 3405A1518 F05 Figure 4. LTC3405A-1.8 Layout Diagram Figure 5. LTC3405A-1.8 Suggested Layout sn3405a1518 3405a1518fs 11 LTC3405A-1.5/LTC3405A-1.8 U W U U APPLICATIO S I FOR ATIO VIN 2.7V TO 4.2V 4 CIN** 4.7µF CER VIN 3 SW 6.8µH* VOUT 1.8V LTC3405A-1.8 1 6 COUT** 4.7µF CER RUN MODE 5 VOUT GND 2 *SUMIDA CMD4D11-6R8MC ** TAIYO YUDEN JMK212BJ475MG 3405A1518 F06a Figure 6a. 100 VIN = 2.7V 90 VIN = 4.2V EFFICIENCY (%) 60 VIN = 3.6V 70 60 50 40 30 0.1 1 10 100 OUTPUT CURRENT (mA) 1000 3405A1518 F06b Figure 6b. VOUT 100mV/DIV AC COUPLED IL 200mA/DIV IL 200mA/DIV VIN = 3.6V 20µs/DIV VOUT = 1.8V ILOAD = 100mA TO 300mA 3405A1518 F06c Figure 6c. sn3405a1518 3405a1518fs 12 LTC3405A-1.5/LTC3405A-1.8 U TYPICAL APPLICATIO S Single Li-Ion to 1.5V/300mA Regulator for High Efficiency and Small Footprint VIN 2.7V TO 4.2V 4 CIN** 4.7µF CER VIN 3 SW 4.7µH* VOUT 1.5V LTC3405A-1.5 1 6 COUT1** 4.7µF CER RUN MODE 5 VOUT GND 3405A1518 TA02a 2 *MURATA LQH32CN4R7M33 **TAIYO YUDEN CERAMIC JMK212BJ475MG 100 VIN = 2.7V 90 VIN = 4.2V VIN = 3.6V EFFICIENCY (%) 60 70 60 50 40 30 0.1 1 10 100 OUTPUT CURRENT (mA) 1000 3405A1518 TA02b VOUT 100mV/DIV AC COUPLED IL 200mA/DIV IL 200mA/DIV VIN = 3.6V 20µs/DIV VOUT = 1.5V ILOAD = 100mA TO 300mA 3405A1518 TA02c sn3405a1518 3405a1518fs 13 LTC3405A-1.5/LTC3405A-1.8 U TYPICAL APPLICATIO S Single Li-Ion to 1.5V/150mA Regulator Using All Ceramic Capacitors Optimized for Smallest Footprint VIN 2.7V TO 4.2V 4 CIN** 2.2µF CER VIN SW 3 2.2µH* LTC3405A-1.5 1 6 COUT1** 2.2µF CER RUN MODE VOUT GND VOUT 1.5V 5 3405A1518 TA03a 2 *TAIYO YUDEN LB2012T2R2M **TAIYO YUDEN CERAMIC LMK212BJ225MG 90 VOUT = 1.5V VIN = 2.7V EFFICIENCY (%) 80 VIN = 3.6V 70 VIN = 4.2V 60 50 40 30 0.1 1 100 10 OUTPUT CURRENT (mA) 1000 3405A1518 TA03b VOUT 100mV/DIV AC COUPLED IL 200mA/DIV IL 100mA/DIV VIN = 3.6V 20µs/DIV VOUT = 1.5V ILOAD = 50mA TO 150mA 3405A1518 TA03c sn3405a1518 3405a1518fs 14 LTC3405A-1.5/LTC3405A-1.8 U PACKAGE DESCRIPTIO S6 Package 6-Lead Plastic TSOT-23 (Reference LTC DWG # 05-08-1636) 0.62 MAX 2.90 BSC (NOTE 4) 0.95 REF 1.22 REF 3.85 MAX 2.62 REF 1.4 MIN 2.80 BSC 1.50 – 1.75 (NOTE 4) PIN ONE ID RECOMMENDED SOLDER PAD LAYOUT PER IPC CALCULATOR 0.30 – 0.45 6 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) 1.90 BSC S6 TSOT-23 0302 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 sn3405a1518 3405a1518fs 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 LTC3405A-1.5/LTC3405A-1.8 