Datasheet
t
SS
=
C
CDHC
x 2.25V
120 PA
www.ti.com
PC Board Layout
C
VCC
: The capacitor on the VCC pin provides noise filtering and stabilizes the LDO regulator. It also
prevents false triggering of the VCC UVLO. C
VCC
is recommended to be a 1 µF good quality and low ESR
ceramic capacitor.
C
CDHC
: The capacitor at the CDHC pin mainly determines the soft-start time t
SS
, i.e. the time for the output
voltage to reach its maximum. t
SS
is determined from the following equation:
(7)
In this example, C
CDHC
is recommended to be a 0.47 µF good quality and low ESR ceramic capacitor.
R
RT
and R
IREF
: The resistors R
RT
and R
IREF
set the switching frequency f
SW
of the boost converter and the
LED current I
LED
respectively. From the LM3492HC datasheet, R
RT
is selected to be 274 kΩ if f
SW
is 500
kHz (Figure 3), and R
IREF
is selected to be 8.25 kΩ if I
LED
is 150 mA (Figure 4 ).
R
COMM
: Since the COMM pin is open drain, a resistor R
COMM
of 52.3 kΩ is used to connect the VCC and
COMM pins to implement a pull-up function.
4 PC Board Layout
The layout of the printed circuit board is critical to optimize the performance of the LM3492HC application
circuit. In general, external components should be placed as close to the LM3492HC and each other as
possible in order to make copper traces short and direct. In particular, components of the boost converter
C
IN
, L
1
, D
1
, C
OUT
, and the LM3492HC should be closed. Also, the output feedback capacitor C
FB1
should be
closed to the output capacitor C
OUT
. The ground plane connecting the GND, PGND, and LGND pins and
the exposed pad of the LM3492HC and the ground connection of the C
IN
and C
OUT
should be placed on
the same copper layer.
Good heat dissipation helps optimize the performance of the LM3492HC. The ground plane should be
used to connect the exposed pad of the LM3492HC, which is internally connected to the LM3492HC die
substrate. The area of the ground plane should be extended as much as possible on the same copper
layer around the LM3492HC. Using numerous vias beneath the exposed pad to dissipate heat of the
LM3492HC to another copper layer is also a good practice.
5
SNVA639A–May 2012–Revised April 2013 AN-2231 LM3492HC Evaluation Board Reference Design
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