SC461 View Datasheet(PDF) - Semtech Corporation
Part Name
Description
MFG CO.
'SC461' PDF : 31 Pages View PDF
SC461
Applications Information (continued)
minimum off-time of the one-shot. When working with
low input voltages, the duty-factor limit must be calcu-
lated using worst-case values for on and off times.
The duty-factor limitation is shown by the following
equation.
DUTY
TON(MIN)
T T ON(MIN)
OFF(MAX)
The inductor resistance and MOSFET on-state voltage
drops must be included when performing worst-case
dropout duty-factor calculations.
System DC Accuracy (VOUT Controller)
Three factors affect VOUT accuracy: the trip point of the FB
error comparator, the ripple voltage variation with line
and load, and the external resistor tolerance. The error
comparator offset is trimmed so that under static condi-
tions it trips when the feedback pin is 600mV, + 1%.
The on-time pulse from the SC461 in the design example
is calculated to give a pseudo-fixed frequency of 220kHz.
Some frequency variation with line and load is expected.
This variation changes the output ripple voltage. Because
adaptive on-time converters regulate to the valley of the
output ripple, ½ of the output ripple appears as a DC regu-
lation error. For example, if the output ripple is 50mV with
VIN = 6 volts, then the measured DC output will be 25mV
above the comparator trip point. If the ripple increases to
80mV with VIN = 25V, then the measured DC output will be
40mV above the comparator trip. The best way to mini-
mize 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.
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
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. An adaptive 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). Because the on-time is
essentially constant for a given VOUT/VIN combination, to
offset the losses the off-time will reduce slightly as load
increases. The net effect is that switching frequency
increases slightly with increasing load.
PCB Layout Guidelines
A switch-mode converter requires good PCB layout which
is essential to achieving high performance. The following
guidelines will provide an optimum PCB layout.
The device layout recommendations consist of four parts.
• Grounding for PGND and AGND
• Power components
• Low-noise analog circuits
• Bypass capacitors
Grounding for PGND and AGND
• A ground plane layer for PGND is recommended
to minimize the effects of switching noise, resis-
tive losses, and to maximize heat removal from
the power components.
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