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1616 500mA (IOUT), 1.4MHz, High Efficiency Step-Down DC/DC Converter 90% Efficiency, VIN = 3.6V to 25V, VOUT = 1.25V, IQ = 1.9mA ISD = <1µA, ThinSOT Package LT1676 450mA (IOUT), 100kHz, High Efficiency Step-Down DC/DC Converter 90% Efficiency, VIN = 7.4V to 60V, VOUT = 1.24V, IQ = 3.2mA ISD = 2.5µA, S8 Package LT1765 25V, 2.75A (IOUT), 1.25MHz, High Efficiency Step-Down DC/DC Converter 90% Efficiency, VIN = 3.0V to 25V, VOUT = 1.20V, IQ = 1mA ISD = 15µA, S8, TSSOP16E Packages LT1776 500mA (IOUT), 200kHz, High Efficiency Step-Down DC/DC Converter 90% Efficiency, VIN = 7.4V to 40V, VOUT = 1.24V, IQ = 3.2mA ISD = 30µA, N8,S8 Packages LTC1878 600mA (IOUT), 550kHz, Synchronous Step-Down DC/DC Converter 95% Efficiency, VIN = 2.7V to 6V, VOUT = 0.8V, IQ = 10µA ISD = <1µA, MS8 Package LTC1879 1.20A (IOUT), 550kHz, Synchronous Step-Down DC/DC Converter 95% Efficiency, VIN = 2.7V to 10V, VOUT = 0.8V, IQ = 15µA ISD = <1µA, TSSOP16 Package LTC3404 600mA (IOUT), 1.4MHz, Synchronous Step-Down DC/DC Converter 95% Efficiency, VIN = 2.7V to 6V, VOUT = 0.8V, IQ = 10µA ISD = <1µA, MS8 Package LTC3405/LTC3405A 300mA (IOUT), 1.5MHz, Synchronous Step-Down DC/DC Converter 95% Efficiency, VIN = 2.7V to 6V, VOUT = 0.8V, IQ = 20µA ISD = <1µA, ThinSOT Package LTC3406/LTC3406B 600mA (IOUT) 1.5MHz, Synchronous Step-Down DC/DC Converter 95% Efficiency, VIN = 2.5V to 5.5V, VOUT = 0.6V, IQ = 20µA ISD = <1µA, ThinSOT Package LTC3411 1.25A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 95% Efficiency, VIN = 2.5V to 5.5V, VOUT = 0.8V, IQ = 60µA ISD = <1µA, MS10 Package LTC3412 2.5A (IOUT), 4MHz, Synchronous Step-Down DC/DC Converter 95% Efficiency, VIN = 2.5V to 5.5V, VOUT = 0.8V, IQ = 60µA ISD = <1µA, TSSOP16E Package LTC3413 3A (IOUT), Sink/Source, 2MHz, Monolithic Synchronous Regulator for DDR/QDR Memory Termination 90% Efficiency, VIN = 2.25V to 5.5V, VOUT = VREF/2, IQ = 280µA ISD = <1µA, TSSOP16E Package LT3430 60V, 2.75A (IOUT), 200kHz, High Efficiency Step-Down DC/DC Converter 90% Efficiency, VIN = 5.5V to 60V, VOUT = 1.20V, IQ = 2.5mA ISD = 25µA, TSSOP16E Package LTC3440 600mA (IOUT), 2MHz, Synchronous Buck-Boost DC/DC Converter 95% Efficiency, VIN = 2.5V to 5.5V, VOUT = 2.5V, IQ = 25µA ISD = <1µA, MS Package sn3405a1518 3405a1518fs 16 Linear Technology Corporation LT/TP 1102 2K • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com LINEAR TECHNOLOGY CORPORATION 2002