OPA369 Datasheet, PDF (10/19 Pages) Burr-Brown (TI) – 1.8V, 1μA max, Zerø-Crossover RAIL-TO-RAIL I/O OPERATIONAL AMPLIFIER

OPA369
OPA2369
SBOS414B – AUGUST 2007 – REVISED DECEMBER 2008 ............................................................................................................................................ www.ti.com
BATTERY MONITORING
The low operating voltage and quiescent current of
the OPA369 series make it an excellent choice for
battery monitoring applications, as shown in
Figure 25. In this circuit, V_STATUS is high as long as
the battery voltage remains above 2V. A low-power
reference is used to set the trip point. Resistor values
are selected as follows:
1. Selecting R_F: Select R_F such that the current
through R_F is approximately 1000x larger than
the maximum bias current over temperature:
V
R_F
=
REF
1000(I
)
BMAX
= ^1.2V
1000(50pA)
= 24MW » 20MW
(1)
2. Choose the hysteresis voltage, V_HYST. For
battery-monitoring applications, 50mV is
adequate.
3. Calculate R₁ as follows:
R
1
=
R_F
V
HYST
V
= 20MW
50mV
2.4V
= 420kW
BATT
(2)
4. Select a threshold voltage for V_IN rising (V_THRS) =
2.0V
5. Calculate R₂ as follows:
1
R=
2
( )´ V
THRS
- ¹- ¹
VR
REF
1
R₁ R_F
1
=
( ) ^2V
1.2V ´ 420kW
-¹-¹
420kW 20MW
= 650kW
(3)
6. Calculate R_BIAS: The minimum supply voltage for
this circuit is 1.8V. The REF1112 has a current
requirement of 1.2µA (max). Providing the
REF1112 with 2µA of supply current assures
proper operation. Therefore:
R_BIAS
=
(V_BATTMIN -
I
BIAS
V_REF)
=
(1.8V - 1.2V)
2mA
=
0.3MW
(4)
+
V_BATT
R₁
I_BIAS
R_BIAS
R₂
REF1112
R_F
+IN
OUT
OPA369
-IN
V_REF
V_STATUS
Figure 25. Battery Monitor
10
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