SLUSDV2A May 2020 – May 2021 BQ25798
The BQ25798 is a fully integrated, switch-mode buck-boost charger for a 1 cell ~ 4 cell Li-ion battery and Li-polymer battery. For compact design and minimum component count, the charger integrates the 4 switching MOSFETs (Q1, Q2, Q3, Q4), input and charging current sensing circuits, the battery FET (BATFET) and all the loop compensation of the buck-boost converter. It provides high power density and design flexibility to charge batteries across the full input voltage range for USB Type-C™ and USB-PD applications such as digital cameras, drones and mobile printers.
The charger supports narrow VDC (NVDC) power path management, in which the system is regulated at a voltage slightly higher than the battery voltage, without dropping below a configurable minimum system voltage. The system keeps operating even when the battery is completely discharged or removed. When load power exceeds the input source rating, the battery gets into supplement mode and prevents the input source from being overloaded and the system from crashing.
The device charges a battery from a wide range of input sources including legacy USB adapter to high voltage USB-PD adapter and traditional barrel adapter. The charger seamlessly transitions between buck, boost and buck-boost modes based on input voltage and battery voltage without host control. The optional dual-input source selector manages the power flowing from two different input sources, prioritizing the first available input source. The host may manually transition between input sources using I2C.
To support fast charging using adjustable high voltage adapter (HVDCP), the device provides D+/D- handshake. The device is compliant with USB 2.0 and USB 3.0 power delivery specification with input current and voltage regulation. In addition, the Input Current Optimizer (ICO) allows the detection of maximum power point of an unknown input source. The BQ25798 also features a Maximum Power Point Tracking (MPPT) algorithm to optimize the energy drawn from a low-power photovoltaic panel. This MPPT algorithm measures the open-circuit voltage of the panel on a user-configurable period, then adjusts the VINDPM using the VOC_PCT setting to calculate the maximum power point as a percentage of the measured open-circuit voltage.
In addition to the I2C host controlled charging mode, BQ25798 also supports autonomous charging mode. After power up, the charging is defaulted enabled with all the registers default settings. The device can complete a charging cycle without any software engagements. It detects battery voltage and charges the battery in different phases: trickle charging, pre-charging, constant current (CC) charging and constant voltage (CV) charging. At the end of the charging cycle, the charger automatically terminates when the charge current is below a pre-set limit (termination current) in the constant voltage phase. When the full battery falls below the recharge threshold, the charger will automatically start another charging cycle.
In the absence of input sources, BQ25798 supports USB On-the-Go (OTG) function, discharging the battery to generate an adjustable 2.8V~22V voltage on VBUS with 10mV step size. This is compliant with the USB PD 3.0 specification defined PPS feature. The BQ25798 also supports a backup feature using this same mechanism in which a system load connected at PMID may be supplied with the adjustable OTG voltage when the adapter is removed. Once configured, the integrated backup comparator automatically triggers the converter to discharge the battery, holding up the PMID node and transitioning into backup mode without host intervention.
The charger provides various safety features for battery charging and system operations, including battery temperature negative thermistor (NTC) monitoring, trickle charge, pre-charge and fast charge timers and over-voltage/over-current protections on the battery and the charger power input pin. The thermal regulation reduces charge current when the die temperature exceeds a programmable threshold. The STAT output of the device reports the charging status and any fault conditions. The INT pin immediately notifies the host when a fault occurs.
The device also provides a 16-bit analog-to-digital converter (ADC) for monitoring charge current and input/battery/system voltages, the TS pin voltage and the die temperature. It is available in a 29-pin 4.0 mm x 4.0 mm QFN package.