SC417/SC427
28
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
V
IN
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
IN
as losses increase). The on-time is essen-
tially constant for a given V
OUT
/V
IN
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.