LPV521 datasheet(19/29 Pages) TI

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10 M:

VOUT

10 M:

VIN

10 M:

270 pF

CR2032 Coin Cell

225 mAh = 5 circuits @ 9.5 yrs. each

60 Hz Twin T Notch Filter

AV = 2 V/V

10 M:

270 pF

VBATT

VBATT = 3V o 2V @ end of life

LPV521

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SNOSB14C – AUGUST 2009 – REVISED FEBRUARY 2013

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 RSENSE is connected in series with the battery. The theoretical output voltage of the circuit is VOUT = [

(RSENSE × R3) / R1 ] × ICHARGE. In reality, however, due to the finite Current Gain, β, of the transistor the current

that travels through R3 will not be ICHARGE, but instead, will be α × ICHARGE or β/( β+1) × ICHARGE. 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 VOUT ≈ 4000 Ω × ICHARGE. This is ideal to amplify a 1 mA ICHARGE to near full scale of an

ADC with VREF 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 RSENSE will

minimize error due to resistor tolerance, however, this will also decrease VSENSE = ICHARGE × RSENSE, 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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