DS2784_12 datasheet(11/43 Pages) MAXIM

Part #	DS2784

Description	1-Cell Stand-Alone Fuel Gauge IC with Li Protector and SHA-1 Authentication

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Manufacturer	MAXIM [Maxim Integrated Products]

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DS2784: 1-Cell Stand-Alone Fuel Gauge IC with Li+ Protector and SHA-1 Authentication

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POWER MODES

The DS2784 has two power modes: Active and Sleep. On initial power-up, the DS2784 defaults to Active mode. In

Active mode, the DS2784 is fully functional with measurements and capacity estimation registers continuously

updated. The protector circuit monitors the battery voltages and current for unsafe conditions. The protection FET

gate drivers are enabled when conditions are deemed safe. Also, the SHA-1 authentication function is available in

Active mode. When a SHA-1 computation is performed, the supply current increases to IDD3 for tSHA. In Sleep mode,

the DS2784 conserves power by disabling measurement and capacity estimation functions, but preserves register

contents. Gate drive to the protection FETs is disabled in Sleep. And the SHA-1 authentication feature is not

operational.

Sleep mode is entered under two different conditions: bus low and undervoltage. An enable bit makes entry into

Sleep optional for each condition. Sleep mode is not entered if a charger is connected (VPLS > VDD + 50mV) or if a

charge current of 1.6mV / RSNS is measured from SNS to VSS. The DS2784 exits Sleep mode upon charger

connection and VIN ≥ VUV or a low to high transition on DQ.

The bus-low condition, where the DQ pin is low for tSLEEP, indicates pack removal or system shutdown in which the

1-Wire bus pullup voltage, VPULLUP, is not present. The Power mode (PMOD) bit must be set to enter Sleep when a

bus-low condition occurs. After the DS2784 enters Sleep due to a bus-low condition, it is assumed that no charge

or discharge current will flow and that coulomb counting is unnecessary.

The second condition to enter Sleep is an undervoltage condition, which reduces battery drain due to the DS2784

supply current and prevents over discharging the cell. The DS2784 transitions to Sleep if the VIN voltage is less

than VUV (2.45V typical) and the undervoltage enable (UVEN) bit is set. The 1-Wire bus must be in a static state,

that is, with DQ either high or low for tSLEEP. The DS2784 transitions from UVEN Sleep to Active mode when DQ

changes logic state.

The DS2784 has the “power switch” capability for waking the device and enabling the protection FETs when the

host system is powered down. A simple dry-contact switch on the PIO pin or DQ pin can be used to wake up the

battery pack. The power switch function is enabled using the PSPIO and PSDQ configuration bits in the control

register. When PSPIO or PSDQ is set and a Sleep condition is satisfied, the PIO and DQ pins pull high weakly,

then become armed to detect a low-going transition. A 100ms debounce period filters out glitches that can be

caused when a sleeping battery is inserted into a system.

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