SC417 datasheet(24/29 Pages) SEMTECH

24 / 29 page

SC417/SC427

Applications Information (continued)

250kHz. Some frequency variation with line and load is

expected. This variation changes the output ripple

voltage. Because constant on-time converters regulate to

the valley of the output ripple, ½ of the output ripple

appears as a DC regulation error. For example, if the

output ripple is 50mV with V

= 6 volts, then the measured

DC output will be 25mV above the comparator trip point.

If the ripple increases to 80mV with V

= 25V, then the

measured DC output will be 40mV above the comparator

trip. The best way to minimize this effect is to minimize

the output ripple.

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 capaci-

tor. 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 capaci-

tance 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

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 induc-

tor. For a conventional PWM constant-frequency con-

verter, 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 V

as losses increase). The on-time is essen-

tially constant for a given V

OUT

combination, to offset

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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