DS2438
VOLTAGE REGISTER FORMAT
Table 2
2
7
2
6
2
5
2
4
2
3
2
2
2
1
2
0
LSB
MSb
0
0
0
(unit = 10 mV)
0
0
0
2
9
LSb
2
8
MSB
BATTERY
VOLTAGE
0.05V
2.7V
3.6V
5V
7.2V
9.99V
10V
DIGITAL OUTPUT (Binary)
0000 0000 0000 0101
0000 0001 0000 1110
0000 0001 0110 1000
0000 0001 1111 0100
0000 0010 1101 0000
0000 0011 1110 0111
0000 0011 1110 1000
DIGITAL OUTPUT (Hex)
0005h
010Eh
0168h
01F4h
02D0h
03E7h
03E8H
For applications requiring a general purpose voltage A/D converter, the DS2438 can be configured so that
the result of a Convert V command will place the scaled binary representation of the voltage on the V
AD
input (as opposed to the V
DD
input) into the Voltage Register in the same format described in Table 2.
Depending upon the state of the Status/Configuration Register, either (but not both) the V
DD
or V
AD
voltage will be stored in the Voltage Register upon receipt of the Convert V command. Refer to the
description of the Status/Configuration Register in the Memory Map section for details. If the V
AD
input
is used as the voltage input, the A/D will be accurate for 1.5V < V
AD
< 2V
DD
over the range 2.4V < V
DD
<
5.0V. This feature gives the user the ability to have a voltage A/D that meets spec accuracy for inputs
over the entire range of 1.5V < V
AD
< 10V for V
DD
= 5.0V.
OPERATION - MEASURING BATTERY CURRENT
The DS2438 features an A/D converter that effectively measures the current flow into and out of the
battery pack by measuring the voltage across an external sense resistor. It does so in the background at a
rate of 36.41 measurements/sec; thus, no command is required to initiate current flow measurements.
However, the DS2438 will only perform current A/D measurements if the IAD bit is set to “1” in the
Status/Configuration Register. The DS2438 measures current flow in and out of the battery through the
V
SENS
pins; the voltage from the V
SENS+
pin to the V
SENS-
pin is considered to be the voltage across the
current sense resistor, R
SENS
. The V
SENS+
terminal may be tied directly to the R
SENS
resistor, however, for
V
SENS-
, we recommend use of an RC low pass filter between it and the GND end of R
SENS
(see the block
diagram in Figure 1). Using a 100 kΩ=(min) resistor (R
F
) and a 0.1
µF
tantalum capacitor (C
F
), the filter
cutoff is approximately 15.9 Hz. The current A/D measures at a rate of 36.41 times per second, or once
every 27.46 ms. This filter will capture the effect of most current spikes, and will thus allow the current
accumulators to accurately reflect the total charge which has gone into or out of the battery.
The voltage across current sense resistor R
SENS
is measured by the ADC and the result is placed in the
Current Register in two’s complement format. The sign (S) of the result, indicating charge or discharge,
resides in the most significant bit of the Current Register, as shown in Table 3. See “Memory Map” in
Figure 7 for the Current Register address location.
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DALLAS [ DALLAS SEMICONDUCTOR ]
