ZHCSGB2 June 2017
The bq27750 gas gauge is a fully integrated battery manager that employs flash-based firmware and integrated hardware protection to provide a complete solution for battery-stack architectures composed of 1-series cells. The bq27750 device interfaces with a host system via an I2C protocol. High-performance, integrated analog peripherals enable support for a sense resistor down to 1 mΩ and simultaneous current/voltage data conversion for instant power calculations. The following sections detail all of the major component blocks included as part of the bq27750 device.
The Functional Block Diagram depicts the analog (AFE) and digital (AGG) peripheral content in the bq27750 device.
The bq27750 device measures cell voltage and current simultaneously, and also measures temperature to calculate the information related to remaining capacity, full charge capacity, state-of-health, and other gauging parameters.
The bq27750 device uses a custom TI-proprietary processor design that features a Harvard architecture and operates at frequencies up to 4.2 MHz. Using an adaptive, three-stage instruction pipeline, the bq27750 processor supports variable instruction length of 8, 16, or 24 bits.
The first ADC is an integrating converter designed specifically for coulomb counting. The converter resolution is a function of its full-scale range and number of bits, yielding a 3.74-µV resolution.
The CC digital filter generates a 16-bit conversion value from the delta-sigma CC front end. Its FIR filter uses the LFO clock output, which allows it to stop the HFO clock during conversions. New conversions are available every 250 ms while CCTL[CC_ON] = 1. Proper use of this peripheral requires turning on the CC modulator in the AFE.
The ADC multiplexer provides selectable connections to the VCx inputs, TS1 inputs, internal temperature sensor, internal reference voltages, internal 1.8-V regulator, PACK input, and VSS ground reference input. In addition, the multiplexer can independently enable the TS1 input connection to the internal thermistor biasing circuitry, and also enables the user to short the multiplexer inputs for test and calibration purposes.
The second ADC is a 16-bit delta-sigma converter designed for general-purpose measurements. The ADC automatically scales the input voltage range during sampling based on channel selection. The converter resolution is a function of its full-scale range and number of bits, yielding a 38-µV resolution. The default conversion time of the ADC is 31.25 ms, but is user-configurable down to 1.95 ms. Decreasing the conversion time presents a tradeoff between conversion speed and accuracy, as the resolution decreases for faster conversion times.
The ADC digital filter generates a 24-bit conversion result from the delta-sigma ADC front end. Its FIR filter uses the LFO clock, which allows it to stop the HFO clock during conversions. The ADC digital filter is capable of providing two 24-bit results: one result from the delta-sigma ADC front end and a second synchronous result from the delta-sigma CC front end.
An internal temperature sensor is available on the bq27750 device to reduce the cost, power, and size of the external components necessary to measure temperature. It is available for connection to the ADC using the multiplexer, and is ideal for quickly determining pack temperature under a variety of operating conditions.
The TS1 input is enabled with an internal 18-kΩ (typical) linearization pull-up resistor to support the use of a 10-kΩ (25°C) NTC external thermistor, such as the Semitec 103AT-2. The NTC thermistor should be connected between VSS and the individual TS1 pin. The analog measurement is then taken via the ADC through its input multiplexer. If a different thermistor type is required, then changes to configurations may be required.
The bq27750 device manages its supply voltage dynamically according to operating conditions. When VBAT > VSWITCHOVER– + VHYS, the AFE connects an internal switch to BAT and uses this pin to supply power to its internal 1.8-V LDO, which subsequently powers all device logic and flash operations. Once BAT decreases to VBAT < VSWITCHOVER–, the AFE disconnects its internal switch from BAT and connects another switch to PACK, allowing sourcing of power from a charger (if present). An external capacitor connected to PBI provides a momentary supply voltage to help guard against system brownouts due to transient short-circuit or overload events that pull BAT below VSWITCHOVER–.
In the event of a power-cycle, the bq27750 AFE holds its internal RESET output pin high for tRST duration to allow its internal 1.8-V LDO and LFO to stabilize before running the AGG. The AFE enters power-on reset when the voltage at VREG falls below VREGIT– and exits reset when VREG rises above VREGIT– + VHYS for tRST time. After tRST, the bq27750 AGG will write its trim values to the AFE.
The bq27750 device has an I2C bus communication interface. This device has the option to broadcast information to a smart charger to provide key information to adjust the charging current and charging voltage based on the temperature or individual cell voltages.
If the device is configured as a single-master architecture (an application processor) and an occasional NACK is detected in the operation, the master can resend the transaction. However, in a multi-master architecture, an incorrect ACK leading to accidental loss of bus arbitration can cause a master to wait incorrectly for another master to clear the bus. If this master does not get a bus-free signal, then it must have in place a method to look for the bus and assume it is free after some period of time. Also, if possible, set the clock speed to be 100 kHz or less to significantly reduce the issue described above for multi-mode operation.
