LM5574, LM5574-Q1

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SNVS478F – JANUARY 2007 – REVISED APRIL 2013

PCB LAYOUT AND THERMAL CONSIDERATIONS

The circuit in Figure 9 serves as both a block diagram of the LM5574 and a typical application board schematic

for the LM5574. 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 trace. Place

several vias in this trace to the ground plane.

The two highest power dissipating components are the re-circulating diode and the LM5574 regulator IC. The

easiest method to determine the power dissipated within the LM5574 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 most significant variables that affect the power dissipated by the LM5574 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 LM5574 evaluation board has been

designed for 300kHz. When operating at 0.5A output current with a 70V input the power dissipation of the

LM5574 regulator is approximately 0.6W.

The junction-to-ambient thermal resistance of the LM5574 will vary with the application. The most significant

variables are the area of copper in the PC board, and the amount of forced air cooling provided. The junction-to-

ambient thermal resistance of the LM5574 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 ambient temperature and no airflow, the predicted

junction temperature for the LM5574 will be 25 + ((90 x 0.6) = 79°C. If the evaluation board is operated at 0.5A

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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Product Folder Links: LM5574 LM5574-Q1