AN2785 datasheet(43/51 Pages) STMICROELECTRONICS

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AN2785

Below-ground voltage on the OUT pin

Doc ID 14785 Rev 1

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Figure 32.

Different bootstrap network characteristics

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

duration (less than 100 ns), therefore it is not enough to further overcharge the bootstrap

capacitor since the time constant of the bootstrap charging 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 overcharging during BVG spikes.

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

constant would be about 12 µs, much higher than the typical below-ground voltage spike

duration: with the internal bootstrap diode it is very difficult to overcharge the floating section

up to dangerous voltages in the very short duration of the undershoot spike. More attention

must be paid to the below-ground voltage of the VBOOT during this spike, because, as

already explained above, the internal junction VBOOT to substrate could turn on; for very

short periods (some tens of nanoseconds) this is typically not a problem.

Figure 33

summarizes the main conditions to avoid any issues related to the below-ground

voltage phenomenon in steady-state. As explained in Section 9.1, typically the static below-

ground voltage is hardly higher than 2 V, so for most applications these constraints are not

usually necessary.

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