AS1975 datasheet(10/18 Pages) AMSCO

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AS1970

Data Sheet - Ap p lica tio n In fo r m a tio n

9 Application Information

Figure 17 shows a typical application circuit for the AS1970 - AS1975 comparators.

Figure 17. Typical Application Diagram – Threshold Detector

Hysteresis (AS1970/AS1972/AS1974)

The AS1970/AS1972/AS1974 have 3mV internal hysteresis. Additional hysteresis can be generated with three resis-

tors using positive feedback (Figure 18), however this method also slows hysteresis response time.

Figure 18. Additional Hysteresis AS1970/AS1972/AS1974

Resistor Selection Example

For the circuit shown in Figure 18, the following steps can be used to calculate values for R1, R2, and R3.

1. Select R3 first. The current through R3 should be at least 1µA to minimize errors caused by leakage current. The

current through R3 at the trip point is:

(VREF - VOUT)/R3

(EQ 1)

The two possible output states in solving for R3 yields these two formulas:

R3 = VREF/1µA

(EQ 2)

R3 = (VREF - VCC)/1µA

(EQ 3)

For example, for VREF = 1.2V and VCC = 5V, the two R3 resistor values are 1.2M

Ω and 3.8MΩ. Use the smaller of

the two resulting resistor values; in this case a standard 1.2M

Ω resistor should be used for R3.

2. Choose the hysteresis band (VHB). For this example, use VHB = 50mV.

3. Calculate R1 according to the following equation:

R1 = R3(VHB/VCC)(EQ 4)

Substituting the example values for R3 and VHB gives:

R1 = 1.2M

Ω(50mV/5V) = 12kΩ

AS1970 - AS1975

RPULLUP

†

† AS1971/AS1973/AS1975 only

VIN

IN-

OUT

VCC

VEE

IN+

OUT

VCC

VIN

VREF

VEE

VCC

–

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