SNVS480J January 2007 – July 2020 LM5022
The boost regulator requires an output diode D1 (see Figure 14) to carrying the inductor current during the MOSFET off-time. The most efficient choice for D1 is a Schottky diode due to low forward drop and near-zero reverse recovery time. D1 must be rated to handle the maximum output voltage plus any switching node ringing when the MOSFET is on. In practice, all switching converters have some ringing at the switching node due to the diode parasitic capacitance and the lead inductance. D1 must also be rated to handle the average output current, IO.
The overall converter efficiency becomes more dependent on the selection of D1 at low duty cycles, where the boost diode carries the load current for an increasing percentage of the time. This power dissipation can be calculating by checking the typical diode forward voltage, VD, from the I-V curve on the data sheet of the diode and then multiplying it by IO. Diode data sheets also provides a typical junction-to-ambient thermal resistance, RθJA, which can be used to estimate the operating die temperature of the Schottky. Multiplying the power dissipation (PD = IO × VD) by RθJA gives the temperature rise. The diode case size can then be selected to maintain the Schottky diode temperature below the operational maximum.
In this example, a Schottky diode rated to 60 V and 1 A is suitable, as the maximum diode current is 0.5 A. A small case such as SOD-123 can be used if a small footprint is critical. Larger case sizes generally have lower RθJA and lower forward voltage drop, so for better efficiency the larger SMA case size is used.