Datasheet

LTM8042/LTM8042-1
18
80421fa
APPLICATIONS INFORMATION
PCB Layout
Most of the headaches associated with PCB layout have
been alleviated or even eliminated by the high level of
integration of the LTM8042/LTM8042-1. The device is
nevertheless a switching power supply, and care must be
taken to minimize EMI and ensure proper operation. Even
with the high level of integration, you may fail to achieve
specified operation with a haphazard or poor layout. See
Figures 2, 3 and 4 for suggested layouts of boost, buck
and buck-boost operating modes.
Ensure that the grounding and heat sinking are acceptable.
A few rules to keep in mind are:
1. Place the R
T
resistor as close as possible to its re-
spective pins.
2. Place the C
IN
and C
VCC
capacitor as close as possible
to the V
IN
and GND connections of the LTM8042/
LTM8042-1.
3. Place the C
OUT
capacitor as close as possible to the
BSTOUT/BKIN or BSTIN/BKLED
–
and GND connection
of the LTM8042/LTM8042-1.
BSTOUT/BKIN
BSTIN/BKLED
–
LED
+
LED
+
GND
V
IN
RUN
SYNC
R
T
SS
PWM
TGEN
80421 F02
TG
TO LED STRING
CTL
V
CC
C
VCC
C
IN
C
OUT
THERMAL VIAS TO GROUND PLANE
GND
GND
Figure 2. Suggested Layout for Boost Operation
4. Place the C
IN
, C
VCC
and C
OUT
capacitors such that their
ground current flows directly adjacent to or underneath
the LTM8042/LTM8042-1.
5. Connect all of the GND connections to as large a copper
pour or plane area as possible on the top layer. Avoid
breaking the ground connection between the external
components and the LTM8042/LTM8042-1.
Use vias to connect the GND copper area to the board’s
internal ground planes. Liberally distribute these GND vias
to provide both a good ground connection and thermal
path to the internal planes of the printed circuit board.
Pay attention to the location and density of the thermal
vias in Figures 2 through 4. The LTM8042/LTM8042-1
can benefit from the heat sinking afforded by vias that
connect to internal GND planes at these locations, due to
their proximity to internal power handling components.
The optimum number of thermal vias depends upon the
printed circuit board design. For example, a board might
use very small via holes. It should employ more thermal
vias than a board that uses larger holes.