8.3.1.1 Power-On-Reset (POR)

The internal bias circuits are powered from the higher voltage of VBUS and BAT. When VBUS or VBAT rises above UVLOZ, the sleep comparator, battery depletion comparator and BATFET driver are active. I²C interface is ready for communication and all the registers are reset to default value. The host can access all the registers after POR. By default, the BATFET driver is inactive when battery power is first applied. The BATFET driver can be enabled by plugging in DC source, by clearing BATFET_Disable bit (REG07[5]), or logic low to high transition on QON pin.

8.3.1.2 Power Up from Battery without DC Source

If only battery is present and the voltage is above depletion threshold (V_{BAT_DEPL}), the BATFET turns on and connects battery to system. The REGN LDO stays off to minimize the quiescent current. The low R_DSON in BATFET and the low quiescent current on BAT minimize the conduction loss and maximize the battery run time. During both boost and charge mode, the device always monitors the discharge current through BATFET. When the system is overloaded or shorted, the device will immediately turn off BATFET and keep BATFET off until the input source plugs in again.

8.3.1.2.2 Shipping Mode

To extend battery life and minimize power when system is powered off during system idle, shipping, or storage, the device can turn off BATFET so that the system voltage is zero to minimize the leakage. The BATFET can be turned off by setting REG07[5] (BATFET_DISABLE) bit.

In order to keep BATFET off during shipping mode, the host has to disable the watchdog timer (REG05[5:4] = 00) and disable BATFET (REG07[5] = 1) at the same time. Once the BATFET is disabled, one of the following events can turn on BATFET and clear REG07[5] (BATFET_DISABLE) bit.

1. Plug in adapter
2. Write REG07[5] = 0
3. watchdog timer expiration
4. Register reset (REG01[7] = 1)
5. A logic low to high transition on QON pin (refer to Figure 11 for detail timing)

Figure 11. QON Timing

8.3.1.3 Boost Mode Operation from Battery

The device supports boost converter operation to deliver power from the battery to other portable devices through PMID pin. The boost mode output current rating meets the 1.5-A charging requirements for smartphone and tablet. The boost operation is enabled by default if the conditions are valid:

1. BAT above BATLOWV threshold (V_BATLOWV set by REG04[1])
2. VBUS less than BAT+V_SLEEP (in sleep mode)
3. Boost mode operation is enabled (OTG pin HIGH and REG01[5:4] = 10)
4. After 30ms delay from boost mode enable

In battery boost mode, the device employs a 1.5-MHz step-up switching regulator. During boost mode, the status register REG08[7:6] is set to 11, the PMID output voltage is 5.1 V. In addition, the device provides adjustable boost voltage from 4.55 V to 5.5 V by changing BOOSTV bits in REG06[7:4]. Any fault during boost operation, including PMID over-voltage, sets the fault register REG09[6] to 1 and an INT is asserted.

For power bank applications, the boost current is supported from PMID pin as in the application diagram. It is recommended to use the minimum PMID cap value 20 uF for boost current. Please note that there is no boost current limit setting when the boost current is sourced from PMID pin, hence it is important not to overload the boost current under this condition.

8.3.1.4 Power Up from DC Source

When the DC source plugs in, the charger device checks the input source voltage to turn on REGN LDO and all the bias circuits. It also checks the input current limit before starts the buck converter.

8.3.1.4.4 D+/D– Detection Sets Input Current Limit

The bq24295 contains a D+/D– based input source detection to program the input current limit. The D+/D- detection has three steps: data contact detect (DCD), primary detection, and non-standard adapter detection. When the charging source passes data contact detect, the device would proceed to run primary detection. Otherwise the charger would proceed to run non-standard adapter detection.

Figure 12. USB D+/D- Detection

DCD (Data Contact Detection) uses a current source to detect when the D+/D– pins have made contact during an attach event. The protocol for data contact detect is as follows:

• Detect VBUS present and REG08[2] = 1 (power good)
• Turn on D+ I_{DP_SRC} and the D– pull-down resistor R_{DM_DWN} for 40 ms
• If the USB connector is properly attached, the D+ line goes from HIGH to LOW, wait up to 0.5 sec.
• Turn off I_{DP_SRC} and disconnect R_{DM_DWN}

The primary detection is used to distinguish between USB host (Standard Down Stream Port, or SDP) and different type of charging ports (Charging Down Stream Port, or CDP, and Dedicated Charging Port, or DCP). The protocol for primary detection is as follows:

• Turn on V_{DP_SRC} on D+ and I_{DM_SINK} on D– for 40 ms
• If PD is attached to a USB host (SDP), the D– is low. If PD is attached to a charging port (CDP or DCP), the D– is high
• Turn off V_{DP_SRC} and I_{DM_SINK}

Table 2 shows the input current limit setting after D+/D– detection.

