EDN Access -- 04.13.95 8A switcher powers high-performance µP

-April 13, 1995

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Design Ideas:April 13, 1995

8A switcher powers high-performance µPs

Chester Simpson,
National Semiconductor, Santa Clara, CA, (408) 721-7501.

The circuit in Fig 1 delivers 8A for driving as many as two of today's high-performance, high-speed µPs. You can also optimize the design for lower current by changing a few components. Most systems use a low-current 12V bias supply and a poorly regulated high-current 5V to power the logic. However, the µP requires 3.3V for its main power voltage. Thus, an efficient dc/dc converter that can convert 5 to 3.3V and provide a tightly regulated output voltage is necessary. The primary design objectives for this circuit were minimizing cost, eliminating the need for heat sinks, and providing short-circuit protection to maximize reliability.

A step-down (buck) regulator that operates at 80 kHz provides the 3.3V/8A output. IC1 drives the main switching transistor, the N-channel Q3, through the Q1 and Q2 gate-drive transistors. Active drive for the gate of Q3 is necessary to produce very fast switching transitions, which minimize switching losses.

The open-collector output of IC1's PWM chip controls Q1 and Q2. When pin 6 pulls low, Q1 turns on through R9, and Q2 turns off, because D1 holds Q2's base high. This action pulls Q3's FET switch up to 12V, which turns on Q3. When pin 6 of IC1 is high, R7 turns on Q2, and R8 turns off Q1. In this state, the circuit pulls Q3's gate to ground, which turns it off. Q3, catch diode D2, L1, C12, and C13 provide a dc output capable of sourcing 8A of load current.

Note that you can use an 8A diode for D2 with only a slight increase in power dissipation in this device. Also, you can use a single 4700-µF output capacitor at the regulator output if output ripple and transient response are not critical.

IC2, a precision reference whose setpoint voltage is 3.3Vñ0.5%, provides feedback and voltage control. When the regulator output reaches 3.3V, IC2 sources current through R2 to ground, providing feedback to the input of IC1 and holding the regulator output at 3.3V.

IC1 and sense resistor R5 easily implement overcurrent protection. When the voltage drop across R5 exceeds 110 mV, IC1 activates its built-in pulse-by-pulse control, which limits the output current. R5 consists of a piece of AWG 17 resistance wire (type MWS-294: MWS Industries, West Lake, CA, (818) 991-8553) with an effective length of 1.5 in. You can also build R5 using the same length and gauge of manganin wire.

C16 must sit very near the drain of Q3, and C16's equivalent series resistance (ESR) is critical. Do not substitute a capacitor with higher ESR. The same warning applies to C12 and C13 Take care to check ESR specs if you use a substitute.

R1, R3, C3, and C6 set the loop compensation of IC1; C7 sets the oscillator frequency. C7 should be a good quality ceramic capacitor. C1, C2, C4, and C8 are necessary for input bypassing, and they reduce the amount of switching noise the circuit conducts onto the input lines. A snubber consisting of R10 and C10 is necessary to eliminate high-frequency ringing when Q3 switches off. These components must be located as close as possible to Q3. R13 and C17 perform the same function for D2.

Bench testing indicates the following circuit performance. For VIN5V, efficiency was 88% for a load current of 4A and 84% for a load current of 8.2A. For VIN5V and ILOAD8.2A, noise was 15-mV p-p and 5.5-mV rms using an HP34401A meter. The switcher stayed in regulation with no changes in the output voltage (1-mV measurement accuracy) when the load varied from 0.1 to 8A and when the input voltage varied from 4.5 to 5.5V. With no heat sinks or airflow, the temperature of Q3 was 70oC and of D2 was 50oC, when operating at 8.2A continuous load current.

All high-current switchers are sensitive to board layout; take care when designing the pc board. You'll obtain the best results by using a multilayer board, so you can dedicate a single layer to ground plane. You should tie the "power" components (C1, C2, C16, D2, C12, C13, C14) directly to this ground plane and group these components so that the connection paths are as short as possible. You should connect the low-current ground, as Fig 1 shows, and then tie it to the power ground at a single point near C13. Finally, all traces in the switching power path (R5, Q3, and L1) must be as short as possible. (DI #1688)

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