Power modes, Voltage measurement, Figure 4. voltage register format – Rainbow Electronics DS2745 User Manual
Page 7: Temperature measurement
DS2745 Low-Cost I
2
C Battery Monitor
7 of 14
POWER MODES
The DS2745 operates in one of two power modes: active and sleep. While in active mode, the DS2745 operates as
a high-precision battery monitor with voltage, temperature, current and accumulated current measurements
acquired continuously and the resulting values updated in the measurement registers. Read and write access is
allowed to all registers.
In sleep mode, the DS2745 operates in a low-power mode with no measurement activity. Serial access to current,
accumulated current, and status/control registers is allowed in sleep mode if V
DD
> 2V.
The DS2745 operating mode transitions from SLEEP to ACTIVE when:
SDA > V
IH
OR SCL > V
IH
The DS2745 operating mode transitions from ACTIVE to SLEEP when:
SMOD = 1 AND (SDA < VIL AND SCL < VIL) for t
SLEEP
.
CAUTION: If SMOD = 1, pull-up resistors are required on SCL and/or SDA in order to ensure that the DS2745
transitions from SLEEP to ACTIVE mode when the battery is charged. If the bus is not pulled up, the DS2745
remains in SLEEP and cannot accumulate the charge current. This caution statement applies particularly to on a
battery that is charged on a standalone charger.
VOLTAGE MEASUREMENT
Battery voltage is measured at the VIN input with respect to VSS over a range of 0V to 4.992V and with a
resolution of 4.88mV. The result is updated every 440ms and placed in the VOLTAGE register in two’s compliment
form. Voltages above the maximum register value are reported as 7FFFh.
Figure 4. VOLTAGE REGISTER FORMAT
MSB—Address
0C
LSB—Address
0D
S 2
9
2
8
2
7
2
6
2
5
2
4
2
3
2
2
2
1
2
0
X X X X X
MSb
LSb
MSb LSb
“S”: sign bit(s), “X”: reserved
Units: 4.88mV
The input impedance of VIN is sufficiently large (>15M
W) to be connected to a high impedance voltage divider in
order to support multiple cell applications. The pack voltage should be divided by the number of series cells to
present a single cell average voltage to the VIN input. Note that the first voltage measurement made after the
DS2745 is powered or after the ACR register is written will not be valid. The host should wait one measurement
cycle after either of these two conditions occur before reading voltage.
TEMPERATURE MEASUREMENT
The DS2745 uses an integrated temperature sensor to measure battery temperature with a resolution of 0.125°C.
Temperature measurements are updated every 440ms and placed in the Temperature Register in two’s
complement form.