Rainbow Electronics DS2788 - Manual (Page 7)

nection or system shutdown, in which no charge or dis-
charge current flows. A PMOD SLEEP condition transi-
tions back to ACTIVE mode when DQ is pulled high.
The second option for entering SLEEP is an undervolt-
age condition. When the UVEN bit is set, the DS2788
transitions to SLEEP if the voltage on V
IN
is less than
V
SLEEP
(2.45V nominal) and DQ is stable at a low or
high logic level for t
SLEEP
. An undervoltage condition
occurs when a pack is fully discharged, where loading
on the battery should be minimized. UVEN SLEEP
relieves the battery of the I
ACTIVE
load until communi-
cation on DQ resumes.
Note: PMOD and UVEN SLEEP features must be dis-
abled when a battery is charged on an external charger
that does not connect to the DQ pin. PMOD SLEEP can
be used if the charger pulls DQ high. UVEN SLEEP can
be used if the charger toggles DQ. The DS2788
remains in SLEEP and therefore does not measure or
accumulate current when a battery is charged on a
charger that fails to properly drive DQ.
Initiating Communication
in Sleep
When beginning communication with a DS2788 in
PMOD SLEEP, DQ must be pulled up first and then a
1-Wire reset pulse must be issued by the master. In
UVEN SLEEP, the procedure depends on the state of
DQ when UVEN SLEEP was entered. If DQ was low,
DQ must be pulled up and then a 1-Wire reset pulse
must be issued by the master as with PMOD SLEEP. If
DQ was high when UVEN SLEEP was entered, then the
DS2788 is prepared to receive a 1-Wire reset from the
master. In the first two cases with DQ low during
SLEEP, the DS2788 does not respond to the first rising
edge of DQ with a presence pulse.
Voltage Measurement
Battery voltage is measured at the V IN input with
respect to V SS over a range of 0 to 4.992V, with a reso-
lution of 4.88mV. The result is updated every 440ms
and placed in the Voltage (VOLT) register in two’s com-
plement form. Voltages above the maximum register
value are reported at the maximum value; voltages
below the minimum register value are reported at the
minimum value. Figure 3 shows the format of the
Voltage register.
V IN is usually connected to the positive terminal of a
single-cell Li+ battery by a 1k Ω resistor. The input
impedance is sufficiently large (15M Ω) 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 sin-
gle-cell average voltage to the V
IN
input. In Figure 2,
the value of R can be up to 1M Ω without incurring sig-
nificant error due to input loading. The VMA pin is dri-
ven high t
PRE
before the voltage conversion begins.
This allows an external switching element to enable the
voltage-divider, and allows settling to occur before the
start of the conversion.
D
S
2
7
8
8
Stand-Alone Fuel-Gauge IC with
LED Display Drivers
_______________________________________________________________________________________ 7
VOLT READ ONLY
MSB—ADDRESS 0Ch LSB—ADDRESS 0Dh
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
Figure 3. Voltage Register Format