DESIGN FEATURES Dual DC/DC Controller Brings 2-Phase Benefits to Low Input Voltage by Jason Leonard Applications Introduction The LTC3701 is an efficient, low input voltage, dual DC/DC controller that fits into the tight spaces required by the latest portable electronics. It uses 2-phase switching techniques to reduce required input capacitance (saving space and cost) and increase efficiency. The versatile LTC3701 accepts a wide range of input voltages, from 2.5V to 9.8V, making it useful for single lithium-ion cell and many multicell systems. It can provide output voltages as low as 0.8V and output currents as high as 5A. The 100% duty cycle allows low dropout for maximum energy extraction from a battery, and the optional Burst Mode operation enhances efficiency at low load currents. It also includes other popular features, such as a Power Good voltage monitor, a phase-locked loop, and an internal soft start. Its small 16-lead narrow SSOP package and relatively high operating frequency (300kHz–750kHz) allow the use of small, surface mount components, making for a compact overall power supply solution. Operation Figure 1 shows the LTC3701 used in a step-down converter with an input 3 2 4 6 80.6k 10k 220pF 5 7 8 100k D1, D2: IR10BQ015 SENSE1 – SENSE1 + VFB1 VIN 15 4.7µH 0.03Ω 14 ITH/RUN1 PGATE1 13 LTC3701 SGND PGND 12 ITH/RUN2 PGATE2 11 PGOOD VFB2 10 PLLLPF EXTCLK/MODE 9 – + SENSE2 SENSE2 L1, L2: LQN6C-4R7 M1 SW1 16 + D1 47µF VIN 2.5V TO 9.8V VOUT1 2.5V 2A 10µF GND D2 47µF + 220pF The LTC3701 offers the benefits of 2phase operation, which include lower input filtering requirements, reduced electromagnetic interference (EMI) and increased efficiency. In a single phase dual switching regulator, both top-side P-channel MOSFETS are turned on at the same time, causing current pulses of up to twice the amplitude of those from a single regulator to be drawn from the input capacitor. These large amplitude pulses increase the total RMS current flowing into the input capacitor, requiring the use of more expensive input capacitors, and increasing both EMI and losses in the input capacitor and input power supply. With 2-phase operation, the two channels of the LTC3701 are operated 180 degrees out of phase. This effectively interleaves the current During normal operation, each external P-channel power MOSFET is turned on every cycle when the oscillator for that controller sets a latch and turned off when the current comparator resets the latch. The peak inductor current at which the current comparator resets the latch is controlled by the voltage on the ITH/RUN pin, which is the output of the error amplifier. The VFB pin receives the output voltage feedback signal, which is compared to the internal 0.8V ref- 1 10k 2-Phase Operation The LTC3701 offers the benefits of 2-phase operation, which include lower input filtering requirements, reduced electromagnetic interference (EMI) and increased efficiency. 169k 78.7k erence by the error amplifier. When the load current increases, it causes a slight decrease in VFB relative to the reference, which, in turn, causes the ITH/RUN voltage to increase until the average inductor current matches the load current. of from 2.5V to 9.8V and two outputs of 2.5V at 2A and 1.8V at 2A. Figure 2 shows its efficiency versus load current. The LTC3701 uses a constant frequency, current mode architecture with the two controllers operating 180 degrees out of phase. 4.7µH 0.03Ω M1, M2: FDC638P M2 SW2 VOUT2 1.8V 2A 3701 F01a Figure 1. 