LPV521MGX Datasheet, PDF (19/29 Pages) Texas Instruments – LPV521 Nanopower, 1.8V, RRIO, CMOS Input, Operational Amplifier

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LPV521
SNOSB14C – AUGUST 2009 – REVISED FEBRUARY 2013
V_BATT = 3V o 2V @ end of life
CR2032 Coin Cell
225 mAh = 5 circuits @ 9.5 yrs. each
10 M:
10 M:
10 M:
10 M:
V_IN
270 pF
270 pF
V_BATT
-
+
V_OUT
10 M:
10 M:
60 Hz Twin T Notch Filter
A_V = 2 V/V
270 pF 270 pF
Figure 62. 60 Hz Notch Filter
Figure 63. 60 Hz Notch Filter Waveform
BATTERY CURRENT SENSING
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 64 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. The theoretical output voltage of the circuit is V_OUT = [
(R_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 64 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 4.1V. A resistor, R2 is used at the non-inverting input of the amplifier, with the same value as
R1 to minimize offset voltage. Selecting values per Figure 64 will limit the current traveling through the R₁ – Q1 –
R₃ leg of the circuit to under 1 µA which is on the same order as the LPV521 supply current. Increasing resistors
R₁ , R₂ , and R₃ will decrease the measuring circuit supply current and extend battery life. Decreasing R_SENSE will
minimize error due to resistor tolerance, however, this will also decrease V_SENSE = I_CHARGE × R_SENSE, and in turn
the amplifier offset voltage will have a more significant contribution to the total error of the circuit. With the
components shown in Figure 64 the measurement circuit supply current can be kept below 1.5 µA and measure
100 µA to 1 mA..
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