LM5575MHX National Semiconductor, LM5575MHX Datasheet - Page 18

• Manufacturer Part Number: LM5575MHX
• Description: IC,SMPS CONTROLLER,CURRENT-MODE,TSSOP,16PIN,PLASTIC
• Manufacturer: National Semiconductor

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PCB LAYOUT AND THERMAL CONSIDERATIONS
The circuit in Figure 1 serves as both a block diagram of the
LM5575 and a typical application board schematic for the
LM5575. 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 LM5575 regulator IC. The easiest
method to determine the power dissipated within the LM5575
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
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
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 LM5575 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 LM5575 eval-
uation board has been designed for 300kHz. When operating
at 1.5A output current with a 70V input the power dissipation
of the LM5575 regulator is approximately 1.25W.
The junction-to-ambient thermal resistance of the LM5575 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 LM5575 (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 LM5575
mounted in the evaluation board varies from 50°C/W with no
airflow to 28°C/W with 900 LFM (Linear Feet per Minute). With
a 25°C ambient temperature and no airflow, the predicted
junction temperature for the LM5575 will be 25 + (50 x 1.25)
= 88°C. If the evaluation board is operated at 1.5A output
current, 70V 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 capac-
itor reset to zero.