SC403BEVB View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC403BEVB Datasheet PDF : 32 Pages
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Applications Information (continued)
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 upon line
and load conditions. The line variation is a result of fixed
propagation delays in the on-time one-shot, as well as
unavoidable delays in the external MOSFET switching. As
VIN 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 VIN as losses increase). The on-time is essen-
tially constant for a given VOUT/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.
SC403B
27
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