7.2.3.2 Detailed Design Procedure

The theoretical output voltage of the circuit is VOUT = [ (RSENSE × R3) / R1 ] × ICHARGE. In reality, however, as a

result of the finite current gain of the transistor (β), the current that travels through R3 is not ICHARGE. Instead, R3

is 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 7-7 results in VOUT ≈ 4000Ω × ICHARGE. This result is needed to amplify a

1mA ICHARGE to near full-scale of an analog-to-digital converter (ADC) with VREF at 4.1V. A resistor, R2 is used at

the noninverting input of the amplifier, with the same value as R1 to minimize offset voltage.

Selecting values per Figure 7-7 limits the current traveling through the R1 – Q1 – R3 leg of the circuit to under

1µA, which is on the same order as the LPV521 supply current. Increasing resistors R1, R2, and R3 decreases

the measuring circuit supply current and extends battery life.

Decreasing RSENSE minimizes error due to resistor tolerance; however, this decrease also decreases VSENSE =

ICHARGE × RSENSE, and in turn, the amplifier offset voltage has a more significant contribution to the total error of

the circuit. With the components shown in Figure 7-7, the measurement circuit supply current can be kept below

1.5µA and measure 100µA to 1mA.

7.2.3.3 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

0.25

0.5

0.75

1

1.25

1.5

ICHARGE (mA)

C001

Figure 7-8. Calculated High-Side Current Sense Circuit Output

7.3 Power Supply Recommendations

The LPV521 is specified for operation from 1.6V to 5.5V (±0.8V to ±2.75V) over a –40°C to +125°C temperature

range. Parameters that can exhibit significant variance with regard to operating voltage or temperature are

presented in the Section 5.6.

CAUTION

Supply voltages greater than 6 V can permanently damage the device.

Low bandwidth nanopower devices do not have good high frequency (> 1kHz) ac PSRR rejection against

high-frequency switching supplies and other kHz and greater noise sources; therefore, extra supply filtering is

recommended if kHz-range noise is expected on the power-supply lines.

LPV521

SNOSB14E – AUGUST 2009 – REVISED JULY 2024

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