OPA379AIDBVR Datasheet, PDF (10/29 Pages) Texas Instruments – 1.8V, 2.9mA, 90kHz, Rail-to-Rail I/O PERATIONAL AMPLIFIERS

OPA379
OPA2379
OPA4379
SBOS347D – NOVEMBER 2005 – REVISED MAY 2008 ................................................................................................................................................... www.ti.com
In unity-gain inverter configuration, phase margin can
be reduced by the reaction between the capacitance
at the op amp input and the gain setting resistors.
Best performance is achieved by using smaller
valued resistors. However, when large valued
resistors cannot be avoided, a small (4pF to 6pF)
capacitor, C_FB, can be inserted in the feedback, as
shown in Figure 22. This configuration significantly
reduces overshoot by compensating the effect of
capacitance, C_IN, which includes the amplifier input
capacitance (3pf) and printed circuit board (PC)
parasitic capacitance.
C_FB
R_IN
V_IN
R_F
OPA379
C_IN
V_OUT
Figure 22. Improving Stability for Large R_F and R_IN
R_F
=
V_REF
1000(I_BMAX)
=
1.2V
1000(100pA)
= 12MW » 10MW
(1)
2. Choose the hysteresis voltage, V_HYST. For battery
monitoring applications, 50mV is adequate.
3. Calculate R₁ as follows:
R =R
1
F
V
HYST
V_BATT
= 10MW
50mW
2.4V
= 210kW
(2)
4. Select a threshold voltage for V_IN rising (V_THRS) =
2.0V
5. Calculate R₂ as follows:
1
R₂ =
V_THRS
- ¹-¹
V_REF ´ R₁
R₁
R_F
1
=
2V
- ¹-¹
1.2V ´ 210kW 210kW 10MW
BATTERY MONITORING
The low operating voltage and quiescent current of
the OPA379 series make it an excellent choice for
battery monitoring applications, as shown in
Figure 23. 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. R_F Selecting: Select R_F such that the current
through R_F is approximately 1000x larger than
the maximum bias current over temperature:
= 325kW
(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 2µA of
supply current assures proper operation.
Therefore:
R_BIAS =
(V_BATTMIN - V_REF) ₌
I_BIAS
(1.8V - 1.2V) _{= 0.3MW}
2mA
(4)
+
V_BATT
R₁
I_BIAS
R_BIAS
R₂
REF1112
R_F
+IN
OUT
OPA379
-IN
V_REF
V_STATUS
Figure 23. Battery Monitor
10
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