The following is the discriminating algorithm be-

tween fast and slow Vcc transients. The transient

voltage clamp is normally set at 70V. If Vcc rises

above the Vcc_ov=19V typ. over voltage shut-

down threshold, both Td_ov_1 and Td_ov_2 tim-

ers start. When the first timer stops (after 130

µs

typ. delay) the clamp status is evaluated and

locked. If the transient has been fast enough and

the voltage clamp activated, then it remains 70V

active until the second timer stops (after 1ms de-

lay), then it deactivates by rising to 80V. If the

transient has been slow and the voltage clamp

unreached when the first timer stops, then

it de-

activates by rising to 80V. A new 70V clamp cycle

may restart only by lowering Vcc below the 19V

over voltage shutdown threshold.

The VFB and VCOM input pins may connect to

the Vcc or lower voltage during the power supply

transients of Figs. 4 and 5.

L9909

7/9