Performance
Efficiency vs. I
LOAD(VIN
= 5V)
20201802
0.5A to 3A Load Step Response (VIN= 5V, V
OUT
= 1.8V)
20201805
Horizontal Resolution: 200 µs/Div.
Trace 1: V
OUT
(100 mV/Div.)
Trace 2: I
LOAD
(1 A /Div.)
Component Selection
CINand C
BYP
The necessary RMS current rating of the input capacitor can
be estimated by the following equation:
where the variable D refers to the duty cycle, and can be
approximated by:
From this equation, it follows that the maximum I
RMS
will
occur at a full 3A load current with the system operating at
50% duty cycle. Under this condition, the maximum I
RMS
is
given by:
Ceramic capacitors feature a very large I
RMS
rating in a small
footprint, making a ceramic capacitor ideal for this application. A 47 µF ceramic capacitor from Murata with a 4.9A I
RMS
rating provides the necessary input capacitance for the
evaluation board. For improved load regulation and transient
performance, an extra 1 µF ceramic capacitor is placed near
to the AVIN pin from V
IN
to GND. This small capacitor helps
to filter high frequency noise pulses on the supply, and
prevent those pulses from disturbing the analog control circuitry of the chip.
C
SS
The soft-start capacitor has been chosen to provide a softstart time of roughly 3 ms. Using the internal soft-start resistance of 450 kΩ and the external soft-start capacitor value of
2.2 nF, the approximate soft-start time can be calculated as
follows:
T
SS
=3xCSSxRSS=
3 x 2.2 nF x 450 kΩ = 2.97 ms ≈ 3ms
A 3 ms soft-start time will allow the LM2853 to start up
gracefully without triggering over-current protection, regardless of the operating conditions.
AN-1513
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