In normal operation, the average current of the
catch diode is the load current times the catch diode duty cycle, 1-D (D is the
switch duty cycle, which is approximately VOUT/VIN). The
largest value of the catch diode average current occurs at the maximum input
voltage (minimum D). For normal operation, the catch diode current rating must
be at least 1.3 times greater than its maximum average current. However, if the
power supply design must withstand a continuous output short, the diode must
have a current rating greater than the maximum current limit of the LM2674. The
most stressful condition for this diode is a shorted output condition. Schottky
diodes provide the best performance, and in this example a 500-mA, 40-V Schottky
diode is a good choice. If the circuit must withstand a continuous shorted
output, TI recommends a higher current (at least 1.2 A) Schottky diode.
The reverse voltage rating of the diode must be at least 1.25 times the maximum input voltage.
Because of their fast switching speed and low forward voltage drop, Schottky diodes provide the best performance and efficiency. The Schottky diode must be placed close to the LM2674 using short leads and short printed-circuit traces.