SC174MLTRT View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
SC174MLTRT
Semtech Corporation
'SC174MLTRT' PDF : 27 Pages View PDF
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 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 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 VIN as losses increase). The on-time is essential-
ly constant for a given VOUT 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.
© 2010 Semtech Corporation
SC174
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