DS2781: Two-Cell Standalone Fuel Gauge IC

6 of 30

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

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

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.