LM5575

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SNVS471G – JANUARY 2007 – REVISED APRIL 2013

PCB LAYOUT AND THERMAL CONSIDERATIONS

The circuit in Figure 9 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 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

regulator AGND pin. Connect the AGND and PGND pins together through the topside 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 inductor and snubber resistor. An

approximation for the Schottky diode loss is P = (1-D) x Iout x Vfwd. An approximation for the output inductor

Fsw x Csnub, where Fsw is the switching frequency 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 LM5575 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 LM5575 evaluation 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 provided. 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 capacitor reset to zero.

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