AN2785 datasheet(45/51 Pages) STMICROELECTRONICS

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AN2785

Below-ground voltage on the OUT pin

Doc ID 14785 Rev 1

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The L6393 IC provides the full switching functionality of the high-side section until the level

shifter is operating. Because the two pull-up resistors are connected to the floating supply

VBOOT of the high-side floating section, the driver still works properly if the OUT voltage

begins to go below ground. This means that the operating limit for the level-shifter structure

has to be referred to the absolute value of the BOOT voltage with respect to ground. In fact,

the BOOT voltage can be considered as the supply of the level-shifter block. The minimum

operating value of the level-shifter supply is 5 V, so the VBOOT value must be at least 5 V.

The VBOOT - VOUT voltage can have any value in the range of 12.4 V ÷ 20 V. If the BOOT

voltage is between 0 and 5 V, the functionality of the level shifter is not certain, but internal

structures should not be damaged.

The following section provides some examples.

9.4.1

Steady-state (DC) conditions

When the LVG is OFF, the bootstrap diode only turns on when the BOOT voltage is about

2 V below VCC (worst case: minimum voltage drop value), so it could overcharge the

bootstrap capacitor up to a maximum voltage of about VCC - 2 V - VOUT. In order to not

overcome the value of 20V on the VBOOT - VOUT voltage, the OUT pin could be forced

permanently to the value of VCC - 2 V - 20 V; the L6393 IC would still operate safely, making

the HVG output switching as the HIN logic input.

In any case it must be emphasized, as already explained in the previous paragraph, that in

most applications the static below-ground voltage of the OUT pin is very rarely lower than

approximately - 2 V.

9.4.2

Transient conditions

If the time during which the OUT pin goes below ground is limited, the bootstrap overcharge

will probably not overcome the safe operating range (20 V) of VBOOT - VOUT, even if the

VOUT voltage is lower than the limit of VCC - 2 V - 20 V mentioned in the previous

paragraph. In such a case, the gate driver and the high-side section are fully operational if

the VBOOT voltage remains above 5 V, as explained previously for the proper functioning of

the high-side level shifter. In any case, the logic state is maintained as long as the VBOOT

remains above ground.

Table 3.

Minimum VOUT in DC condition providing safe and full operation of the

high-side section

Example 1

Example 2

Example 3

VCC

12.5

VBOOT

10.5

VOUT (min)

-9.5

-7

-5

VBOOT - VOUT (max)

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