August 15, 1997

Single resistor improves V² converter

Dimitry Goder, Switch Power Inc, San Jose, CA

Step-down switching regulators commonly provide myriad digital-circuit voltages that range from traditional logic voltages of 5 and 3.3V to processor-core voltages of 1.8V and below. The main advantage of these regulators is high efficiency. These switching regulators must be able to quickly react to abrupt changes in load current. Compared with a variety of control architectures, V² control offers a significant improvement in transient response by using two voltage feedbacks. A conventional feedback path ensures output-voltage dc accuracy, and an additional fast feedback path delivers output-voltage ripple right to the PWM-comparator input, thereby bypassing the error amplifier. Thus, under transient conditions, the ramp signal changes, and the error amplifier's output remains constant. Also, the amplifier's compensation has no effect on the loop-response time.

A disadvantage of this approach is that the circuit relies on the existence of the output ripple. If the switching regulator needs to provide a well-filtered output ripple (less than 7 mV), the operation could become unstable. You can solve this problem by adding a small artificial ramp to the output ripple. If the converter operates with low output ripple or in a high-noise environment, this artificial ramp can significantly improve circuit performance and simplify board layout.

A typical application uses a V² controller, IC₁, in a synchronous-buck-regulator topology to convert 5V into 3.3V at 10A (Figure 1). An operating frequency of 500 kHz and state-of-the-art components minimize pc-board area. Note that the 0.33-µF compensation capacitor from the COMP pin of IC₁ to ground is relatively large, which is common only for the V² control architecture. This capacitor provides good output dc accuracy and noise immunity. The fast feedback path to IC₁'s Pin 8, which connects directly to the output voltage, provides the circuit with excellent transient response. A small RC filter (R₁×C₁=330 nsec) causes the only external delay in this path, but the delay is negligible compared with the switching frequency.

The artificial ramp requires only one resistor, R₂, between the switching node and the fast-feedback pin. R₂ and C₁ integrate the square-wave pulses present at the switching node; note that C₁ is the same capacitor that filters noise spikes at the fast-feedback pin. This approach superimposes a quasisawtooth signal on the output voltage. You can adjust the magnitude of the artificial ramp by varying R₂. (DI #2069)

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