AS1976 datasheet(13/18 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 (AS1976)

Additional hysteresis can be added to the AS1976 and AS1978 with three resistors and positive feedback (see Figure

27), however, this positive feedback method slows hysteresis response time.

Figure 27. AS1976 Additional Hysteresis

Resistor Selection Example

For the circuit shown in Figure 27, use the following steps to calculate values for R1, R2, and R3.

1. First select the value for R3. Leakage current at IN is less than 2nA, thus the current through R3 should be at least

0.2µA to minimize errors due to leakage current. The current through R3 at the trip point is:

(VREF - VOUT)/R3

(EQ 1)

Taking into consideration the two possible output states, solving for R3 yields two formulas:

R3 = VREF/IR3

(EQ 2)

R3 = (VCC - VREF)/IR3

(EQ 3)

Use the smaller of the two resulting values for R3. For example, for VREF = 1.245V, VCC = 3.3V, and IR3 = 1µA, the

two resistor values are 1.2M

Ω and 2.0MΩ, therefore choose a 1.2MΩ standard resistor for R3.

2. Choose the required hysteresis band (VHB). For this example, choose 33mV.

3. Calculate R1 as:

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

Substituting the R1 and VHB example values gives:

R1 = 1.2M

Ω(50mV/3.3V) = 12kΩ

4. Choose the trip point for VIN rising (VTHR) such that VTHR > VREF(R1 + R3)/R3. For this example, choose 3V.

5. Calculate R2 as:

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

(EQ 5)

Substituting the R1 and R3 example values gives:

R2 = 1/[3.0V/(1.2V x 12k

Ω) - (1/12kΩ) - (1/1.2MΩ)] = 8.05kΩ

In this example, a standard 8.2k

Ω resistor should be used for R2.

6. Verify the trip voltages and hysteresis as:

VTHR = VREF x R1[(1/R1) + (1/R2) + (1/R3)]

(EQ 6)

VTHF = VTHR - (R1 x VCC/R3)(EQ 7)

Hysteresis = VTHR - VTHF

(EQ 8)

OUT

VCC

VIN

VREF

VEE

VCC

–

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