Dual power supply delivers 8A with no heat sinks
John Seago, Linear Technology Corp, Milpitas, CA -- EDN, March 30, 2000
The circuit in Figure 1 is a high-current dual supply that provides 5 and 3.3V at currents as high as 8A. The circuit uses a fixed-frequency, two-output, current-mode synchronous-buck- controller, IC1, to regulate both 5 and 3.3V outputs. The circuit uses separate regulator circuits for each output voltage. However, both circuits are identical except for the lower feedback resistors, R4 and R7, which determine the 5 and 3.3V output voltages, respectively.
IC1 regulates the 5V output, VOUT1, by controlling the duty cycle of the top MOSFET for VOUT1, Q1, so that average input voltage to the buck inductor, L1, is equal to the output voltage. The buck inductor and output capacitors C3 to C5-three 330-µF capacitors in parallel-integrate and filter the energy pulses from Q1 to generate the dc output. After Q1 turns off, the bottom MOSFET for VOUT1, Q2, turns on to conduct inductor current to the load. To avoid shoot-through current, a short dead time occurs before each MOSFET turns on. During this dead time, inductor current flows through the commutating diode, D1, to the load. Feedback resistors R3 and R4 connect IC1's internal error amplifier to the output. Loop-compensation components R1, C1, and C2 control the frequency response of the error amplifier. The internal current comparator senses inductor current by the voltage developed across the current-sense resistor, R2.
The 3.3 regulator, which produces VOUT2, functions exactly like the 5V regulator. Q3 and Q4 are the top and bottom MOSFETs, respectively, and L2 is the buck inductor. D2 is the commutating diode. Feedback resistors R7 and R8 connect the error amplifier to the output. R5, C6, and C7 are the loop-compensation components, and R6 is the sense resistor. C8, C9, and C10 make up the output capacitor.
The circuit in Figure 1 has some features that add versatility. The low-battery comparator in IC1 flags a low-input-voltage condition. Normally, the LBO pin is high but goes low when the input voltage is low. IC1 includes a complete power-on-reset circuit. At startup, the POR2 pin is low. This pin goes high 65,536 oscillator cycles after Channel 2's output voltage reaches 95% of its programmed value. The POR2 pin goes low if the output voltage falls 7.5% from nominal. Each output has a RUN/SS pin that provides output-voltage delay, output-current soft-start, and on/off control. The value of the capacitor connected to the RUN/SS pin determines the output voltage delay and the inductor-current ramp time, both at a rate of 0.5 sec/µF. Pulling a RUN/SS pin low turns off that output voltage. Pulling both RUN/SS pins low shuts down IC1, turns off all internal circuitry, and limits the input current to 16 µA. (DI #2494)
