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AN2785

Below-ground voltage on the OUT pin

Doc ID 14785 Rev 1

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9.3

Issues related to the below-ground voltage phenomenon

The issues resulting from significant below-ground voltages on the OUT pin are not related

to the maximum absolute voltage values that the OUT pin is able to withstand, but related

mainly to the voltage value of the BOOT pin, which is indirectly bound with the OUT pin

voltage. There are two main issues to be carefully taken in consideration, both related to the

absolute maximum ratings of the IC.

●

VBOOT absolute minimum voltage.

●

VBOOT-VOUT absolute maximum voltage.

9.3.1

VBOOT voltage safe operating condition

Electrically, the OUT pin could stand safely below ground of many volts without problems,

but the BOOT pin cannot. The absolute value of the VBOOT voltage must not go steadily

below -0.3 V in order to avoid the turn-on of the built-in junction between the BOOT pin and

the IC gate driver substrate (connected to GND), normally in reverse condition. The turn-on

of this junction could in fact cause a very high current that could in turn cause damage to the

device.

9.3.2

Bootstrap capacitor overcharging

Another important point to consider is the overcharging of the bootstrap capacitor. Even

before the OUT pin approaches the zero voltage value, the bootstrap diode (internal or

external) tends to tie VBOOT close to the VCC supply voltage; since the OUT pin is below

ground, the bootstrap capacitor is overcharged through the current coming from the

bootstrap diode and then the VBOOT - VOUT voltage increases. It is very important that the

bootstrap overcharging does not overcome the recommended maximum value for the

VBOOT - VOUT voltage (see the relevant datasheet), because the high-side floating section of

the gate driver could be damaged.

Note that a significant overcharging of the bootstrap capacitor is really only possible through

the static contribution of the below-ground voltage, because the dynamic below-ground

voltage is very brief and the overcharging transition is limited by the time constant RC of the

bootstrap capacitor and by the overall resistance in series with the bootstrap diode.

Figure 31

shows the overcharging phenomenon in detail.

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