lm25088mhx-2 National Semiconductor Corporation, lm25088mhx-2 Datasheet - Page 20

• Manufacturer Part Number: lm25088mhx-2
• Description: Wide Input Range Non-synchronous Buck Controller
• Manufacturer: National Semiconductor Corporation

www.national.com
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 LM25088 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 LM25088 is to measure the total con-
version losses (Pin-Pout), then subtract the power losses in
the Schottky diode, MOSFET, output inductor and snubber
resistor. When operating at 7A of output current and at 36V,
the power dissipation of the LM25088 is approximately 550
mW. The junction to ambient thermal resistance of the
LM25088 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*0.55
= 47°C. The LM25088 has an exposed thermal pad to aid in
power dissipation. Adding several vias under the device will
greatly reduce the controller junction temperature. The junc-
tion 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 con-
nection from the IC exposed pad to the PC board is critical.
Excessive voids will greatly diminish the thermal dissipation
capacity.