SC3303 datasheet(21/26 Pages) SEMTECH

21 / 26 page

SC3303

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 Variation

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 power FET switching. As V

increases, these factors make the actual DH on-time

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

that frequency tends to fall 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. An adaptive 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

and V

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.

Applications Information (continued)

SC3303 Datasheet(PDF) 21 Page - Semtech Corporation