CS70 datasheet(19/24 Pages) STMICROELECTRONICS

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DocID026041 Rev 1

19/24

CS70

Application information

The CS70 can be used to measure current and to feed back the information to a

microcontroller.

Figure 22. Single-supply configuration schematic

The current from the supply flows to the load through the Rsense resistor, causing a voltage

drop equal to Vsense across Rsense. The amplifier’s input currents are negligible, therefore its

inverting input voltage is equal to Vm. The amplifier's open-loop gain forces its non-inverting

input to the same voltage as the inverting input. Consequently, the amplifier adjusts the

current flowing through Rg1 so that the voltage drop across Rg1 matches Vsense exactly.

Therefore, the drop across Rg1 is:

VRg1 =Vsense =Rsense.Iload

If IRg1 is the current flowing through Rg1, then IRg1 is given by the formula:

IRg1 =Vsense/Rg1

The IRg1 current flows entirely into resistor Rg3 (the input bias current of the buffer is

negligible). Therefore, the voltage drop on the Rg3 resistor can be calculated as follows.

VRg3 =Rg3.IRg1 =(Rg3/Rg1).Vsense= K1.Vsense with K1=Rg3/Rg1.

The voltage across the Rg3 resistor is buffered to the Out pin by the voltage buffer, featuring

a gain equal to K2. Therefore Vout can be expressed as:

Vout =K1.K2.Vsense = Av.Vsense with Av= K1.K2

or: Vout = Av .Rsense.Iload

AM04517v1

5 V

Vsense

Iload

Rsense

Common-mode voltage: 2.9 V to 70 V

Rg1

Rg2

Vcc+

Vcc

SEL1

Vout

Iload

Rsense

Rg3

Out

Rg1

Rg2

Voltage

buffer

Sense

amplifier

Vcc+

CS70

Gnd

Vcc-

μ Controller

ADC

Vcc

GPIO2

Gnd

SEL1

SEL2

GPIO1

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