AS1976 datasheet(13/17 Pages) AMSCO

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AS1976/AS1977

Data Sheet - A p p l i c a t i o n I n f o r m a t i o n

Additional Hysteresis (AS1977)

Additional hysteresis can be added to the AS1977 and AS1979 with 4 resistors and positive feedback (see Figure 28).

Figure 28. AS1977 Additional Hysteresis

Resistor Selection Example

For the circuit shown in Figure 28, use the following steps to calculate values for R1, R2, R3, and R4.

1. Select R3 according to one of these formulas:

R3 = VREF/1µA

(EQ 9)

R3 = (VCC - VREF)/1µA - R4

(EQ 10)

Use the smaller of the two resulting resistor values for R3.

2. Choose the hysteresis band required (VHB).

3. Calculate R1 as:

R1 = (R3 + R4)(VHB/VCC)

(EQ 11)

4. Choose the trip point for VIN rising (VTHR).

5. Calculate R2 as:

R2 = 1/[VTHR/(VREF x R1) - (1/R1) - 1/R3](EQ 12)

6. Verify the trip voltages and hysteresis as:

VIN rising: VTHR = VREF[R1(1/R1 + 1/R2 + 1/R3)]

(EQ 13)

VIN falling: VTHF = VREF[R1(1/R1 + 1/R2 + 1/(R3+R4))] - [1/(R3+R4)]VCC

(EQ 14)

Hysteresis = VTHR - VTHF

(EQ 15)

Zero-Crossing Detector

Figure 29 shows the AS1976 in a zero-crossing detector circuit. The inverting input (IN-) is connected to ground, and

the non-inverting input (IN+) is connected to a 100mVp-p signal source. When the signal at IN- crosses 0V, the signal

at OUT changes states.

Figure 29. Zero Crossing Detector

VEE

VCC

–

VCC

VREF

VIN

OUT

–

100mVp-p

–

AS1976

VCC

IN+

OUT

VEE

IN-

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