The bq27750 device controls two external N-Channel MOSFETs in a back-to-back configuration for battery protection. The charge (CHG) and discharge (DSG) FETs are automatically disabled if a safety fault (AOLD, ASSC, ASCD, SOV) is detected, and can also be manually turned off using AFE_CONTROL[CHGEN, DSGEN] = 0, 0. When the gate drive is disabled, an internal circuit discharges CHG to BAT and DSG to PACK.
The bq27750 AFE includes a low frequency oscillator (LFO) running at 262.144 kHz. The AFE monitors the LFO frequency and indicates a failure via LATCH_STATUS[LFO] if the output frequency is much lower than normal.
The bq27750 AGG includes a high frequency oscillator (HFO) running at 16.78 MHz. It is synthesized from the LFO output and scaled down to 8.388 MHz with 50% duty cycle.
The bq27750 AFE contains an integrated 1.8-V LDO that provides regulated supply voltage for the device CPU and internal digital logic.
The bq27750 AFE provides two internal voltage references with VREF1, used by the ADC and CC, while VREF2 is used by the LDO, LFO, current wake comparator, and OCD/SCC/SCD1/SCD2 current protection circuitry.
The overcurrent in discharge (OCD) function detects abnormally high current in the discharge direction. The overload in discharge threshold and delay time are configurable via the OCD_CONTROL register. The thresholds and timing can be fine-tuned even further based on a sense resistor with lower resistance or wider tolerance via the PROTECTION_CONTROL register. The detection circuit also incorporates a filtered delay before disabling the CHG and DSG FETs. When an OCD event occurs, the LATCH_STATUS[OCD] bit is set to 1 and is latched until it is cleared and the fault condition has been removed.
The short-circuit current in charge (SCC) function detects catastrophic current conditions in the charge direction. The short-circuit in charge threshold and delay time are configurable via the SCC_CONTROL register. The thresholds and timing can be fine-tuned even further based on a sense resistor with lower resistance or wider tolerance via the PROTECTION_CONTROL register. The detection circuit also incorporates a blanking delay before disabling the CHG and DSG FETs. When an SCC event occurs, the LATCH_STATUS[SCC] bit is set to 1 and is latched until it is cleared and the fault condition has been removed.
The short-circuit current in discharge (SCD) function detects catastrophic current conditions in the discharge direction. The short-circuit in discharge thresholds and delay times are configurable via the SCD1_CONTROL and SCD2_CONTROL registers. The thresholds and timing can be fine-tuned even further based on a sense resistor with lower resistance or wider tolerance via the PROTECTION_CONTROL register. The detection circuit also incorporates a blanking delay before disabling the CHG and DSG FETs. When an SCD event occurs, the LATCH_STATUS[SCD1] or LATCH_STATUS[SCD2] bit is set to 1 and is latched until it is cleared and the fault condition has been removed.
The bq27750 gas gauge supports the following battery and system level protection features, which can be configured using firmware:
This device uses the Impedance Track™ technology to measure and determine the available charge in battery cells. The accuracy achieved using this method is better than 1% error over the lifetime of the battery. There is no full charge/discharge learning cycle required. See the Theory and Implementation of Impedance Track Battery Fuel-Gauging Algorithm Application Report (SLUA364) for further details.
This device supports charge control features, such as:
This device supports security by:
This device supports three modes, but the current consumption varies, based on firmware control of certain functions and modes of operation:
The device supports data logging of several key parameters for warranty and analysis:
The device supports accurate data measurements and data logging of several key parameters.
The device uses an integrating delta-sigma analog-to-digital converter (ADC) for current measurement. The ADC measures charge/discharge flow of the battery by measuring the voltage across a very small external sense resistor. The integrating ADC measures a bipolar signal from a range of –100 mV to 100 mV, with a positive value when V(SRP) – V(SRN), indicating charge current and a negative value indicating discharge current. The integration method uses a continuous timer and internal counter, which has a rate of 0.65 nVh.
The bq27750 gas gauge measures the individual cell voltages at 250-ms intervals using an ADC. This measured value is internally scaled for the ADC and is calibrated to reduce any errors due to offsets. This data is also used for calculating the impedance of the individual cell for Impedance Track gas gauging.
The current measurement is performed by measuring the voltage drop across the external sense resistor (1 mΩ to 3 mΩ) and the polarity of the differential voltage determines if the cell is in the CHARGE or DISCHARGE mode.
The auto-calibration feature helps to cancel any voltage offset across the SRP and SRN pins for accurate measurement of the cell voltage, charge/discharge current, and thermistor temperature. The auto-calibration is performed when there is no communication activity for a minimum of 5 s on the bus lines.
This device has an internal sensor for on-die temperature measurements, and the ability to support external temperature measurements via the external NTC on the TS1 pin. These two measurements are individually enabled and configured.