Table 2. bq24295 USB D+/D– Detection

D+/D– DETECTION	INPUT CURRENT LIMIT	REG08[7:6]
0.5 sec timer expired in DCD (D+/D- floating)	Proceed to non-standard adapter detection	00
USB host	500 mA	01
Charging port	3 A	10

When DCD 0.5 sec timer expires, the non-standard adapter detection is used to distinguish three different divider bias conditions on D+/D- pins. When non-standard adapter is detected, the input current limit (REG0[2:0]) is set based on the table shown below and REG08[7:6] is set to 10 (Adapter port). If non-standard adapter is not detected, REG08[7:6] is set to 00 (Unknown) and the input current limit is set in REG0[2:0] to 500mA by default.

Table 3. bq24295 Non-Standard Adapter Detection

NON-STANDARD ADAPTER	D+ THRESHOLD	D- THRESHOLD	INPUT CURRENT LIMIT
Divider 1	Vadpt1_lo < VD+ < Vadpt1_hi For VBUS = 5 V, typical range 2.4 V < VD+ < 3.1 V	VD- < Vadpt1_lo or VD- > Vadpt1_hi For VBUS = 5 V, typical range VD- < 2.4 V or VD- > 3.1 V	2.0 A
Divider 2	Vadpt2_lo < VD+ < Vadpt2_hi For VBUS = 5 V, typical range 0.85 V < VD+ < 1.5 V	NA	2.0 A
Divider 3	VD+< Vadpt3_lo or VD+> Vadpt3_hi For VBUS = 5 V, typical range VD+ < 2.4 V or VD+ > 3.1 V	Vadpt3_lo < VD- < Vadpt3_hi For VBUS = 5 V, typical range 2.4 V < VD- < 3.1 V	1 A

After D+/D- detection is completed with an input source already plugged in, the input current limit is not changed unless DPDM_EN (REG07[7]) bit is set to force detection.

8.3.1.5 Converter Power-Up

After the input current limit is set, the converter is enabled and the HSFET and LSFET start switching. If battery charging is disabled, BATFET turns off. Otherwise, BATFET stays on to charge the battery.

The device provides soft-start when ramp up the system rail. When the system rail is below 2.2 V, the input current limit is forced to 100mA. After the system rises above 2.2 V, the charger device sets the input current limit set by the lower value between register and ILIM pin.

As a battery charger, the charger deploys a 1.5-MHz step-down switching regulator. The fixed frequency oscillator keeps tight control of the switching frequency under all conditions of input voltage, battery voltage, charge current and temperature, simplifying output filter design.

A type III compensation network allows using ceramic capacitors at the output of the converter. An internal saw-tooth ramp is compared to the internal error control signal to vary the duty cycle of the converter. The ramp height is proportional to the PMID voltage to cancel out any loop gain variation due to a change in input voltage.

In order to improve light-load efficiency, the device switches to PFM control at light load when battery is below minimum system voltage setting or charging is disabled. During the PFM operation, the switching duty cycle is set by the ratio of SYS and VBUS.

8.3.1.6 Low Power HIZ State

The host can configure the converter to go into HIZ State by setting EN_HIZ (REG00[7]) to 0. The device is in the lowest quiescent state with REGN LDO and the bias circuits off, the VBUS current during HIZ state will be less than 30 µA while the system is supplied by the battery. Once the charger device enters HIZ state in host mode, it stays in HIZ until the host writes REG00[7] = 0. When the processor host wakes up, it is recommended to first check if the charger is in HIZ state.

8.3.2.1 Narrow VDC Architecture

The device deploys Narrow VDC architecture (NVDC) with BATFET separating system from battery. The minimum system voltage is set by REG01[3:1]. Even with a fully depleted battery, the system is regulated above the minimum system voltage (default 3.5 V).

When the battery is below minimum system voltage setting, the BATFET operates in linear mode (LDO mode), and the system is 150 mV above the minimum system voltage setting. As the battery voltage rises above the minimum system voltage, BATFET is fully on and the voltage difference between the system and battery is the V_DS of BATFET.

When the battery charging is disabled or terminated, the system is always regulated at 150 mV above the minimum system voltage setting. The status register REG08[0] goes high when the system is in minimum system voltage regulation.

Figure 13. V(SYS) vs V(BAT)

8.3.2.2 Dynamic Power Management

To meet maximum current limit in USB spec and avoid over loading the adapter, the device features Dynamic Power Management (DPM), which continuously monitors the input current and input voltage.

