OPA369 Datasheet, PDF (10/20 Pages) Burr-Brown (TI) – 1.8V, 1μA max, Zerø-Crossover RAIL-TO-RAIL I/O OPERATIONAL AMPLIFIER

OPA369
OPA2369
SBOS414A – AUGUST 2007 – REVISED SEPTEMBER 2007
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 larger valued
resistors cannot be avoided, a small (4pF to 6pF)
capacitor, C_FB, can be inserted in the feedback, as
shown in Figure 25. This configuration significantly
reduces overshoot by compensating the effect of
capacitance, C_IN, which includes the amplifier input
capacitance and printed circuit board (PCB) parasitic
capacitance.
C_FB
R_I
V_IN
RF
OPA369
C_IN
V_OUT
Figure 25. Improving Stability for Large R_F and R_IN
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 26. 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:
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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:
R_F
=
V_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:
R₁ = R_F
V_HYST
V_BATT
= 20MW
50mV
2.4V
= 420kW
(2)
4. Select a threshold voltage for V_IN rising (V_THRS) =
2.0V
5. Calculate R₂ as follows:
R=
1
2
( ) VTHRS
V_BATT
-
1
R₁
-
1
R₁
=
1
( ) ^2V
1.2V ´ 420kW
-
1
420kW
-
1
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
=
1.8V
2mA
=
0.9MW
(4)
+
V_BATT
R₁
I_BIAS
R_BIAS
R₂
REF1112
R_F
+IN
OPA369 ^OUT
-IN
V_REF
V_STATUS
Figure 26. Battery Monitor
10
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