2-phase step-down converter with an input of 2.5V to 9.8V and two outputs: 2.5V at 2A and 1.8V at 2A 12 Linear Technology Magazine • May 2002 DESIGN FEATURES 100 2.0 VIN = 3.3V SW1 5V/DIV 90 EFFICIENCY (%) 1.8 INPUT CAPACITOR RMS CURRENT VIN = 4.2V 80 SW2 5V/DIV VIN = 6V VIN = 8.4V 70 INPUT CURRENT 1A/DIV 60 50 1 10 100 1000 LOAD CURRENT (mA) 2-PHASE DUAL CONTROLER 1.0 0.8 0.6 0.4 VOUT1 = 2.5V/2A VOUT2 = 1.8V/2A 0 10000 Figure 3. These input waveforms for the circuit in Figure 1 show how 2phase operation reduces ripple. Reduced ripple translates directly to less expensive input capacitors, reduced shielding requirements for EMI, and improved efficiency. 3701 F01b Figure 2. Efficiency vs load current pulses coming from the switches, greatly reducing the amount of time where they overlap and add together. The dead bands in the input current waveform are “filled up,” so to speak. The result is a significant reduction in the total RMS input current, which in turn allows for the use of less expensive input capacitors, reduces shielding requirements for EMI, and improves efficiency. Figure 3 shows the input waveforms for the circuit in Figure 1. The RMS input current is significantly reduced by the interleaving current pulses. Of course, the improvement afforded by 2-phase operation is a function of the dual switching regulator’s relative duty cycles, which are dependent on the input voltage VIN. Figure 4 shows how the RMS input current varies for single-phase and 2-phase operation for 2.5V and 1.8V regulators over a wide input voltage range. 1.4 1.2 0.2 VOUT = 2.5V 40 SINGLE PHASE DUAL CONTROLER 1.6 tor. When the ITH/RUN voltage goes above 0.925V, the sleep signal goes low and normal operation resumes. For frequency sensitive applications, Burst Mode operation can be inhibited by connecting the EXTCLK/ MODE pin to ground. In this case, constant frequency operation is maintained at a lower load current with a lower output voltage ripple. If the load current is low enough, cycle skipping occurs to maintain regulation. Frequency Selection/Synchronization (Phase-Locked Loop) The LTC3701 operates at a constant frequency between 300kHz and 2 3 4 8 6 5 7 INPUT VOLTAGE (V) 9 10 3701 F04 Figure 4. RMS input current comparison 750kHz. The frequency can be selected by forcing a voltage at the PLLLPF pin. Grounding the PLLLPF pin selects 300kHz, while tying it to VIN or a voltage greater than 2V selects 750kHz. Floating the PLLLPF pin selects 550kHz operation. The LTC3701 can also be synchronized to an external clock source (300kHz to 750kHz) using the LTC3701’s true phase-locked loop. The clock signal is applied to the EXTCLK/MODE pin and an RC filter is connected between the PLLLPF pin and ground. Burst Mode operation is disabled when synchronized to an external clock. Run/Soft Start Either controller can be shutdown by pulling its respective ITH/RUN pin below 0.35V, which turns off most circuits associated with that control- Burst Mode Operation The LTC3701 can be enabled to enter Burst Mode operation at low load currents by connecting the EXTCLK/ MODE pin to VIN. In this mode, the minimum peak current is set as if VITH/RUN = 1V, even though the voltage at the ITH/RUN pin is at a lower value. If the inductor’s average current is greater than the load requirement, the voltage at the ITH/ RUN pin will drop as VOUT rises slightly. When the ITH/RUN voltage goes below 0.85V, a sleep signal is generated, turning off the external MOSFET and much of the LTC3701’s internal circuitry. The load current is then supported by the output capaciLinear Technology Magazine • May 2002 VIN 2.5V TO 9.8V R7 169k 1 R6 78.7k 2 3 4 5 R5 10k C4 220pF 6 7 8 SENSE1 – SENSE1 + ITH/RUN1 VIN VFB1 SGND PGATE1 LTC3701 PGND VFB2 PGATE2 ITH/RUN2 PGOOD PLLLPF EXTCLK/MODE SENSE2 – C1: C2: D1, D2: L1, L2: M1, M2: R1, R2: 16 15 14 M1 R1 0.03Ω L1 4.7µH D1 + 13 VOUT 2.5V 4A C1 47µF C2 10µF 12 D2 11 10 9 SENSE2 + L2 4.7µH R2 0.03Ω SANYO 6TPA47M TAIYO YUDEN LMK325BJ106K-T INTERNATIONAL RECTIFIER IR10BQ015 MURATA LQN6C-4R7 SILICONIX Si3443DV DALE 0.25W M2 3701 TA03 (619) 661-6835 (408) 573-4150 (310) 322-3331 (814) 237-1431 (800) 554-5565 (605) 665-9301 Figure 5. 