SC427MLTRT【PDF 28/33 页】IC REG DL BUCK/LINEAR 32MLPQ

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Sheet目录46

24页 25页 26页 27页 [28页]29页 30页 31页 32页

SC417/SC427

Applications Information (continued)

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 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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