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AN1625 APPLICATION NOTE

Figure 18. Over Current Operation after a short circuit between an OUT pin and GND. EN and DIAG

pins are connected together.

Figure 18 shows the device operating in overcurrent condition (short to ground). When an over current is de-

tected the internal open drain mosfet pulls the EN pin to GND switching off all 6 power DMOS of the device and

allowing the current to decay. Under a persistent over current condition, like a short to ground or a short between

two output pins, the external RC network on the EN pin (see Figure 17) reduces the r.m.s. value of the output

selected to ensure proper operation of the device under a short circuit condition. When the current flowing

through the high side DMOS reaches the OCD threshold (5.6 A typ.), after an internal propagation delay

decay as it circulates through the freewheeling diodes. Since the DMOS are off, there is no current flowing

(see Figure 18).

The maximum value reached by the current depends on its slew-rate, so on the short circuit nature and supply

imum value reached by the output current becomes lower, because the capacitor on EN and DIAG pins is dis-

The following approximate relations estimate the disable time and the first OCD intervention delay after the short

circuit (worst case).

The time the device remains disabled is:

EN = DIAG

Output Current

EN = DIAG

Output Current

t

D IS ABLE

t

OC D OF F

()

t

EN R ISE

()

t

DO N

()EN

++

=

t

EN R ISE

()

R

EN

C

EN

V

DD

V

EN LOW

()

–

V

DD

V

TH O N

()

–

--------------------------------------------





ln

⋅⋅

=