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DS2776G Datasheet(PDF) 11 Page - Maxim Integrated Products |
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DS2776G Datasheet(HTML) 11 Page - Maxim Integrated Products |
11 / 46 page 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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