SC174 datasheet(19/27 Pages) SEMTECH

19 / 27 page

Applications Information (continued)

The use of 1% feedback resistors may result in up to an

additional 1% error. If tighter DC accuracy is required, re-

sistors with lower tolerances 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 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

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 adaptive on-time converter

must also compensate for the same losses by increasing

the effective duty cycle (more time is spent drawing en-

ergy from V

IN as losses increase). The on-time is essential-

ly constant for a given V

OUT and VIN combination, to offset

the losses the off-time will tend to reduce slightly as load

increases. The net effect is that switching frequency in-

creases slightly with increasing load.

SC174

SC174 Arkusz danych(PDF) 19 Page - Semtech Corporation