DS2438 Dallas Semiconducotr, DS2438 Datasheet - Page 6

• Manufacturer Part Number: DS2438
• Description: Smart Battery Monitor
• Manufacturer: Dallas Semiconducotr

CURRENT REGISTER FORMAT Table 3
The battery pack current is calculated from the Current Register value using the equation:
For example, if 1.25A is flowing into the pack, and the pack uses a 0.025
will write the value 128
to be:
Because small current ADC offset errors can have a large cumulative effect when current is integrated
over time, the DS2438 provides a method for canceling offset errors in the current ADC. After each
current measurement is completed, the measured value is added to the contents of the Offset Register and
the result is then stored in the Current Register. The Offset Register is a two-byte nonvolatile read/write
register formatted in two’s-complement format. The four MSb’s of the register contain the sign of the
offset, as shown in Table 4.
OFFSET REGISTER FORMAT Table 4
The following process can be used to calibrate the current ADC:
1. Write all zeroes to the Offset Register
2. Force zero current through R
3. Read the Current Register value
4. Disable the current ADC by setting the IAD bit in the Status/Configuration Register to “0”
5. Change the sign of the previously-read Current Register value by performing the two’s complement
6. Enable the current ADC by setting the IAD bit in the Status/Configuration Register to “1”
NOTE:
When writing to the Offset Register, current measurement MUST be disabled (IAD bit set to “0”).
The current ADC calibration process is done for each DS2438 device prior to shipment. However, for
best results, battery pack manufacturers should calibrate the current ADC during initial battery pack
testing, and the host system should calibrate whenever possible (during battery charging, for example).
and write the result to the Offset Register
(This register actually stores the voltage measured across current sense resistor R
This value can be used to calculate battery pack current using the equation below.)
I = Current Register / (4096 * R
I = 128 / ( 4096 * 0.025) = 1.25A
MSb
MSb
2
2
X
S
10
7
4
to the Current Register. From this value, battery pack current can be calculated
2
2
2
X
14
6
3
SENS
2
2
2
(unit = 0.2441 mV)
X
13
5
2
(unit = 0.2441mV)
SENS
2
2
2
S
4
1
12
)
2
2
2
2
11
3
0
8
2
2
2
0
(where R
10
2
7
6 of 29
2
2
2
0
1
9
6
SENS
LSb
LSb
2
2
0
2
0
8
5
is in )
MSB
MSB
LSB
LSB
sense resistor, the DS2438
SENS
.
DS2438