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| 型號： | AS1977-T |
| 廠商： | AUSTRIAMICROSYSTEMS AG |
| 元件分類： | 電源管理 |
| 英文描述： | nPower Single Comp. Open-Drain; Package Type: SOT23-5 |
| 封裝： | SOT23-5 |
| 文件頁數(shù)： | 12/17頁 |
| 文件大?。?/td> | 422K |
| 代理商： | AS1977-T |
www.austriamicrosystems.com
Revision 1.01
12 - 17
AS1976/AS1977
Data Sheet - Application Information
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 R
1
, R
2
, and R
3
.
1. First select the value for R
3
. Leakage current at IN is less than 2nA, thus the current through R
3
should be at least
0.2μA to minimize errors due to leakage current. The current through R
3
at the trip point is:
(V
REF
- V
OUT
)/R
3
(EQ 1)
Taking into consideration the two possible output states, solving for R
3
yields two formulas:
R
3
= V
REF
/I
R3
R
3
= (V
CC
- V
REF
)/I
R3
(EQ 2)
(EQ 3)
Use the smaller of the two resulting values for R
3
. For example, for V
REF
= 1.245V, V
CC
= 3.3V, and I
R3
= 1μA, the
two resistor values are 1.2M
Ω
and 2.0M
Ω
, therefore choose a 1.2M
Ω
standard resistor for R
3
.
2. Choose the required hysteresis band (V
HB
). For this example, choose 33mV.
3. Calculate R
1
as:
R
1
= R
3
(V
HB
/V
CC
)
(EQ 4)
Substituting the R
1
and V
HB
example values gives:
R
1
= 1.2M
Ω
(50mV/3.3V) = 12k
Ω
4. Choose the trip point for V
IN
rising (V
THR
) such that V
THR
> V
REF
(R
1
+ R
3
)/R
3
. For this example, choose 3V.
5. Calculate R
2
as:
R
2
= 1/[V
THR
/(V
REF
x R
1
) - (1/R
1
) - (1/R
3
)]
(EQ 5)
Substituting the R
1
and R
3
example values gives:
R
2
= 1/[3.0V/(1.2V x 12k
Ω
) - (1/12k
Ω
) - (1/1.2M
Ω
)] = 8.05k
Ω
In this example, a standard 8.2k
Ω
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)
OUT
R
1
R
3
V
CC
V
IN
V
REF
R
2
+
V
EE
V
CC
–
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