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DS2776G Datasheet(PDF) 11 Page - Maxim Integrated Products

Part # DS2776G
Description  2-Cell, Stand-Alone, Li Fuel-Gauge IC with Protector and Optional SHA-1 Authentication
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Manufacturer  MAXIM [Maxim Integrated Products]
Direct Link  https://www.maximintegrated.com/en.html
Logo MAXIM - Maxim Integrated Products

DS2776G Datasheet(HTML) 11 Page - Maxim Integrated Products

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2-Cell, Stand-Alone, Li+ Fuel-Gauge IC with
Protector and Optional SHA-1 Authentication
______________________________________________________________________________________
11
Detailed Description
The DS2775–DS2778 function as an accurate fuel
gauge, Li+ protector, and SHA-1-based authentication
token (SHA-1-based authentication available only on
the DS2776/DS2778). The fuel gauge provides accu-
rate estimates of remaining capacity and reports timely
voltage, temperature, and current measurement data.
Capacity estimates are calculated from a piecewise lin-
ear model of the battery performance over load and
temperature along with system parameters for charge
and end-of-discharge conditions. The algorithm para-
meters are user programmable and can be modified
within the pack. Critical capacity and aging data are
periodically saved to EEPROM in case of short-circuit
or deep-depletion events.
The Li+ protection function ensures safe, high-perfor-
mance operation. nFET protection switches are driven
with a charge pump that maintains gate drive as the
cell voltage decreases. The high-side topology pre-
serves the ground path for serial communication while
eliminating the parasitic charge path formed when the
fuel-gauge IC is located inside the protection FETs in a
low-side configuration. The thresholds for overvoltage,
undervoltage, overcurrent, and short-circuit current are
user programmable for customization to each cell and
application.
The 32-bit-wide SHA-1 engine with 64-bit secret and
64-bit challenge words resists brute force and other
attacks with financial-level HMAC security. The chal-
lenge of managing secrets in the supply chain is
addressed with the compute next secret feature. The
unique serial number or ROM ID can be used to assign
a unique secret to each battery.
Power Modes
The DS2775–DS2778 have two power modes: active
and sleep. On initial power-up, the DS2775–DS2778
default to active mode. In active mode, the DS2775–
DS2778 are fully functional with measurements and
capacity estimation registers continuously updated.
The protector circuit monitors battery pack, cell volt-
ages, and battery current for safe conditions. The pro-
tection FET gate drivers are enabled when conditions
are deemed safe. Also, the SHA-1 authentication func-
tion is available in active mode. When an SHA-1 com-
putation is performed, the supply current increases to
IDD2 for tSHA. In sleep mode, the DS2775–DS2778 con-
serve power by disabling measurement and capacity
estimation functions, but preserve register contents.
Gate drive to the protection FETs is disabled in sleep;
the SHA-1 authentication feature is not operational.
The IC enters sleep mode under two different condi-
tions: 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 +
VCD) or if a charge current of 1.6mV/RSNS measured
from SNS to VSS. The DS2775–DS2778 exit sleep mode
upon charger connection or a low-to-high transition on
any communication line. The bus-low condition, where
all communication lines are low for tSLEEP, indicates
pack removal or system shutdown in which the 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 DS2775–DS2778
enter sleep due to a bus-low condition, it is assumed
that no charge or discharge current flows and that
coulomb counting is unnecessary.
The second condition to enter sleep is an undervoltage
condition, which reduces battery drain due to the
DS2775–DS2778 supply current and prevents overdis-
charging the cell. The DS2775–DS2778 transition to
sleep mode if the VIN1 or VIN2 voltage is less than VUV
and the undervoltage enable (UVEN) bit is set. The
communication bus must be in a static state, that is,
with DQ (SDA and SCL for 2-wire) either high or low for
tSLEEP. The DS2775–DS2778 transition from sleep
mode to active mode when DQ (SDA and SCL for
2-wire) changes logic state. See Figures 1 and 2 for
more information on sleep-mode state.
The DS2775–DS2778 have a “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 are set and sleep mode is
entered through the PMOD condition*, the PIO and DQ
pins pull high, respectively. Sleep mode is exited upon
the detection of a low-going transition on PIO or DQ.
PIO has a 100ms debounce period to filter out glitches
that can be caused when a sleeping battery is inserted
into a system.
*
The “power switch” feature is disabled if sleep mode is
entered because of a UV condition.


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