LM25576MH National Semiconductor, LM25576MH Datasheet - Page 18

• Manufacturer Part Number: LM25576MH
• Description: IC,SMPS CONTROLLER,CURRENT-MODE,TSSOP,20PIN,PLASTIC
• Manufacturer: National Semiconductor

Specifications of LM25576MH

• Rohs Compliant: NO

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PCB LAYOUT AND THERMAL CONSIDERATIONS
The circuit in Figure 13 serves as both a block diagram of the
LM25576 and a typical application board schematic for the
LM25576. 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 LM25576 regulator IC. The easiest
method to determine the power dissipated within the
LM25576 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
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
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 LM25576 are the output current, input voltage and op-
erating frequency. The power dissipated while operating near
the maximum output current and maximum input volatge can
be appreciable. The operating frequency of the LM25576
evaluation board has been designed for 300kHz. When op-
erating at 3A output current with a 42V input the power
dissipation of the LM25576 regulator is approximately 1.9W.
The junction-to-ambient thermal resistance of the LM25576
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 LM25576 (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 LM25576
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 LM25576 will be 25 + (45 x 1.9)
= 110°C. If the evaluation board is operated at 3A output cur-
rent and 42V input voltage for a prolonged period of time the
thermal shutdown protection within the IC may activate. The
IC will turn off allowing the junction to cool, followed by restart
with the soft-start capacitor reset to zero.