When input source is over-loaded, either the current exceeds the input current limit (REG00[2:0]) or the voltage falls below the input voltage limit (REG00[6:3]). The device then reduces the charge current until the input current falls below the input current limit and the input voltage rises above the input voltage limit.

When the charge current is reduced to zero, but the input source is still overloaded, the system voltage starts to drop. Once the system voltage falls below the battery voltage, the device automatically enters the supplement mode where the BATFET turns on and battery starts discharging so that the system is supported from both the input source and battery.

During DPM mode (either VINDPM or IINDPM), the status register REG08[3] will go high.

Figure 14 shows the DPM response with 5-V/1.2-A adapter, 3.2-V battery, 2.0-A charge current and 3.4-V minimum system voltage setting.

Figure 14. DPM Response

8.3.2.3 Supplement Mode

When the system voltage falls below the battery voltage, the BATFET turns on and the BATFET gate is regulated the gate drive of BATFET so that the minimum BATFET V_DS stays at 30 mV when the current is low. This prevents oscillation from entering and exiting the supplement mode. As the discharge current increases, the BATFET gate is regulated with a higher voltage to reduce R_DSON until the BATFET is in full conduction. At this point onwards, the BATFET V_DS linearly increases with discharge current. shows the V-I curve of the BATFET gate regulation operation. BATFET turns off to exit supplement mode when the battery is below battery depletion threshold.

Figure 15. BATFET V-I Curve

8.3.3.1 Autonomous Charging Cycle

With battery charging enabled at POR (REG01[5:4] = 01), the charger device complete a charging cycle without host involvement. The device default charging parameters are listed in the following table.

A new charge cycle starts when the following conditions are valid:

• Converter starts
• Battery charging is enabled by I²C register bit (REG01[5:4]) = 01 and CE is low
• No thermistor fault on TS
• No safety timer fault
• BATFET is not forced to turn off (REG07[5])

The charger device automatically terminates the charging cycle when the charging current is below termination threshold and charge voltage is above recharge threshold. When a full battery voltage is discharged below recharge threshold (REG04[0]), the device automatically starts another charging cycle. After the charge done, either toggle /CE pin or REG01[5:4] will initiate a new charging cycle.

The STAT output indicates the charging status of charging (LOW), charging complete or charge disable (HIGH) or charging fault (Blinking). The status register REG08[5:4] indicates the different charging phases: 00-charging disable, 01-precharge, 10-fast charge (constant current) and constant voltage mode, 11-charging done. Once a charging cycle is complete, an INT is asserted to notify the host.

The host can always control the charging operation and optimize the charging parameters by writing to the registers through I²C.

8.3.3.2 Battery Charging Profile

The device charges the battery in three phases: preconditioning, constant current and constant voltage. At the beginning of a charging cycle, the device checks the battery voltage and applies current.

If the charger device is in DPM regulation or thermal regulation during charging, the actual charging current will be less than the programmed value. In this case, termination is temporarily disabled and the charging safety timer is counted at half the clock rate.

Figure 16. Battery Charging Profile

8.3.3.3 Thermistor Qualification

The charger device provides a single thermistor input for battery temperature monitor.

8.3.3.3.1 Cold/Hot Temperature Window

The device continuously monitors battery temperature by measuring the voltage between the TS pin and ground, typically determined by a negative temperature coefficient thermistor and an external voltage divider. The device compares this voltage against its internal thresholds to determine if charge or boost is allowed.

To initiate a charge cycle, the battery temperature must be within the V_LTF to V_HTF thresholds. During the charge cycle the battery temperature must be within the V_LTF to V_TCO thresholds, else the device suspends charging and waits until the battery temperature is within the V_LTF to V_HTF range.

For battery protection during boost mode, the device monitors the battery temperature to be within the VBCOLDx to VBHOTx thresholds unless boost mode temperature is disabled by setting BHOT bits (REG06[3:2]) to 11. When temperature is outside of the temperature thresholds, the boost mode and BATFET are disabled and BATFET_Disable bit is set (REG07[5] bit) to reduce leakage current on PMID. Once temperature returns within thresholds, the host can clear BATFET_Disable bit (REG07[5]) or provide logic low to high transition on QON pin to enable BATFET and boost mode.

Figure 17. TS Resistor Network

When the TS fault occurs, the fault register REG09[2:0] indicates the actual condition on each TS pin and an INT is asserted to the host. The STAT pin indicates the fault when charging is suspended.

