Datasheet

t
SS
=
C
SS
x V
REF
I
SS
PVIN
AVIN
EN
AGND
PGND
FB
SW
LM2854
V
IN
SS
C
F
R
F
C
IN
'V
in
=
V
OUT
D(1-D)
f
SW
C
IN
D =
V
OUT
V
IN
I
Cin(RMS)
= I
OUT
D(1-D)
LM2854
SNVS560C –MARCH 2008–REVISED APRIL 2013 www.ti.com
Design Guidelines
INPUT FILTER CAPACITOR
Fast switching currents place a large strain on the input supply to a buck regulator. A capacitor placed close to
the PVIN and PGND pins of the LM2854 helps to supply the instantaneous charge required when the regulator
demands a pulse of current every switching cycle. In fact, the input capacitor conducts a square-wave current of
peak-to-peak amplitude equal to I
OUT
. With this high AC current present in the input capacitor, the RMS current
rating becomes an important parameter. The necessary RMS current rating of the input capacitor to a buck
regulator can be estimated by
(1)
where the PWM duty cycle, D, is given by
(2)
Neglecting capacitor ESR, the resultant input capacitor AC ripple voltage is a triangular waveform with peak-to-
peak amplitude specified as follows
(3)
The maximum input capacitor ripple voltage and RMS current occur at 50% duty cycle. A 22 µF or 47 µF high
quality dielectric (X5R, X7R) ceramic capacitor with adequate voltage rating is typically sufficient as an input
capacitor to the LM2854. The input capacitor should be placed as close as possible to the PVIN and PGND pins
to substantially eliminate the parasitic effects of any stray inductance or resistance on the PC board and supply
lines. Additional bulk capacitance with higher ESR may be required to damp any resonance effects of the input
capacitance and parasitic inductance.
AVIN FILTERING COMPONENTS
In addition to the large input filter capacitor, a smaller ceramic capacitor such as a 0.1 µF or 1.0 µF is
recommended between AVIN and AGND to filter high frequency noise present on the PVIN rail from the quiet
AVIN supply. For additional filtering in noisy environments, a small RC filter can be used on the AVIN pin as
shown below.
In general, R
F
is typically selected between 1Ω and 10Ω so that the steady state voltage drop across the resistor
due to the AVIN bias current does not affect the UVLO level. Recommended filter capacitor, C
F
, is 1.0 µF in X5R
or X7R dielectric.
SOFT-START CAPACITOR
When the LM2854 is enabled, the output voltage will ramp up linearly in the time dictated by the following
relationship
(4)
where V
REF
is the internal reference voltage (nominally 0.8V), I
SS
is the soft-start charging current (nominally 2
µA) and C
SS
is the external soft-start capacitance. Rearranging this equation allows for the necessary soft-start
capacitor for a given startup time to be calculated as follows
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