AN3395
Application note
Sensing resistor selection and usage
in STC310x battery monitoring applications
Introduction
Voltage measurement and coulomb counting are the two most common methods used to
implement battery monitoring for gas gauge applications. Although the use of voltage
measurement has been a popular method, it does not produce the most accurate results.
The STC310x series battery monitor ICs developed by STMicroelectronics combine the two
methods into one integrated solution. It updates the battery State-of-Charge (SOC) at light
load (relaxation/standby period) with the real battery Open-Circuit-Voltage (OCV) while
using coulomb counting to track the battery capacity under heavy load to provide the most
accurate SOC value under all application conditions.
In coulomb counting, the sensing resistor is used to measure the battery current. The
specified maximum voltage drop on the sensing resistor is only 80 mV, thus it plays an
important role in the gas gauge accuracy and merits careful attention. This document
describes:
■
■
■
the sensing resistor (Rcg) selection
the Rcg power considerations
the Rcg layout recommendations
December 2011
Doc ID 018779 Rev 1
1/11
www.st.com
Contents
AN3395
Contents
1
2
STC310x external components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Rcg resistance selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1
2.2
2.3
2.4
Maximum peak current in the application . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Power rating of the resistor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
ADC code usage efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
Selection of Rcg (example) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3
4
5
6
Rcg power loss consideration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Rcg layout considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2/11
Doc ID 018779 Rev 1
AN3395
STC310x external components
1
STC310x external components
Figure 1
illustrates the typical connections for a gas gauge application using the STC3105.
The SDA, SCL and ALM (I/O0 in the STC3100) pins are open drain and require external
pull-up resistors to either system I/O voltage or V
CC
(pull up to battery voltage). The
components shown in
Figure 1
connected to the V
CC
and V
IN
pins are used to provide
additional ESD protection and input filtering, please refer to AN3064 for more information.
The resistor (Rcg) connected between the CG and GND pins is the sensing resistor. In
order to obtain higher accuracy, refer to the following application guidelines.
Figure 1.
STC3105 typical connections
System
I/O
supply
Rpu1
Rpu2
Rpu3
STC3105
C1 D1
ALM
VCC
R2
MCU
SDA
VIN
C2
SCL
GND
Gnd
AM045291v1
System supply
R1
Battery
CG
R
cg
Doc ID 018779 Rev 1
3/11
Rcg resistance selection
AN3395
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.
2.
3.
Maximum peak current
Power rating
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
is Rcg x I
PEAK
must be
≤
80 mV. This gives a maximum limit for the Rcg resistor value in
Equation 1.
Equation 1
80
(
mv
)
Rcg
(
mΩ
) ≤
-----------------------
I
PEAK
(
A
)
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
Power dissipation = Rcg x I
RMS2
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 x I
PEAK
must be > 40 mV for a reasonable ADC code usage.
4/11
Doc ID 018779 Rev 1
AN3395
Rcg resistance selection
2.4
Selection of Rcg (example)
Assume I
PEAK
= 2.2 A and I
RMS
= 1.5 A in a mobile phone.
According to
Equation 1,
the maximum limit of Rcg is obtained, that is Rcg < 36 mΩ.
Let's choose a 33 mΩ resistor.
Power rating = 33 mΩ x 1.5
2
A = 74 mW
Therefore, a 1/8 W (125 mW) resistor is sufficient, however, it is possible to use a 20 mΩ
resistor that will only dissipate 45 mW instead of 74 mW.
A 20 mΩ resistor is optimal because 20 mΩ x 2.2 A = 44 mV, which is acceptable in
comparison with the full scale range of 80 mV.
Doc ID 018779 Rev 1
5/11
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