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Document Number: 66550

S11-1638-Rev. B, 15-Aug-11

Vishay Siliconix

SiC403

This document is subject to change without notice.

THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000

To compensate for valley regulation, it may be desirable to

use passive droop. Take the feedback directly from the

output side of the inductor and place a small amount of trace

resistance between the inductor and output capacitor.

This trace resistance should be optimized so that at full load

the output droops to near the lower regulation limit. Passive

droop minimizes the required output capacitance because

the voltage excursions due to load steps are reduced as

seen at the load.

The use of 1 % feedback resistors contributes up to 1 %

error. If tighter DC accuracy is required, 0.1 % resistors

should be used.

The output inductor value may change with current. This will

change the output ripple and therefore will have a minor

effect on the DC output voltage. The output ESR also affects

the output ripple and thus has a minor effect on the DC

output voltage.

Switching Frequency Variations

The switching frequency will vary depending on line and load

conditions. The line variations are a result of fixed

propagation delays in the on-time one-shot, as well as

unavoidable delays in the external MOSFET switching. As

VIN increases, these factors make the actual DH on-time

slightly longer than the ideal on-time. The net effect is that

frequency tends to falls slightly with increasing input voltage.

The switching frequency also varies with load current as a

result of the power losses in the MOSFETs and the inductor.

For a conventional PWM constant-frequency converter, as

load increases the duty cycle also increases slightly to

compensate for IR and switching losses in the MOSFETs

and inductor.

A constant on-time converter must also compensate for the

same losses by increasing the effective duty cycle (more

time is spent drawing energy from VIN as losses increase).

The on-time is essentially constant for a given VOUT/VIN

combination, to off set the losses the off-time will tend to

reduce slightly as load increases. The net effect is that

switching frequency increases slightly with increasing load.

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