LPV521_15 Datasheet, PDF (24/33 Pages) Texas Instruments – LPV521 NanoPower, 1.8-V, RRIO, CMOS Input, Operational Amplifier

LPV521
SNOSB14D – AUGUST 2009 – REVISED DECEMBER 2014
Typical Applications (continued)
8.2.2.3 Application Curve
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0
10
20
30
40
50
VSENSOR (mV)
C001
Figure 67. Calculated Oxygen Sensor Circuit Output (Single 5V Supply)
8.2.3 High-Side Battery Current Sensing
I_CHARGE
R_SENSE
V⁺
LOAD
R₂
24.9 k:
10:
R₁
24.9 k:
V⁺
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R_SENSE X R₃
V_OUT =
R₁
X I_CHARGE
R₃
10 M:
Q1
2N2907
V_OUT
Figure 68. High-Side Current Sensing
8.2.3.1 Design Requirements
The rail-to-rail common mode input range and the very low quiescent current make the LPV521 ideal to use in
high-side and low-side battery current sensing applications. The high-side current sensing circuit in Figure 68 is
commonly used in a battery charger to monitor the charging current in order to prevent over charging. A sense
resistor R_SENSE is connected in series with the battery.
8.2.3.2 Detailed Design Procedure
The theoretical output voltage of the circuit is V_OUT = [ ®_SENSE × R₃) / R₁ ] × I_CHARGE. In reality, however, due to
the finite Current Gain, β, of the transistor the current that travels through R₃ will not be I_CHARGE, but instead, will
be α × I_CHARGE or β/( β+1) × I_CHARGE. A Darlington pair can be used to increase the β and performance of the
measuring circuit.
Using the components shown in Figure 68 will result in V_OUT ≈ 4000 Ω × I_CHARGE. This is ideal to amplify a 1 mA
I_CHARGE to near full scale of an ADC with V_REF at 4.1 V. A resistor, R2 is used at the noninverting input of the
amplifier, with the same value as R1 to minimize offset voltage.
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