Figure 18. TS Pin Thermistor Sense Thresholds in Charge Mode

Figure 19. TS Pin Thermistor Sense Thresholds in Boost Mode

Assuming a 103AT NTC thermistor is used on the battery pack Figure 18, the value RT1 and RT2 can be determined by using the following equation:

Equation 1.

Select 0°C to 45°C range for Li-ion or Li-polymer battery,
RTH_COLD = 27.28 kΩ
RTH_HOT = 4.911 kΩ
RT1 = 5.25 kΩ
RT2 = 31.23 kΩ

8.3.3.4 Charging Termination

The device terminates a charge cycle when the battery voltage is above recharge threshold, and the current is below termination current. After the charging cycle is complete, the BATFET turns off. The converter keeps running to power the system, and BATFET can turn back on to engage supplement mode.

When termination occurs, the status register REG08[5:4] is 11, and an INT is asserted to the host. Termination is temporarily disabled if the charger device is in input current/voltage regulation or thermal regulation. Termination can be disabled by writing 0 to REG05[7].

8.3.3.5 Charging Safety Timer

The device has safety timer to prevent extended charging cycle due to abnormal battery conditions. The safety timer is 4 hours when the battery is below batlowv threshold. The user can program fast charge safety timer (default 12 hours) through I²C (REG05[2:1]). When safety timer expires, the fault register REG09[5:4] goes 11 and an INT is asserted to the host. The safety timer feature can be disabled via I²C (REG05[3]).

The following actions restart the safety timer after safety timer expires:

• Toggle the CE pin HIGH to LOW to HIGH (charge enable)
• Write REG01[5:4] from 00 to 01 (charge enable)
• Write REG05[3] from 0 to 1 (safety timer enable)

During input voltage/current regulation, thermal regulation, or FORCE_20PCT bit (REG02[0]) is set , the safety timer counting at half clock rate since the actual charge current is likely to be below the register setting. For example, if the charger is in input current regulation (IINDPM) throughout the whole charging cycle, and the safety time is set to 5 hours, the safety timer will expire in 10 hours. This feature can be disabled by writing 0 to REG07[6].

8.3.4.1 Charging Status Indicator (STAT)

The device indicates charging state on the open drain STAT pin. The STAT pin can drive LED as the application diagram shows.

8.3.4.2 Interrupt to Host (INT)

In some applications, the host does not always monitor the charger operation. The INT notifies the system on the device operation. The following events will generate a 256-µs INT pulse.

1. USB/adapter source identified (through DPDM detection)
2. Good input source detected
   • not in sleep
   • VBUS below V_ACOV threshold
   • current limit above I_BADSRC
3. Input removed or VBUS above V_ACOV threshold
4. Charge Complete
5. Any FAULT event in REG09

For the first four events, INT pulse is always generated. For the last event, when a fault occurs, the charger device sends out INT and latches the fault state in REG09 until the host reads the fault register. If a prior fault exists, the charger device would not send any INT upon new faults except NTC fault (REG09[2:0]). The NTC fault is not latched and always reports the current thermistor conditions. In order to read the current fault status, the host has to read REG09 two times consecutively. The 1^st reads fault register status from the last read and the 2^nd reads the current fault register status.

8.3.5.1 Input Current Limit on ILIM

For safe operation, the device has an additional hardware pin on ILIM to limit maximum input current on ILIM pin. The input maximum current is set by a resistor from ILIM pin to ground as:

Equation 2.

The actual input current limit is the lower value between ILIM setting and register setting (REG00[2:0]). For example, if the register setting is 111 for 3 A, and ILIM has a 316-Ω resistor to ground for 1.5 A, the input current limit is 1.5 A. ILIM pin can be used to set the input current limit rather than the register settings.

The device regulates ILIM pin at 1 V. If ILIM voltage exceeds 1 V, the device enters input current regulation (Refer to Dynamic Power Path Management section).

The voltage on ILIM pin is proportional to the input current. ILIM pin can be used to monitor the input current following Equation 3:

Equation 3.

For example, if ILIM pin sets 2 A, and the ILIM voltage is 0.75 V, the actual input current 1.5 A. If ILIM pin is open, the input current is limited to zero since ILIM voltage floats above 1 V. If ILIM pin is short, the input current limit is set by the register.

8.3.5.2 Thermal Regulation and Thermal Shutdown

During charge operation, the device monitors the internal junction temperature T_J to avoid overheat the chip and limits the IC surface temperature. When the internal junction temperature exceeds the preset limit (REG06[1:0]), the device lowers down the charge current. The wide thermal regulation range from 60°C to 120°C allows the user to optimize the system thermal performance.

During thermal regulation, the actual charging current is usually below the programmed battery charging current. Therefore, termination is disabled, the safety timer runs at half the clock rate, and the status register REG08[1] goes high.

