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17

RD-0608

RS2

()

Ω

≤

Ω

=

−

=

−

=

−

=

k

RS

W

V

V

W

V

V

RS

RS

V

V

W

Out

Out

1

512

1

2

1

14

30

2

1

14

2

2

14

2

1

2

2

2

max

max

RS1 may be rounded up to 1K

7. VBS Supply

The bootstrap diode (DBOOT) and supply capacitor (CBOOT) comprise the supply voltage for the high side driver

circuitry. To guarantee that the high-side supply is charged up before operation commences, the first pulse from the

output drivers comes from the LO pin. During under voltage lock-out mode, the high and low-side outputs are both

held low.

During an open circuit condition, without the watchdog timer, the HO output would remain high at all times and the

charge stored in the bootstrap capacitor (CBOOT) would slowly leak until reaching zero, thus eliminating the

floating power supply for the high side driver. To maintain sufficient charge on CBOOT, a watchdog timer has been

implemented. In the condition where VIFB remains below VIFBTH, the HO output will be forced low roughly after

20µs and the LO output forced high. This toggling of the outputs will last for 1µs to maintain and replenish

sufficient charge on CBOOT.

Fig. 21 Illustration of Watchdog Timer

The bootstrap capacitor value needs to be chosen so that it maintains sufficient charge for at least the 20µs interval

until the watchdog timer allows the capacitor to recharge. If the capacitor value is too small, the charge will fully

dissipate in less than 20µs. The bootstrap capacitor should be at least 100nF. A larger value within reason can be

used if preferred.