LMV7255M7 datasheet(7/11 Pages) NSC

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Application Info

Basic Comparators

A comparator is quite often used to convert an analog signal

to a digital signal. The comparator compares an input volt-

age (V

IN) at the non-inverting pin to the reference voltage

REF) at the inverting pin. If VIN is less than VREF the output

O) is low (VOL). However, if VIN is greater than VREF, the

output voltage (V

O) is high (VOH).

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 volt-

age). 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.

Hysteresis can easily be added to a comparator in a

non-inverting configuration with two resistors and positive

feedback

Figure 3. The output will switch from low to high

when V

IN rises up to VIN1, where VIN1 is calculated by

IN1 =(VREF (R1 + R2))/ R2

The output will switch from high to low when V

IN falls to VIN2,

where V

IN2 is calculated by

IN2 =(VREF (R1+R2)-VCC R1)/R2

The Hysteresis is the difference between V

IN1 and VIN2.

∆V

IN =VIN1-VIN2 = ((VREF (R1 + R2)) / R2) - ((VREF (R1 +

R2)-V

CC R1) / R2) = VCC R1/ R2.

For an inverting configured comparator, hysteresis can be

added with a three resistor network and positive feedback.

When input voltage (V

IN) at the inverting node is less than

non-inverting node (V

T), 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 V

T1 is

calculated by:

T1 = ((VCC R2) / ((R1 R3) / (R1+ R3)) + R2)

When V

IN 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 V

T2 is calculated by:

T2 =VCC ((R2 R3) / (R2 + R3)) / ((R1 + ((R2 R3) / (R2 +

R3))

The hysteresis is defined as

∆V

IN =VT1 -VT2 = ((VCC R2) / ((R1 R3) / (R1+ R3)) + R2) –

CC ((R2 R3) / (R2 + R3)) / ((R1 + ((R2 R3) / (R2 + R3)))

LMV7251

DS200057-15

LMV7255

DS200057-16

Input/Output

DS200057-17

FIGURE 2. Basic Comparator

DS200057-18

DS200057-19

FIGURE 3. Non-Inverting Comparator Configuration —

LMV7251

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