AN2738 datasheet(43/54 Pages) STMICROELECTRONICS

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AN2738

The below-ground voltage on the OUT pin

Doc ID 14589 Rev 2

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11.3

Issues related to the below-ground voltage phenomenon

Contrary to common belief, 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 are instead related mainly to the voltage value of BOOT pin, which is

indirectly bound to the OUT pin voltage. There are two main issues to be taken into

consideration, all related to the absolute maximum ratings of the IC:

●

VBOOT absolute minimum voltage

●

VBOOT-VOUT absolute maximum voltage

11.3.1

VBOOT voltage safe operating condition

Electrically, the OUT pin could safely tolerate even many volts below ground 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 BOOT pin

and the IC gate driver substrate (connected to GND), which is normally in reverse condition.

The turn-on of this junction could, in fact, cause a current large enough to damage the

device.

11.3.2

Bootstrap capacitor over-charging

Another very important consideration is bootstrap capacitor over-charging. Actually, 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 the VBOOT - VOUT voltage increases. It is very important that the

bootstrap over-charge does not exceed the recommended maximum value for the VBOOT -

VOUT voltage (see the relevant datasheet), as the high side floating section of the gate driver

could be damaged.

Note that a significant over-charging of the bootstrap capacitor is only possible through the

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

is very brief and the over-charging transition is limited by the time constant RC of the

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

In Figure 39, the over-charging phenomenon is illustrated in detail.

AN2738 Datasheet(PDF) 43 Page - STMicroelectronics