2.5V–9.8V to 2.5V/4A 2-phase step-down converter operating at 550kHz 13 DESIGN FEATURES VIN 2.7V to 4.2V R7 249k R8 80.6k R9 100k 1 3 R10 C6 470pF 47k SENSE1 – SENSE1 + VFB1 10k 15 14 2 R5 10k C4 220pF VIN 16 ITH/RUN1 PGATE1 13 4 LTC3701 SGND PGND 12 6 ITH/RUN2 PGATE2 11 5 PGOOD VFB2 10 7 PLLLPF EXTCLK/MODE 9 8 SENSE2 + SENSE2 – • • L1A D1 GND 100k D2 VIN L2 4.7µH 550kHz R2 0.03Ω 10nF L1B M1 R1 0.025Ω + R6 78.7k C3 10µF VOUT1 3.3V 1A + C1 47µF C2 22µF C5 47µF VOUT2 1.8V 2A M2 3701 TA06 C1, C5: SANYO 6TPA47M C2: TAIYO YUDEN JMK325BJ226MM C3: TAIYO YUDEN JMK316BJ106ML D1, D2: INTERNATIONAL RECTIFIER IR10BQ015 L1A, L1B: COILTRONICS CTX5-2 L2: MURATA LQN6C-4R7 M1, M2: SILICONIX Si3443DV R1, R2: DALE 0.25W (619) 661-6835 (408) 573-4150 (310) 322-3331 (561) 752-5000 (814) 237-1431 (800) 554-5565 (605) 665-9301 Figure 6. Single cell Li-Ion to 3.3V/1A and 1.8V/2A DC/DC converter ler and holds its external MOSFET off. If both ITH/RUN pins are pulled low, the LTC3701 is shutdown and draws only 9µA. The LTC3701 has separate internal soft start functions that allow each output to power up gently. The maximum allowed inductor current is stepped up from 0 to 120mV/RSENSE in four equal steps of 30mV/ RSENSE, with each step lasting 512 clock cycles (just under 1ms per step at 550kHz). Power Good Output Voltage Monitor A window comparator monitors both output voltages and the open-drain PGOOD output is pulled low when the divided down output voltages are not within ±8% of the reference voltage of 0.8V. 2-Phase 2.5V/2A and 1.8V/ 2A Step-Down Regulator Figure 1 shows a typical application of the LTC3701. This circuit supplies a 2A load at 2.5V and a 2A load at 1.8V with an input supply from 2.5V to 9.8V. Due to the reduced input current ripple associated with 2-phase operation, only a single 10µF ceramic 14 input capacitor is required. The 0.03Ω sense resistors ensure that both outputs are capable of supplying 2A with a low input voltage. The circuit operates at the internally set frequency of 550KHz. 4.7µH inductors are chosen so that the inductor currents remain continuous during burst periods at low load current. 2-Phase Single Output 2.5V/ 4A Step-Down Regulator In addition to dual output applications, the LTC3701 can also be used in a single output configuration to take advantage of the benefits of 2phase operation, as shown in Figure 5. This circuit provides a 2.5V output with up to 4A of load current. In this case, 2-phase operation reduces both the input and output current ripple, in turn reducing the required input and output capacitances. Single Cell Li-Ion to 3.3V/1A (Zeta Converter) and 1.8V/2A In addition to step-down applications, the LTC3701 can also be used in a zeta converter configuration that will do both step-down and step-up conversions, as shown in Figure 6. This circuit delivers 1A at 3.3V (zeta converter) and 2A at 1.8V (step-down converter) from an input of 2.7V to 4.2V (Li-Ion voltage range). The circuit takes advantage of the LTC3701’s true phase-locked loop by synchronizing to an external clock source. Conclusion The LTC3701 brings the benefits of 2phase operation to low-voltage dual power supply systems. It offers flexibility, high efficiency, and many other popular features in a small 16-pin narrow SSOP package. for the latest information on LTC products, visit www.linear.com Linear Technology Magazine • May 2002