LM397 datasheet(8/19 Pages) TI1

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VIN

VT2

VT1

100

OUTPUT SINK CURRENT (mA)

0.5

1.5

2.5

3.5

4.5

85°C

25°C

-40°C

LM397

SNOS977E – MAY 2001 – REVISED OCTOBER 2015

www.ti.com

Feature Description (continued)

Figure 7. Output Saturation Voltage vs Output Sink Current

7.4 Device Functional Modes

7.4.1 Hysteresis

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

the input offset voltage of the comparator. 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 Equation 1:

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 Equation 2:

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

(2)

The hysteresis is defined in Equation 3:

ΔVIN = VT1 – VT2

(3)

Figure 8. Inverting Configured Comparator - LM397

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