LM5576MH National Semiconductor, LM5576MH Datasheet - Page 18

• Manufacturer Part Number: LM5576MH
• Description: IC,SMPS CONTROLLER,CURRENT-MODE,TSSOP,20PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM5576MH

• Rohs Compliant: NO

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PCB LAYOUT AND THERMAL CONSIDERATIONS
The circuit in Figure 1 serves as both a block diagram of the
LM5576 and a typical application board schematic for the
LM5576. In a buck regulator there are two loops where cur-
rents 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 LM5576 regulator IC. The easiest
method to determine the power dissipated within the LM5576
is to measure the total conversion losses (Pin – Pout) then
subtract the power losses in the Schottky diode, output in-
ductor and snubber resistor. An approximation for the Schot-
tky diode loss is P = (1-D) x Iout x Vfwd. An approximation for
the output inductor power is P = I
the DC resistance of the inductor and the 1.1 factor is an ap-
proximation for the AC losses. If a snubber is used, an ap-
proximation for the damping resistor power dissipation is P =
Vin
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 LM5576 are the output current, input voltage and op-
2
x Fsw x Csnub, where Fsw is the switching frequency
SS
, R
T
, C
RAMP
) directly to the regulator AGND pin.
OUT
2
x R x 1.1, where R is
18
erating frequency. The power dissipated while operating near
the maximum output current and maximum input volatge can
be appreciable. The operating frequency of the LM5576 eval-
uation board has been designed for 300kHz. When operating
at 3A output current with a 70V input the power dissipation of
the LM5576 regulator is approximately 2.5W.
The junction-to-ambient thermal resistance of the LM5576 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 pro-
vided. Referring to the evaluation board artwork, the area
under the LM5576 (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 thermal dissipation capacity.
The junction-to-ambient thermal resistance of the LM5576
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 LM5576 will be 25 + (45 x 2.5) =
137.5°C. If the evaluation board is operated at 3A output cur-
rent 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 LM5576 thermal resis-
tance 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 dissipa-
tion reduces to 2W (at 3A output current). With the same
forced air cooling the junction temperature reduces to 25 + (2
x 30) = 85°C.