ZHCSHU8F October 2008 – July 2019 LM5575-Q1
PRODUCTION DATA.
The circuit in Functional Block Diagram serves as both a block diagram of the LM5575-Q1 and a typical application board schematic for the LM5575-Q1. 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, and then 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. The user can minimize the loop area of these two loops to reduce the stray inductance and to minimize noise and possible erratic operation. A ground plane in the printed-circuit board (PCB) 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 connections (CSS, RT, CRAMP) directly to the regulator AGND pin. Connect the AGND and PGND pins together through the top-side copper area that covers 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-Q1 regulator IC. The easiest method to determine the power dissipated within the LM5575-Q1 is to measure the total conversion losses (Pin – Pout) then subtract the power losses in the Schottky diode, output inductor, and snubber resistor.
Equation 17 calculates an approximation for the Schottky diode loss.
Equation 18 calculates an approximation for the output inductor power.
where
If a snubber is used, Equation 19 calculates an approximation for the damping resistor power dissipation.
where
The regulator has an exposed thermal pad to help power dissipation. Add several vias under the device to the ground plane to greatly reduce the regulator junction temperature. Select a diode with an exposed pad to help the power dissipation of the diode.
The most significant variables that affect the power dissipated by the LM5575-Q1 are the output current, input voltage, and operating frequency. The power dissipated while the device operates near the maximum output current and maximum input voltage can be appreciable. The operating frequency of the LM5575-Q1 evaluation board has been designed for 300 kHz. When the device operates at 1.5-A output current with a 70-V input, the power dissipation of the LM5575-Q1 regulator is approximately 1.25 W.