7.2.2.2 Detailed Design Procedure

Figure 7-5 shows a typical circuit used to amplify the output of an oxygen detector. The oxygen sensor outputs

a known current through the load resistor. This value changes with the amount of oxygen present in the air

sample. Oxygen sensors usually recommend a particular load resistor value or specify a range of acceptable

values for the load resistor. The use of the nanopower LPV521 means minimal power usage by the op amp,

and enhanced battery life. With the components shown in Figure 7-5, the circuit consumes less than 0.5µA

of current, so that even batteries used in compact portable electronics, with low mAh charge ratings, can last

beyond the life of the oxygen sensor. The precision specifications of the LPV521, such as the very low offset

voltage, low TCVOS, low input bias current, high CMRR, and high PSRR are other factors that make the LPV521

a great choice for this application.

7.2.2.3 Application Curve

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

0

10

20

30

40

50

VSENSOR (mV)

C001

Figure 7-6. Calculated Oxygen Sensor Circuit Output (Single 5V Supply)

7.2.3 High-Side Battery Current Sensing

10:

VOUT

LOAD

Q1

2N2907

RSENSE

R1

24.9 k:

R2

24.9 k:

R3

10 M:

ICHARGE

V

+

V

+

VOUT =

X ICHARGE

RSENSE X R3

R1

Figure 7-7. High-Side Current Sensing

7.2.3.1 Design Requirements

The rail-to-rail common-mode input range and the very low quiescent current make the LPV521 an excellent

choice for use in high-side and low-side battery current-sensing applications. The high-side current-sensing

circuit in Figure 7-7 is commonly used in a battery charger to monitor the charging current to prevent

overcharging. A sense resistor RSENSE is connected in series with the battery.

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LPV521

SNOSB14E – AUGUST 2009 – REVISED JULY 2024

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