AN2738 datasheet(45/54 Pages) STMICROELECTRONICS

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AN2738

The below-ground voltage on the OUT pin

Doc ID 14589 Rev 2

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Figure 40.

Different bootstrap network characteristics

Regarding the below-ground voltage spike, typically this undershoot voltage has a very brief

duration (less than 100 ns) and is not enough to further over-charge the bootstrap capacitor,

because the charging time constant of the bootstrap is equal to the product of the CBOOT

and the RBOOT resistance in series with the diode. Moreover, the higher this resistance, the

lower the risk of bootstrap over-charging during BVG spikes.

For example, with a RBOOT of 120 Ω and a CBOOT of 100 nF, the associated time constant

would be about 12 µs, much higher than typical below-ground voltage spike duration. Using

an internal bootstrap diode it is very difficult to over-charge the floating section up to

dangerous voltage levels with the very short duration of the undershoot spike. More

attention should be given to the below-ground voltage of the VBOOT during this spike

because, as explained previously, the internal junction VBOOT -to-substrate could turn on;

typically for very short time (a few tenths of nanoseconds), this is not a problem.

In Figure 41, the main conditions to avoid issues related to the below-ground voltage

phenomenon in steady-state are resumed. As explained in paragraph 11.1, typically the

static below-ground voltage is hardly higher than 2 V, so these constraints are usually not a

limitation for most applications.

AN2738 Datasheet(PDF) 45 Page - STMicroelectronics