lm5088mhx-2 National Semiconductor Corporation, lm5088mhx-2 Datasheet - Page 20

• Manufacturer Part Number: lm5088mhx-2
• Description: Wide Input Range Non-synchronous Buck Controller
• Manufacturer: National Semiconductor Corporation

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PCB BOARD LAYOUT AND THERMAL
CONSIDERATIONS
In a buck regulator there are two loops where currents are
switched very fast. The first loop starts from the input capac-
itors, through the buck MOSFET, to the inductor then out to
the load. The second loop starts from the output capacitor
ground, to the regulator PGND pins, to the current sense re-
sistor, through the Schottky diode, to the inductor and then
out to the load. Minimizing the area of these two loops reduces
the stray inductance and minimizes noise which can cause
erratic operation. A ground plane is recommended as a
means to connect the input filter capacitors of the output filter
capacitors and the PGND pin of the regulator. Connect all of
the low power ground connections (C
to the regulator GND pin. Connect the GND pin and PGND
pins together through to topside copper area covering the en-
tire underside of the device. Place several vias in this under-
side copper area to the ground plane. The input capacitor
ground connection should be as close as possible to the cur-
rent sense ground connection.
In a buck converter, most of the losses can be attributed to
MOSFET conduction and switching loss, re-circulating diode
conduction loss, inductor DCR loss and LM5088 VIN and
VCC loss. The other dissipative components in a buck con-
SS
, R
T
, C
RAMP
) directly
20
verter produce losses but these other losses collectively ac-
count for about 2% of the total loss. Formulae to calculate all
the major losses are described in their respective sections of
this datasheet. The easiest method to determine the power
dissipated within the LM5088 is to measure the total conver-
sion losses (Pin-Pout), then subtract the power losses in the
Schottky diode, MOSFET, output inductor and snubber re-
sistor. When operating at 7A of output current and at 55V, the
power dissipation of the LM5088 is approximately 850 mW.
The junction to ambient thermal resistance of the LM5088
mounted in the evaluation board is approximately 40°C with
no airflow. At 25°C ambient temperature and no airflow, the
predicted junction temperature will be 25+40*0.9 = 61°C. The
LM5088 has an exposed thermal pad to aid in power dissi-
pation. Adding several vias under the device will greatly re-
duce the controller junction temperature. The junction to
ambient thermal resistance will vary with application. The
most significant variables are the area of copper in the PC
board; the number of vias under the IC exposed pad and the
amount of forced air cooling. The integrity of solder connec-
tion from the IC exposed pad to the PC board is critical.
Excessive voids will greatly diminish the thermal dissipation
capacity.