SC417/SC427
Applications Information (continued)
250kHz. Some frequency variation with line and load is
expected. This variation changes the output ripple
voltage. Because constant on-time converters regulate to
the valley of the output ripple, ½ of the output ripple
appears as a DC regulation error. For example, if the
output ripple is 50mV with V = 6 volts, then the measured
IN
DC output will be 25mV above the comparator trip point.
If the ripple increases to 80mV with V = 25V, then the
IN
measured DC output will be 40mV above the comparator
trip. The best way to minimize this effect is to minimize
the output ripple.
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 contributes up to 1%
error. If tighter DC accuracy is required, 0.1% resistors
should be used.
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
V increases, these factors make the actual DH on-time
IN
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-
IN
tially constant for a given V /V combination, to offset
OUT IN
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.
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.
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