AN2738 Datasheet, PDF (48/54 Pages) STMicroelectronics – L6390 half-bridge gate driver

The below-ground voltage on the OUT pin
AN2738
Figure 43. OUT below-ground voltage in transient conditions: limited boot over-
charging
V_OUT
V_BOOT
V_BOOT - V_OUT
VCC
0V
V_{BGV_spike}
HIN
V_BOOT - V_OUT
befor overcharge
V_BOOT - V_OUT
safe operating
maximum range
(20V)
V_{BGV_static}
V_OUT
High side OFF time
V_BOOT - V_OUT
real
overcharge
V_BOOT - V_OUT
virtual final value
in steady state
t
11.4.3
Below-ground voltage spikes
As previously stated, the duration of the dynamic contribution of the below-ground voltage
on the OUT pin is not enough to over-charge the bootstrap capacitor. This fact usually
removes the risk of exceeding the recommended maximum value of 20 V for V_BOOT -V_OUT
voltage, but it increases the danger of pulling down the BOOT pin below ground, thus
violating its absolute minimum rating. As mentioned above, this phenomenon should be
avoided for safe IC gate driver operation. However, note that applicative bench tests have
shown the L6390 proper operation even with below-ground spikes on the OUT pin well
above - 50 V (see Figure 44).
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Doc ID 14589 Rev 2