Datasheet

t
SS
=
8 PA
C
SS
x 0.8V
LMR24220
www.ti.com SNVS737E –OCTOBER 2011–REVISED APRIL 2013
C
SS
: The capacitor at the SS pin determines the soft-start time, i.e. the time for the reference voltage at the
regulation comparator and the output voltage to reach their final value. The time is determined from the following
equation:
(13)
C
FB
: If the output voltage is higher than 1.6V, C
FB
is needed in the Discontinuous Conduction Mode to reduce the
output ripple. The recommended value for C
FB
is 10 nF.
PC BOARD LAYOUT
The LMR24220 regulation, over-voltage, and current limit comparators are very fast and may respond to short
duration noise pulses. Layout is therefore critical for optimum performance. It must be as neat and compact as
possible, and all external components must be as close to their associated pins of the LMR24220 as possible.
Refer to , the loop formed by C
IN
, the main and synchronous MOSFET internal to the LMR24220, and the PGND
pin should be as small as possible. The connection from the PGND pin to C
IN
should be as short and direct as
possible. Vias should be added to connect the ground of C
IN
to a ground plane, located as close to the capacitor
as possible. The bootstrap capacitor C
BST
should be connected as close to the SW and BST pins as possible,
and the connecting traces should be thick. The feedback resistors and capacitor R
FB1
, R
FB2
, and C
FB
should be
close to the FB pin. A long trace running from V
OUT
to R
FB1
is generally acceptable since this is a low impedance
node. Ground R
FB2
directly to the AGND pin. The output capacitor C
OUT
should be connected close to the load
and tied directly to the ground plane. The inductor L should be connected close to the SW pin with as short a
trace as possible to reduce the potential for EMI (electromagnetic interference) generation. If it is expected that
the internal dissipation of the LMR24220 will produce excessive junction temperature during normal operation,
making good use of the PC board’s ground plane can help considerably to dissipate heat. Additionally the use of
thick traces, where possible, can help conduct heat away from the LMR24220. Judicious positioning of the PC
board within the end product, along with the use of any available air flow (forced or natural convection) can help
reduce the junction temperature.
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