lm5576q0 National Semiconductor Corporation, lm5576q0 Datasheet - Page 18

• Manufacturer Part Number: lm5576q0
• Description: Simple Switcher 75v, 3a Step-down Switching Regulator
• Manufacturer: National Semiconductor Corporation

www.national.com
PCB LAYOUT AND THERMAL CONSIDERATIONS
The circuit in Figure 1 serves as both a block diagram of the
LM5576Q0 and a typical application board schematic for the
LM5576Q0. 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 LM5576Q0 regulator IC. The easiest
method to determine the power dissipated within the
LM5576Q0 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
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 LM5576Q0 are the output current, input voltage and
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
operating frequency. The power dissipated while operating
near the maximum output current and maximum input volatge
can be appreciable. The operating frequency of the
LM5576Q0 evaluation board has been designed for 300kHz.
When operating at 3A output current with a 70V input the
power dissipation of the LM5576Q0 regulator is approximate-
ly 2.5W.
The junction-to-ambient thermal resistance of the LM5576Q0
will vary with the 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
provided. Referring to the evaluation board artwork, the area
under the LM5576Q0 (component side) is covered with cop-
per 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 thermal dissipation capacity.
The junction-to-ambient thermal resistance of the LM5576Q0
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 LM5576Q0 will be 25 + (45 x 2.5)
= 137.5°C. If the evaluation board is operated at 3A output
current and 70V input voltage for a prolonged period of time
the thermal shutdown protection within the IC will activate.
The IC will turn off allowing the junction to cool, followed by
restart with the soft-start capacitor reset to zero.
One or more of the following modifications will prevent the
thermal shutdown from being activated: apply forced air cool-
ing, reduce the maximum input voltage, lower the maximum
output current, reduce the operating frequency, add more
heat sinking to the PC board. For example, applying forced
air cooling of 225 LFM will reduce the LM5576Q0 thermal re-
sistance to approximately 30°C/W. The junction temperature
will be reduced to 25 + (2.5 x 30) = 100°C. If the maximum
input voltage for the application is 48V, then the IC power
dissipation reduces to 2W (at 3A output current). With the
same forced air cooling the junction temperature reduces to
25 + (2 x 30) = 85°C.