a manner that there is virtually no current flowing into the

current 20 mA maximum for the comparator input stage

for equal potentials on each of the comparator inputs) how-

lowing formula

b

ed to provide transient protection for the Sense pin espe-

cially when driving inductive type loads A minimum value of

output goes high the total bias current for the input stage

transfers from the Sense pin to the Threshold pin thereby

changing the voltages present at the inputs to the compara-

tor For consistent switching of the comparator right at the

desired threshold point the voltage drop that occurs at the

non-inverting input (Threshold) should equal or exceed the

rise in voltage at the inverting input (Sense)

In automotive applications the load supply may be the bat-

switched OFF there is always a concern for the amount of

current drained from the battery The only current drain un-

der this condition is a leakage current into the Threshold pin

which is less than 10 mA

help keep the reference voltage constant in applications

noise This bypass capacitor should be no larger than

01 mF and is not needed for most applications

DELAY TIMER

To allow the MOSFET to conduct currents beyond the pro-

tection threshold for a brief period of time a delay timer

function is provided This timer delays the actual latching

OFF of the MOSFET for a programmable interval This fea-

ture is important to drive loads which require a surge of

current in excess of the normal ON current upon start up or

at any point in time such as lamps and motors

Figure 4

details the delay timer circuitry A capacitor connected from

the Delay pin 8 to ground sets the delay time interval With

the MOSFET turned ON and all conditions normal the out-

put of the protection comparator is low and this keeps the

discharge transistor ON This transistor keeps the delay ca-

pacitor discharged Should a surge of load current trip the

protection comparator high the discharge transistor turns

OFF and an internal 10 mA current source begins linearly

charging the delay capacitor

enough for the capacitor to charge to the timing comparator

threshold of typically 55V the output of the comparator will

go high to set a flip-flop and immediately latch the MOSFET

OFF It will not re-start until the ONOFF Input is toggled

low then high

can be found from

Charging of the delay capacitor is clamped at approximately

75V which is the internal bias voltage for the 10 mA current

source

MINIMUM DELAY TIME

A minimum delay time interval is required in all applications

due to the nature of the protection circuitry At the instant

the MOSFET is commanded ON the voltage across the

cause the source is held at ground by the load This condi-

tion will immediately trip the protection comparator Without

a minimum delay time set the timing comparator will trip

and force the MOSFET to latch OFF thereby never allowing

the load to be energized

TLH12317 – 12

FIGURE 4 Delay Timer

9