AN2785 Datasheet, PDF (43/51 Pages) STMicroelectronics – L6393 half bridge gate driver

AN2785
Below-ground voltage on the OUT pin
Figure 32. Different bootstrap network characteristics
Regarding the below-ground voltage spike, typically this undershoot voltage has a very brief
duration (less than 100 ns), therefore it is not enough to further overcharge the bootstrap
capacitor since the time constant of the bootstrap charging is equal to the product of the
C_BOOT and the R_BOOT resistance in series with the diode. Moreover, the higher this
resistance, the lower the risk of bootstrap overcharging during BVG spikes.
For example, with an R_BOOT of 120 Ω and a C_BOOT equal to 100 nF, the associated time
constant would be about 12 µs, much higher than the typical below-ground voltage spike
duration: with the internal bootstrap diode it is very difficult to overcharge the floating section
up to dangerous voltages in the very short duration of the undershoot spike. More attention
must be paid to the below-ground voltage of the V_BOOT during this spike, because, as
already explained above, the internal junction V_BOOT to substrate could turn on; for very
short periods (some tens of nanoseconds) this is typically not a problem.
Figure 33 summarizes the main conditions to avoid any issues related to the below-ground
voltage phenomenon in steady-state. As explained in Section 9.1, typically the static below-
ground voltage is hardly higher than 2 V, so for most applications these constraints are not
usually necessary.
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