lm25576q0 National Semiconductor Corporation, lm25576q0 Datasheet - Page 18

• Manufacturer Part Number: lm25576q0
• Description: Simple Switcher 42v, 3a Step-down Switching Regulator
• Manufacturer: National Semiconductor Corporation

www.national.com
PCB LAYOUT AND THERMAL CONSIDERATIONS
The circuit in Figure 13 serves as both a block diagram of the
LM25576Q0 and a typical application board schematic for the
LM25576Q0. In a buck regulator there are two loops where
currents are switched very fast. The first loop starts from the
input capacitors, to the regulator VIN pin, to the regulator SW
pin, to the inductor then out to the load. The second loop starts
from the output capacitor ground, to the regulator PGND pins,
to the regulator IS pins, to the diode anode, to the inductor
and then out to the load. Minimizing the loop area of these
two loops reduces the stray inductance and minimizes noise
and possible 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 regulator. Connect all of the low power ground connec-
tions (C
Connect the AGND and PGND pins together through the top-
side copper area covering the entire underside of the device.
Place several vias in this underside copper area to the ground
plane.
The two highest power dissipating components are the re-
circulating diode and the LM25576Q0 regulator IC. The eas-
iest method to determine the power dissipated within the
LM25576Q0 is to measure the total conversion losses (Pin –
Pout) then subtract the power losses in the Schottky diode,
output inductor and snubber resistor. An approximation for
the Schottky diode loss is P = (1-D) x Iout x Vfwd. An approx-
imation for the output inductor power is P = I
where R is the DC resistance of the inductor and the 1.1 factor
is an approximation for the AC losses. If a snubber is used,
an approximation for the damping resistor power dissipation
is P = Vin
quency and Csnub is the snubber capacitor. The regulator
has an exposed thermal pad to aid power dissipation. Adding
several vias under the device to the ground plane will greatly
SS
2
, R
x Fsw x Csnub, where Fsw is the switching fre-
T
, C
RAMP
) directly to the regulator AGND pin.
OUT
2
x R x 1.1,
18
reduce the regulator junction temperature. Selecting a diode
with an exposed pad will aid the power dissipation of the
diode.
The most significant variables that affect the power dissipated
by the LM25576Q0 are the output current, input voltage and
operating frequency. The power dissipated while operating
near the maximum output current and maximum input volatge
can be appreciable. The operating frequency of the
LM25576Q0 evaluation board has been designed for 300kHz.
When operating at 3A output current with a 42V input the
power dissipation of the LM25576Q0 regulator is approxi-
mately 1.9W.
The
LM25576Q0 will vary with the application. The most signifi-
cant 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 provided. Referring to the evaluation board
artwork, the area under the LM25576Q0 (component side) is
covered with copper and there are 5 connection vias to the
solder side ground plane. Additional vias under the IC will
have diminishing value as more vias are added. The integrity
of the solder connection from the IC exposed pad to the PC
board is critical. Excessive voids will greatly diminish the ther-
mal dissipation capacity. The junction-to-ambient thermal re-
sistance of the LM25576Q0 mounted in the evaluation board
varies from 45°C/W with no airflow to 25°C/W with 900 LFM
(Linear Feet per Minute). With a 25°C ambient temperature
and no airflow, the predicted junction temperature for the
LM25576Q0 will be 25 + (45 x 1.9) = 110°C. If the evaluation
board is operated at 3A output current and 42V input voltage
for a prolonged period of time the thermal shutdown protec-
tion within the IC may activate. The IC will turn off allowing the
junction to cool, followed by restart with the soft-start capac-
itor reset to zero.
junction-to-ambient
thermal
resistance
of
the