SNVSB28 December 2017 LM25575-Q1
A Schottky type re-circulating diode is required for all LM25575-Q1 applications. Ultra-fast diodes are not recommended and may result in damage to the IC due to reverse recovery current transients. The near ideal reverse recovery characteristics and low forward voltage drop are particularly important diode characteristics for high input voltage and low output voltage applications common to the LM25575-Q1. The reverse recovery characteristic determines how long the current surge lasts each cycle when the buck switch is turned on. The reverse recovery characteristics of Schottky diodes minimize the peak instantaneous power in the buck switch occurring during turn-on each cycle. The resulting switching losses of the buck switch are significantly reduced when using a Schottky diode. The reverse breakdown rating should be selected for the maximum VIN, plus some safety margin.
The forward voltage drop has a significant impact on the conversion efficiency, especially for applications with a low output voltage. “Rated” current for diodes vary widely from various manufacturers. The worst case is to assume a short circuit load condition. In this case the diode will carry the output current almost continuously. For the LM25575-Q1 this current can be as high as 2.1 A. Assuming a worst case 1 V drop across the diode, the maximum diode power dissipation can be as high as 2.1 W. For the reference design a 60 V Schottky in a SMC package was selected.