LM397MFX datasheet(5/14 Pages) TI

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VOLTS

VREF

TIME

VIN

VREF

VIN

RPULL-UP

LM397

www.ti.com

SNOS977D – MAY 2001 – REVISED MARCH 2013

APPLICATION NOTES

Basic Comparators

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

input voltage (VIN) at the non-inverting 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 9.

Figure 9. Basic Comparator

Hysteresis

The basic comparator configuration may oscillate or produce a noisy output if the applied differential input is near

the comparator’s input offset voltage. This tends to occur when the voltage on the input is equal or very close to

the other input voltage. Adding hysteresis can prevent this problem. Hysteresis creates two switching thresholds

(one for the rising input voltage and the other for the falling input voltage). Hysteresis is the voltage difference

between the two switching thresholds. When both inputs are nearly equal, hysteresis causes one input to

effectively move quickly pass the other. Thus, effectively moving the input out of region that oscillation may

occur.

For an inverting configured comparator, hysteresis can be added with a three resistor network and positive

feedback. When input voltage (VIN) at the inverting node is less than non-inverting node (VT), the output is high.

The equivalent circuit for the three resistor network is R1 in parallel with R3 and in series with R2. The lower

threshold voltage VT1 is calculated by:

VT1 = ((VS R2) / (((R1 R3) / (R1 + R3)) + R2))

(1)

When VIN is greater than VT, the output voltage is low. The equivalent circuit for the three resistor network is R2

in parallel with R3 and in series with R1. The upper threshold voltage VT2 is calculated by:

VT2 = VS ((R2 R3) / (R2 + R3)) / (R1 + ((R2 R3) / (R2 + R3)))

(2)

The hysteresis is defined as

ΔVIN = VT1 – VT2

(3)

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