AN2785 datasheet(40/51 Pages) STMICROELECTRONICS

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Below-ground voltage on the OUT pin

AN2785

40/51

Doc ID 14785 Rev 1

Figure 30.

Use of combination of OUT resistor and OUT diode to limit the below

ground voltage spike on OUT pin

Note that ROUT resistor is also in series with the charging path of the bootstrap capacitor. Its

effects may be more evident during the first charge of the bootstrap capacitor, when it is

completely discharged and a significant charging current may flow into the ROUT resistor,

producing an additional voltage drop between ground and OUT pin. Because during the

bootstrap charging the HVG pin is set to low level, the OUT pin and the HVG pin are shorted

together and have the same voltage, so the voltage drop due to ROUT results directly

transferred to the VGE (or VGS) of the high side IGBT (or MOSFET). Then the risk is that a

weak turn ON of the high side power switch, when the low side power switch is already ON,

could cause a cross-conduction in the power half bridge. Actually in most of cases this

doesn’t represent an issue for the proper working of the application, because the typical

intrinsic resistance RBOOT (~120 Ω) in series to the internal bootstrap diode is much higher

than the ROUT commonly used and the voltage drop on ROUT is negligible.

HVG

H.V.

OUT

LVG

BOOT

CBOOT

VCC

HB_OUT

RGH_ON

RGH_OFF

RGL_ON

RGL_OFF

RBOOT

DBOOT

HB_OUT

OUT

BOOT

DOUT

GND

ROUT

AN2785 Datasheet(PDF) 40 Page - STMicroelectronics