LM5118EVAL/NOPB National Semiconductor, LM5118EVAL/NOPB Datasheet - Page 22

• Manufacturer Part Number: LM5118EVAL/NOPB
• Description: BOARD EVALUATION FOR LM5118
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LM5118EVAL/NOPB

• Main Purpose: DC/DC, Step Up or Down
• Outputs And Type: 1, Non-Isolated
• Voltage - Output: 12V
• Current - Output: 3A
• Voltage - Input: 5 ~ 75V
• Regulator Topology: Buck-Boost
• Frequency - Switching: 300kHz
• Board Type: Fully Populated
• Utilized Ic / Part: LM5118
• Lead Free Status / RoHS Status: Not applicable / Not applicable
• Power - Output: -
• Other names: *LM5118EVAL; LM5118EVAL

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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 ca-
pacitors, 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 re-circulating 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 recom-
mended 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
C
and PGND pins together through topside copper area cover-
ing the entire underside of the device. Place several vias in
this underside copper area to the ground plane of the input
capacitors.
RAMP
) directly to the regulator AGND pin. Connect the AGND
FIGURE 17. VCC Bias with Additional Bias Supply
SS
, R
T
,
22
The highest power dissipating components are the two power
MOSFETs, the re-circulating diode, and the output diode. The
easiest way to determine the power dissipated in the MOS-
FETs is to measure the total conversion losses (P
then subtract the power losses in the Schottky diodes, output
inductor and any snubber resistors. An approximation for the
re-circulating Schottky diode loss is:
P = (1-D) x I
The boost diode loss is
P = I
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. Select-
ing diodes with exposed pads will aid the power dissipation of
the diodes as well. When selecting the MOSFETs, pay careful
attention to R
FETs with low gate charge will result in lower switching loss-
es.
OUT
x V
FWD
OUT
DS(ON)
.
x V
at high temperature. Also, selecting MOS-
FWD
.
30058552
IN
- P
OUT
),