AN731

Vishay Siliconix

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4

Document Number: 71128

28-Jan-00

DYNAMIC RESPONSE PERFORMANCE

Figures 7 and 8 show the dynamic response of the output

response and settling time depend on the following factors:

1. the bandwidth of the converter control loop: the larger the

2. the large signal response slew rate of the converter: the

faster the slew rate, the faster the response

3. the slew rate of the converter control loop error amplifier

4. the bandwidth of the difference amplifier.

The bandwidth of the converter control loop is the most

important parameter. The maximum attainable bandwidth of a

converter control loop is 1/(2*pi) times the switching frequency.

For a converter switching at 100 kHz, the control bandwidth is

limited to 16 kHz. Vishay Siliconix high-frequency switching

regulators (including the Si9165, Si9169, and Si9170) offer

switching frequencies up to 2 MHz and have a theoretical

bandwidth limit of 318 kHz. Thus they would

be good

candidates for this kind of application. Figure 5 shows a

complete example of a dc-to-dc converter using the Si9165

controller IC and the op-amp circuit. The output voltage can be

programmed from 0.4 V to 3.4 V with a control voltage from

0.2 V to 2.7 V. Figure 6 shows a similar example using the

Si9166 controller.

The signal response slew rate of the converter dictates how

fast the converter can slew its output voltage up or down given

an infinitely large control loop bandwidth. The converter slew

rate depends on the converter output averaging LC filter. The

smaller the LC, the faster the slew rate.

With respect to the slew rate of the converter control loop error

amplifier, most of the converter control loop is figured as an

integrator. The error amplifier often drives a fairly large

integrator capacitor. The slew rate of the op-amp should be

sized adequately.

The bandwidth of the difference amplifier requires the

difference amplifier to have its unity gain bandwidth a decade

or more larger than the control loop bandwidth.

CONCLUSION

A simple op-amp circuit is used to program the output voltage

of a typical dc-to-dc converter. The circuit offers flexible voltage

scaling using a linear control voltage. The control voltage can

be hard-wired for fixed-voltage operation. When used together

with a wide bandwidth dc-to-dc converter, the circuit offers a

simple solution for controlling the output voltage dynamically.