SC410AEVB datasheet(21/26 Pages) SEMTECH

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SC410A

Applications Information (continued)

System DC Accuracy — V

OUT Controller

Three factors affect V

OUT accuracy: the trip point of the FB

error comparator, the ripple voltage variation with line

and load, and the external resistor tolerance. The error

comparator offset is trimmed so that under static condi-

tions it trips when the feedback pin is 750mV, +1%.

The on-time pulse from the SC410A in the design example

is calculated to give a pseudo-fixed frequency of 500kHz.

Some frequency variation with line and load is expected.

This variation changes the output ripple voltage. Because

adaptive on-time converters regulate to the valley of the

output ripple, ½ of the output ripple appears as a DC regu-

lation error.

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 may result in up to an

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

resistors 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 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

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

tially 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

increases slightly with increasing load.

SC410AEVB Datasheet(PDF) 21 Page - Semtech Corporation

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