LM612AMN datasheet(8/10 Pages) NSC

8 / 10 page

Application Information (Continued)

of hysteresis can substantially reduce the accuracy of the

comparator since the offset voltage is effectively being in-

creased by the hysteresis when the comparator output is

high

It is often a good idea to decrease the amount of hysteresis

until oscillations are observed then use three times that

minimum hysteresis in the final circuit Note that the amount

of hysteresis needed is greatly affected by layout The

amount of hysteresis should be rechecked each time the

layout is changed such as changing from a breadboard to a

PC board

Input Stage

The input stage uses lateral PNP input transistors which

unlike those of many op amps have breakdown voltage

BVEBO equal to the absolute maximum supply voltage Also

they have no diode clamps to the positive supply nor across

the inputs These features make the inputs look like high

impedances to input sources producing large differential

and common-mode voltages

The guaranteed common-mode input voltage range for an

LM612 is Vb s VCM s (Va b 18V) over temperature

This is the voltage range in which the comparisons must be

made If both inputs are within this range the output will be

at the correct state If one input is within this range and the

other input is less than (Vb a32V) even if this is greater

than Va the output will be at the correct state If however

either or both inputs are driven below Vb and either input

current exceeds 10 mA the output state is not guaranteed

to be correct If both inputs are above (Va b 18V) the

output state is also not guaranteed to be correct

Output Stage

The comparators have a common open-collector output

stage which requires a pull-up resistor to a positive supply

voltage for the output to switch properly When the internal

output transistor is off the output (HIGH) voltage will be

pulled up to this external positive voltage

To ensure that the LOW output voltage is under the TTL-low

threshold the output transistor’s load current must be less

than 08 mA (over temperature) when it turns on This im-

pacts the minimum value of the pull-up resistor

Typical Applications

TLH11058 – 12

Power Supply Monitor with Indicator

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