AN2785 Datasheet, PDF (42/51 Pages) STMicroelectronics – L6393 half bridge gate driver

Below-ground voltage on the OUT pin
AN2785
Figure 31. Bootstrap overcharging due to below-ground voltage on OUT pin
V_BOOT - V_OUT
V_BOOT
BOOTSTRAP NETWORK
&
BELOW GROUND VOLTAGE
DBOOT RBOOT
VCC
BOOT
VCC
V_OUT
Time constant of this transition:
RBOOT * CBOOT >> Tspike typically
OUT
0V
V_{BGV_spike}
V_BOOT - V_OUT
befor overcharge
V_BOOT - V_OUT
during BGV spike
V_BOOT - V_OUT
overcharged
during static BGV
V_{BGV_static}
t
Tspike < 100ns
typically
V_OUT
Note that in the L6393 IC gate driver the internal DMOS in series with the integrated
bootstrap diode is only fully turned on when the LVG is ON. In fact, when the LVG is OFF, the
gate of the bootstrap DMOS is biased at VCC voltage. This means that if the V_BOOT is pulled
down by the bootstrap capacitor (due to a below-ground voltage on the OUT pin) at about 3
V (typical value) below the VCC voltage, the DMOS turns on again. Figure 32 shows some
different (internal and external) bootstrap network characteristics.
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Doc ID 14785 Rev 1