Additionally, the device has thermal shutdown to turn off the converter. The fault register REG09[5:4] is 10 and an INT is asserted to the host.

8.3.5.3 Voltage and Current Monitoring in Buck Mode

The device closely monitors the input and system voltage, as well as HSFET current for safe buck mode operation.

8.3.5.4 Current Monitoring in Boost Mode

The bq24295 closely monitors LSFET current to ensure safe boost mode operation.

8.3.5.5 Battery Protection

8.5.1.1 Data Validity

The data on the SDA line must be stable during the HIGH period of the clock. The HIGH or LOW state of the data line can only change when the clock signal on the SCL line is LOW. One clock pulse is generated for each data bit transferred.

Figure 21. Bit Transfer on the I²C Bus

8.5.1.2 START and STOP Conditions

All transactions begin with a START (S) and can be terminated by a STOP (P). A HIGH to LOW transition on the SDA line while SCl is HIGH defines a START condition. A LOW to HIGH transition on the SDA line when the SCL is HIGH defines a STOP condition.

START and STOP conditions are always generated by the master. The bus is considered busy after the START condition, and free after the STOP condition.

Figure 22. START and STOP Conditions

8.5.1.3 Byte Format

Every byte on the SDA line must be 8 bits long. The number of bytes to be transmitted per transfer is unrestricted. Each byte has to be followed by an Acknowledge bit. Data is transferred with the Most Significant Bit (MSB) first. If a slave cannot receive or transmit another complete byte of data until it has performed some other function, it can hold the clock line SCL low to force the master into a wait state (clock stretching). Data transfer then continues when the slave is ready for another byte of data and release the clock line SCL.

Figure 23. Data Transfer on the I²C Bus

8.5.1.4 Acknowledge (ACK) and Not Acknowledge (NACK)

The acknowledge takes place after every byte. The acknowledge bit allows the receiver to signal the transmitter that the byte was successfully received and another byte may be sent. All clock pulses, including the acknowledge 9^th clock pulse, are generated by the master.

The transmitter releases the SDA line during the acknowledge clock pulse so the receiver can pull the SDA line LOW and it remains stable LOW during the HIGH period of this clock pulse.

When SDA remains HIGH during the 9th clock pulse, this is the Not Acknowledge signal. The master can then generate either a STOP to abort the transfer or a repeated START to start a new transfer.

8.5.1.5 Slave Address and Data Direction Bit

After the START, a slave address is sent. This address is 7 bits long followed by the eighth bit as a data direction bit (bit R/W). A zero indicates a transmission (WRITE) and a one indicates a request for data (READ).

Figure 24. Complete Data Transfer

8.5.1.5.1 Single Read and Write

Figure 25. Single Write

Figure 26. Single Read

If the register address is not defined, the charger IC send back NACK and go back to the idle state.

8.5.1.5.2 Multi-Read and Multi-Write

The charger device supports multi-read and multi-write on REG00 through REG08.

Figure 27. Multi-Write

Figure 28. Multi-Read

The fault register REG09 locks the previous fault and only clears it after the register is read. For example, if Charge Safety Timer Expiration fault occurs but recovers later, the fault register REG09 reports the fault when it is read the first time, but returns to normal when it is read the second time. To verify real time fault, the fault register REG09 should be read twice to get the real condition. In addition, the fault register REG09 does not support multi-read or multi-write.

REG09 is a fault register. It keeps all the fault information from last read until the host issues a new read. For example, if there is a TS fault but gets recovered immediately, the host still sees TS fault during the first read. In order to get the fault information at present, the host has to read REG09 for the second time. REG09 does not support multi-read and multi-write.

8.6.1.1 Input Source Control Register REG00 [reset = 01011000, or 58]

8.6.1.2 Power-On Configuration Register REG01 [reset = 00011011, or 0x1B]

8.6.1.3 Charge Current Control Register REG02 [reset = 00100000, or 0x20]

8.6.1.4 Pre-Charge/Termination Current Control Register REG03 [reset = 00010001, or 0x11]

8.6.1.5 Charge Voltage Control Register REG04 [reset = 10110010, or 0xB2]

8.6.1.6 Charge Termination/Timer Control Register REG05 [reset = 10011100, or 0x9C]

8.6.1.7 Boost Voltage/Thermal Regulation Control Register REG06 [reset = 10010011, or 0x93]

8.6.1.8 Misc Operation Control Register REG07 [reset = 01001011, or 4B]

8.6.1.9 System Status Register REG08

8.6.1.10 New Fault Register REG09

8.6.1.11 Vender / Part / Revision Status Register REG0A