1

10

100

OUTPUT SINK CURRENT (mA)

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

85°C

25°C

-40°C

0

-

+

R

2

LM397

SNOS977D – MAY 2001 – REVISED MARCH 2013

www.ti.com

Figure 10. Inverting Configured Comparator - LM397

Input Stage

Output Stage

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

external pull-up resistor connected between the positive supply voltage and the output. The external pull-up

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

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 10mA. When the maximum current limit is reached,

the output transistor will saturate and the output will rise rapidly (Figure 11).

Figure 11. Output Saturation Voltage vs. Output Sink Current

6

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