lm25574q0 National Semiconductor Corporation, lm25574q0 Datasheet - Page 17

• Manufacturer Part Number: lm25574q0
• Description: Simple Switcher 42v, 0.5a Step-down Switching Regulator
• Manufacturer: National Semiconductor Corporation

PCB LAYOUT AND THERMAL CONSIDERATIONS
The circuit in Figure 1 serves as both a block diagram of the
LM25574Q0 and a typical application board schematic for the
LM25574Q0. 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 trace. Place several vias in this trace to the
ground plane.
The two highest power dissipating components are the re-
circulating diode and the LM25574Q0 regulator IC. The eas-
iest method to determine the power dissipated within the
LM25574Q0 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
SS
, R
T
, C
RAMP
) directly to the regulator AGND pin.
OUT
2
x R x 1.1,
17
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 most significant variables that affect the power dissipated
by the LM25574Q0 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
LM25574Q0 evaluation board has been designed for 300kHz.
When operating at 0.5A output current with a 42V input the
power dissipation of the LM25574Q0 regulator is approxi-
mately 0.36W.
The
LM25574Q0 will vary with the application. The most signifi-
cant variables are the area of copper in the PC board, and the
amount of forced air cooling provided. The junction-to-ambi-
ent thermal resistance of the LM25574Q0 mounted in the
evaluation board varies from 90°C/W with no airflow to 60°C/
W with 900 LFM (Linear Feet per Minute). With a 25°C am-
bient temperature and no airflow, the predicted junction tem-
perature for the LM25574Q0 will be 25 + ((90 x 0.36) = 57°C.
If the evaluation board is operated at 0.5A output current, 42V
input voltage and high ambient temperature 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.
junction-to-ambient
2
x Fsw x Csnub, where Fsw is the switching fre-
thermal
resistance
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