Datasheet
LM5118, LM5118-Q1
www.ti.com
SNVS566H –APRIL 2008–REVISED JANUARY 2014
PCB Layout and Thermal Considerations
In a buck-boost regulator, there are two loops where currents are switched very fast. The first loop starts from the
input capacitors, and then to the buck switch, the inductor, the boost switch then back to the input capacitor. The
second loop starts from the inductor, and then to the output diode, the output capacitor, the recirculating diode,
and back to the inductor. Minimizing the PC board area of these two loops reduces the stray inductance and
minimizes noise and the possibility of erratic operation. A ground plane in the PC board is recommended as a
means to connect the input filter capacitors to the output filter capacitors and the PGND pins of the LM5118.
Connect all of the low current ground connections (C
SS
, R
T
, C
RAMP
) directly to the regulator AGND pin. Connect
the AGND and PGND pins together through topside copper area covering the entire underside of the device.
Place several vias in this underside copper area to the ground plane of the input capacitors.
The highest power dissipating components are the two power MOSFETs, the recirculating diode, and the output
diode. The easiest way to determine the power dissipated in the MOSFETs is to measure the total conversion
losses (P
IN
- P
OUT
), then subtract the power losses in the Schottky diodes, output inductor and any snubber
resistors. An approximation for the recirculating Schottky diode loss is:
P = (1-D) x I
OUT
x V
FWD
. (35)
The boost diode loss is
P = I
OUT
x V
FWD
. (36)
If a snubber is used, the power loss can be estimated with an oscilloscope by observation of the resistor voltage
drop at both turn-on and turn-off transitions. The LM5118 package has an exposed thermal pad to aid power
dissipation. Selecting diodes with exposed pads will aid the power dissipation of the diodes as well. When
selecting the MOSFETs, pay careful attention to R
DS(ON)
at high temperature. Also, selecting MOSFETs with low
gate charge will result in lower switching losses.
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