LM397MFX datasheet(5/8 Pages) NSC

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Application Notes (Continued)

Input Stage

The LM397 has a bipolar input stage. The input common

mode voltage range is from 0 to (V

S – 1.5V).

Output Stage

The LM397 has an open collector grounded-emitter NPN

output transistor for the output stage. This requires an exter-

nal 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 4).

20022107

FIGURE 4. Output Saturation Voltage vs. Output Sink

Current

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