Diagnosis of a Short to VS

During the short current through the sink transistor will rise more rapidly than under normal load condi-

tions. Reaching a peak current of 1.5 times the maximum PWM current between typically 2

µs and 5µs

Detecting a short the low side transistor will try to turn on again the next 7 trigger pulse of the oscillator.

Simultaneously the error flag will updated on each pulse.

t

turn on of the sink transistor

activation of short threshold

PWM threshold

PWM detection

signal (internal)

Short detection

signal (internal)

Error 1

t

t

t

D99AT422

Figure 6. Normal PWM current versus short circuit current and detection of short to VS..

the current of current setting LL).

Overcurrent now will set the error flag.

current will just turn off the sink transistor. This is considered as normal operation. The error flag is de-

tached from the comparator after tPWM so no error flag is set during normal pulse width modulation.

Short from an Output to Ground

The current through the short will be detected by the protection of the source transistor. The source tran-

sistor will turn off exceeding a current of typically 1.8A. Minimum overload detection current is 1.2A. To

obtain proper current regulation (by the sink transistors and not by source transistor shut down) the

maximum current of the PWM regulator should be set to a maximum value of 1.1A.

L9935

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