AS1970
Data Sheet
- Application Information
4. Choose the trip point for V
IN
rising (V
THR
)
This is the threshold voltage at which the AS1970 -
AS1975 switches its output from low to high as V
IN
rises above the trip point. For this example, choose V
THR
= 3V.
5. Calculate R
2
as:
R
2
= 1/[V
THR
/(V
REF
x R
1
) - (1/R
1
) - (1/R
3
)]
Substituting the example values gives:
R
2
= 1/[3.0V/(1.2V x 12kΩ) - (1/12kΩ) - (1/1.2MΩ)] = 8.05kΩ
In this example, a standard 8.2kΩ resistor should be used for R
2
.
6. Verify the trip voltages and hysteresis as:
V
THR
= V
REF
x R
1
[(1/R
1
) + (1/R
2
) + (1/R
3
)]
V
THF
= V
THR
- (R
1
x V
CC
/R
3
)
Hysteresis = V
THR
- V
THF
(EQ 6)
(EQ 7)
(EQ 8)
(EQ 5)
Hysteresis (AS1971/AS1973/AS1975)
The AS1971/AS1973/AS1975 have 3mV internal hysteresis. Their open-drain outputs require an external pullup resis-
tor (Figure
and additional hysteresis can be generated using positive feedback.
Figure 19. Additional Hysteresis AS1971/AS1973/AS1975
V
CC
R
3
R
4
R
1
V
IN
R
2
V
REF
+
–
V
CC
V
EE
OUT
Resistor Selection Example
For the circuit shown in
the following steps can be used to calculate values for R
1
, R
2
, R
3
, and R
4
:
1. Select R
3
according to one of:
R
3
= V
REF
/500µA
R
3
= (V
REF
- V
CC
)/500µA - R
4
Use the smaller of the two resulting resistor values.
2. Choose the hysteresis band required (V
HB
). For this example, use 50mV.
3. Calculate R
1
as:
R
1
= (R
3
+ R
4
)(V
HB
/V
CC
)
(EQ 11)
4. Choose the trip point for V
IN
rising (V
THR
)
This is the threshold voltage at which the comparator
switches its output from low to high as V
IN
rises above the trip point.
5. Calculate R
2
as:
R
2
= 1/[V
THR
/(V
REF
x R
1
) - (1/R
1
) - 1/(R
3
+ R
4
)]
6. Verify the trip voltages and hysteresis as follows:
V
IN
rising: V
THR
= V
REF
x R
1
x [1/R
1
+ 1/R
2
+ 1/(R
3
+ R
4
)]
V
IN
falling: V
THF
= V
REF
x R
1
x [1/R
1
+ 1/R
2
+ 1/(R
3
+R
4
)] - 1/(R
3
+R
4
) x V
CC
Hysteresis = V
THR
- V
THF
(EQ 13)
(EQ 14)
(EQ 15)
(EQ 12)
(EQ 9)
(EQ 10)
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