Rainbow Electronics DS2781 - Manual (Page 6)

DS2781: Two-Cell Standalone Fuel Gauge IC
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Figure 2: Typical Operating Circuit
DQ
SNS
VDD
VSS
RSNS
PACK+
PACK-
DATA
Protection
Circuit
DS2781
PIO
150
0.1uF
VB
VIN
500 1K
OVD
TSSOP-8
5.6V
0.1uF
1 or 2
Cell
Li-Ion
Stack
POWER MODES
The DS2781 has two power modes: ACTIVE and SLEEP. On initial power up, the DS2781 defaults to ACTIVE
mode. While in ACTIVE mode, the DS2781 is fully functional with measurements and capacity estimation
continuously updated.
In SLEEP mode, the DS2781 conserves power by disabling measurement and capacity estimation functions, but
preserves register contents. SLEEP mode is entered under two different conditions. An enable bit makes entry into
SLEEP optional for each condition. The first condition in which SLEEP is entered is a bus low condition. The Power
Mode (PMOD) bit must be set to enter SLEEP when a bus low condition occurs. (PMOD = 1 AND BUS_LOW). A
bus low condition, where the DQ pin is low for t
SLEEP (2s nominal), is used to detect a pack disconnection or
system shutdown in which the bus pull-up voltage, V
PULLUP
, is not present. PMOD type SLEEP assumes that no
charge or discharge current will flow and therefore coulomb counting is not necessary. A system with PMOD
SLEEP enabled must ensure that a standalone or cradle charger includes a pull-up on DQ. The DS2781 transitions
from PMOD SLEEP to ACTIVE mode when DQ is pulled high, as would happen when a battery is inserted into a
system.
The second condition to enter SLEEP is an under voltage condition (measured on VIN). When the Under Voltage
Enable (UVEN) bit is set, the DS2781 will transition to SLEEP if the VIN voltage is less than V
SLEEP (Selectable
2.45 or 4.9V). The bus must be in a static state, that is with DQ either high or low for t SLEEP . UVEN SLEEP reduces
battery drain due to the DS2781 to prevent over discharge. The DS2781 transitions from UVEN SLEEP to ACTIVE
mode when DQ changes logic state. The bus master should initiate communication when charging of a depleted
battery begins to ensure that the DS2781 enters ACTIVE mode from UVEN SLEEP.
NOTE: PMOD and UVEN SLEEP features must be disabled 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 DS2781 remains in SLEEP and therefore does not measure or accumulate
current when a battery is charged on a charger that failures properly drive DQ.
INITIATING COMMUNICATION IN SLEEP
When beginning communication with a DS2781 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 DS2781 is
prepared to receive a 1-Wire reset from the master. In the first two cases with DQ low during SLEEP, the DS2781
does not respond to the first rising edge of DQ with a presence pulse.