AN2390 datasheet(12/42 Pages) STMICROELECTRONICS

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Evaluation board implementation

AN2390

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2.2.2

Current measurement

The current measurement circuit is given in Figure 6.

Figure 6.

Voltage and current measurement circuit

As shown in the above diagram, a shunt (R23) is connected in series to the battery in order

to measure the charging current. The drop across this sense resistor is further amplified by

using the OP-AMP (Operational Amplifier) LM258A for better resolution and this amplified

voltage is measured by the ST7 microcontroller using ADC channel AIN1. The amplification

factor is chosen such that the OP-AMP output has a voltage range between ground and

VDD.

2.2.3

Voltage measurement

The voltage measurement circuit is also given in Figure 6 above, where the ST7 ADC

channel, AIN0 is used for voltage measurement. In the evaluation board, while the input

supply voltage Vsupply = 5 V, the microcontroller is supplied with VDD = 3.3 V. Therefore, it is

not possible to read the battery voltage directly, but this voltage is attenuated by using a

resistor bridge (R19, R20). However, this attenuation must still allow us to make full use of the

whole ADC input range (0 to VDD).

Note:

The ST7 MCU does not measure VBAT, it measures VB, which is proportional to (VBAT + RS*

IBAT). Some calculation must be performed on the conversion results to get the actual

battery voltage.

R19

5k (0.5%)

R20

5k (0.5%)

AIN0

VBAT

V+

V-

AIN8

BTI

LI-ION BATTERY

AIN1

R22

3.3k (0.5%)

13.32k (0.5%)

R24

VCC

GND

LM258AD

R23

0.5,1W (0.5%)

VIN

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