COMPARATORS

Any of the comparators or the reference may be biased in

any way with no effect on the other sections of the LM615

except when a substrate diode conducts (see Electrical

Characteristics Note 3) For example one or both inputs of

one comparator may be outside the input voltage range lim-

its the reference may be unpowered and the other compar-

ators will still operate correctly Unused comparators should

Hysteresis

Any comparator may oscillate or produce a noisy output if

the applied differential input voltage is near the compara-

tor’s offset voltage This usually happens when the input

signal is moving very slowly across the comparator’s switch-

ing threshold This problem can be prevented by the addi-

tion of hysteresis or positive feedback as shown in

Figure

12

TLH11057 – 20

The amount of hysteresis added in

Figure 12 is

A good rule of thumb is to add hysteresis of at least the

maximum specified offset voltage More than about 50 mV

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

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

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

current exceeds 10 mA the output state is not guaranteed

output state is also not guaranteed to be correct

Output Stage

The comparators have open-collector output stages which

require a pull-up resistor from each output pin to a positive

supply voltage of 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

10