The LM5108 incorporates the bootstrap diode necessary to generate the high-side bias for HO to work satisfactorily. The characteristics of this diode are important to achieve efficient, reliable operation. The characteristics to consider are forward voltage drop and dynamic resistance. Generally, low forward voltage drop diodes are preferred for low power loss during charging of the bootstrap capacitor. The device has a boot diode forward voltage drop rated at 0.9 V and dynamic resistance of 2 Ω for reliable charge transfer to the bootstrap capacitor. The dynamic characteristics to consider are diode recovery time and stored charge. Diode recovery times that are specified without operating conditions, can be misleading. Diode recovery times at no forward current (I_F) can be noticeably less than with forward current applied. The LM5108 boot diode recovery is specified as 20 ns at I_F = 20 mA, I_REV = 0.5 A. Dynamic impedance of LM5108 bootstrap diode helps limit the peak forward current.

In applications where switching frequencies are very high, for example in excess of 1 MHz, and the low-side minimum pulse widths are very small, the diode peak forward current could be very high and peak reverse current could also be very high, specifically if high bootstrap capacitor value has been chosen. In such applications it might be advisable to use external Schottky diode as bootstrap diode. It is safe to at least make a provision for such diode on the board if possible.