Rcg resistance selection

AN3395

4/11

Doc ID 018779 Rev 1

2

Rcg resistance selection

The Rcg resistor is used to sense the current flowing "into" or "out of" the battery. The

voltage drop on Rcg is input to the current measurement ADC through the CG pin. There

are three common rules for the selection of the Rcg resistance:

1.

Maximum peak current

2.

Power rating

3.

ADC code usage

2.1

Maximum peak current in the application

As specified in the datasheet (refer to the STC3105 or STC3100 datasheet), the voltage

drop across the Rcg resistor (input voltage range on CG pin) must not exceed ±80 mV. That

Equation 1.

Equation 1

2.2

Power rating of the resistor

The second step is to consider the power dissipation limit of the resistor as given in Equation

2. The power dissipation in the resistor must be kept within the power rating of the resistor

calculated by:

Equation 2

2

Note:

Must be less than the power rating of the resistor

However, it may be better to choose a smaller resistance value with a smaller power rating

to:

●

have a smaller PCB footprint and

●

reduce the power loss in the resistor.

2.3

ADC code usage efficiency

The full scale voltage range of the ADC is designed for the input on the CG pin to reach

±80 mV (max). To make better use of the ADC performance, Rcg must not be too small:

Rcg m

Ω

()

80 mv

()

I

()

-----------------------

≤