AS1972 datasheet(11/18 Pages) AMSCO

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AS1970

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

4. Choose the trip point for VIN rising (VTHR) (see page 12). This is the threshold voltage at which the AS1970 -

AS1975 switches its output from low to high as VIN rises above the trip point. For this example, choose VTHR = 3V.

5. Calculate R2 as:

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

(EQ 5)

Substituting the 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)

Hysteresis (AS1971/AS1973/AS1975)

The AS1971/AS1973/AS1975 have 3mV internal hysteresis. Their open-drain outputs require an external pullup resis-

tor (Figure 19), and additional hysteresis can be generated using positive feedback.

Figure 19. Additional Hysteresis AS1971/AS1973/AS1975

Resistor Selection Example

For the circuit shown in Figure 19, the following steps can be used to calculate values for R1, R2, R3, and R4:

1. Select R3 according to one of:

R3 = VREF/500µA

(EQ 9)

R3 = (VREF - VCC)/500µA - R4

(EQ 10)

Use the smaller of the two resulting resistor values.

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

3. Calculate R1 as:

R1 = (R3 + R4)(VHB/VCC)(EQ 11)

4. Choose the trip point for VIN rising (VTHR) (see page 12). This is the threshold voltage at which the comparator

switches its output from low to high as VIN rises above the trip point.

5. Calculate R2 as:

R2 = 1/[VTHR /(VREF x R1) - (1/R1) - 1/(R3 + R4)]

(EQ 12)

6. Verify the trip voltages and hysteresis as follows:

VIN rising: VTHR = VREF x R1 x [1/R1 + 1/R2 + 1/(R3 + R4)]

(EQ 13)

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

(EQ 14)

Hysteresis = VTHR - VTHF

(EQ 15)

VEE

VCC

–

VCC

VREF

VIN

OUT

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