VIN+

VIN-

VS

GND

OUTPUT

VOLTS

VREF

VO

TIME

VIN

VREF

VIN

VO

-

+

V

-

RPULL-UP

VS

7

LM397

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SNOS977F – MAY 2001 – REVISED MAY 2016

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7 Detailed Description

7.1 Overview

A comparator is often used to convert an analog signal to a digital signal. The comparator compares an input

voltage (VIN) at the noninverting pin to the reference voltage (VREF) at the inverting pin. If VIN is less than VREF

the output (VO) is low (VOL). However, if VIN is greater than VREF, the output voltage (VO) is high (VOH). Refer to

Figure 6.

Figure 5. Basic Comparator

Figure 6. Basic Comparator Output

7.2 Functional Block Diagram

7.3 Feature Description

7.3.1 Input Stage

The LM397 has a bipolar input stage. The input common-mode voltage range is from 0 to (VS – 1.5 V).

7.3.2 Output Stage

The LM397 has an open-collector grounded-emitter NPN output transistor for the output stage. This requires an

external pullup resistor connected between the positive supply voltage and the output. The external pullup

resistor should be high enough resistance so to avoid excessive power dissipation. In addition, the pullup resistor

should be low enough resistance to enable the comparator to switch with the load circuitry connected. Because it

is an open-collector output stage, several comparator outputs can be connected together to create an OR’ing

function output. With an open collector, the output can be used as a simple SPST switch to ground. The amount

of current which the output can sink is approximately 10 mA. When the maximum current limit is reached, the

output transistor will saturate and the output will rise rapidly (Figure 7).