bq297xx Cost-Effective Voltage and Current Protection Integrated Circuit for Single-Cell Li-Ion and Li-Polymer Batteries (Rev. D)
SLUSBU9D – March2014 – revisedMay 2016
UNLESS OTHERWISE NOTED, this document contains PRODUCTION DATA.
9 Detailed Description
This bq2970 device is a primary protector for a single-cell Li-Ion/Li-Polymer battery pack. The device uses a minimum number of external components to protect for overcurrent conditions due to high discharge/charge currents in the application. In addition, it monitors and helps to protect against battery pack overcharging or depletion of energy in the pack. The bq2970 device is capable of having an input voltage of 8 V from a charging adapter and can tolerate a voltage of BAT – 25 V across the two input pins. In the condition when a fault is triggered, there are timer delays before the appropriate action is taken to turn OFF either the CHG or DSG FETs. The recovery period also has a timer delay once the threshold for recovery condition is satisfied. These parameters are fixed once they are programmed. There is also a feature called zero voltage charging that enables depleted cells to be charged to an acceptable level before the battery pack can be used for normal operation. Zero voltage charging is allowed if the charger voltage is above 1.7 V. For Factory Programmable Options, see Table 1.
Table 1. Factory Programmable Options
For available released devices, see the Released Device Configurations table.
9.2 Functional Block Diagram
9.3 Feature Description
The bq2970 family of devices measures voltage drops across several input pins for monitoring and detection of the following faults: OCC, OCD, OVP, and UVP. An internal oscillator initiates a timer to the fixed delays associated with each parameter once the fault is triggered. Once the timer expires due to a fault condition, the appropriate FET drive output (COUT or DOUT) is activated to turn OFF the external FET. The same method is applicable for the recovery feature once the system fault is removed and the recovery parameter is satisfied, then the recovery timer is initiated. If there are no reoccurrences of this fault during this period, the appropriate gate drive is activated to turn ON the appropriate external FET.
9.4 Device Functional Modes
9.4.1 Normal Operation
This device monitors the voltage of the battery connected between BAT pin and VSS pin and the differential voltage between V– pin and VSS pin to control charging and discharging. The system is operating in NORMAL mode when the battery voltage range is between the over-discharge detection threshold (VUVP) and the overcharge detection threshold (VOVP), and the V– pin voltage is within the range for charge overcurrent threshold (VOCC) to over-discharge current threshold (VOCD) when measured with respect to VSS. If these conditions are satisfied, the device turns ON the drive for COUT and DOUT FET control.
When the battery is connected for the first time, the discharging circuit might not be enabled. In this case, short the V– pin to the VSS pin.
Alternatively, connect the charger between the Pack+ and Pack– terminals in the system.
9.4.2 Overcharge Status
This mode is detected when the battery voltage measured is higher than the overcharge detection threshold (VOVP) during charging. If this condition exists for a period greater than the overcharge detection delay (tOVPD) or longer, the COUT output signal is driven low to turn OFF the charging FET to prevent any further charging of the battery.
The overcharge condition is released if one of the following conditions occurs:
- If the V– pin is higher than the overcharge detection voltage (VOCC_Min), the device releases the overcharge status when the battery voltage drops below the overcharge release voltage (VOVP-Hys).
- If the V– pin is higher than or equal to the over-discharge detection voltage (VOCD), the device releases the overcharge status when the battery voltage drops below the overcharge detection voltage (VOVP).
The discharge is initiated by connecting a load after the overcharge detection. The V– pin rises to a voltage greater than VSS due to the parasitic diode of the charge FET conducting to support the load. If the V– pin voltage is higher than or equal to the discharge overcurrent detection threshold (VOCD), the overcurrent condition status is released only if the battery voltage drops lower than or equal to the overcharge detection voltage (VOVP).
- If the battery is overcharged to a level greater than overcharge detection (VOVP) and the battery voltage does not drop below the overcharge detection voltage (VOVP) with a heavy load connected, the discharge overcurrent and load short-circuit detection features do not function until the battery voltage drops below the overcharge detection voltage (VOVP). The internal impedance of a battery is in the order of tens of mΩ, so application of a heavy load on the output should allow the battery voltage to drop immediately, enabling discharge overcurrent detection and load short-circuit detection features after an overcharge release delay.
- When a charger is connected after an overcharge detection, the overcharge status does not release even if the battery voltage drops below the overcharge release threshold. The overcharge status is released when the V– pin voltage exceeds the overcurrent detection voltage (VOCD) by removing the charger.
9.4.3 Over-Discharge Status
If the battery voltage drops below the over-discharge detection voltage (VUVP) for a time greater than (tUVPD) the discharge control output, DOUT is switched to a low state and the discharge FET is turned OFF to prevent further discharging of the battery. This is referred to as an over-discharge detection status. In this condition, the V– pin is internally pulled up to BAT by the resistor RV–D. When this occurs, the voltage difference between V– and BAT pins is 1.3 V or lower, and the current consumption of the device is reduced to power-down level ISTANDBY. The current sink IV–S is not active in power-down state or over-discharge state. The power-down state is released when a charger is connected and the voltage delta between V– and BAT pins is greater than 1.3 V.
If a charger is connected to a battery in over-discharge state and the voltage detected at the V– is lower than –0.7 V, the device releases the over-discharge state and allows the DOUT pin to go high and turn ON the discharge FET once the battery voltage exceeds over-discharge detection voltage (VUVP).
If a charger is connected to a battery in over-discharge state and the voltage detected at the V– is higher than –0.7 V, the device releases the over-discharge state and allows the DOUT pin to go high and turn ON the discharge FET once the battery voltage exceeds over-discharge detection release hysteresis voltage (VUVP +Hys).
9.4.4 Discharge Overcurrent Status (Discharge Overcurrent, Load Short-Circuit)
When a battery is in normal operation and the V– pin is equal to or higher than the discharge overcurrent threshold for a time greater than the discharge overcurrent detection delay, the DOUT pin is pulled low to turn OFF the discharge FET and prevent further discharge of the battery. This is known as the discharge overcurrent status. In the discharge overcurrent status, the V– and VSS pins are connected by a constant current sink IV–S. When this occurs and a load is connected, the V– pin is at BAT potential. If the load is disconnected, the V– pin goes to VSS (BAT/2) potential.
This device detects the status when the impedance between Pack+ and Pack– (see Figure 26) increases and is equal to the impedance that enables the voltage at the V– pin to return to BAT – 1 V or lower. The discharge overcurrent status is restored to the normal status.
Alternatively, by connecting the charger to the system, the device returns to normal status from discharge overcurrent detection status, because the voltage at the V– pin drops to BAT – 1 V or lower.
The resistance RV–D between V– and BAT is not connected in the discharge overcurrent detection status.
9.4.5 Charge Overcurrent Status
When a battery is in normal operation status and the voltage at V– pin is lower than the charge overcurrent detection due to high charge current for a time greater than charge overcurrent detection delay, the COUT pin is pulled low to turn OFF the charge FET and prevent further charging to continue. This is known as charge overcurrent status.
The device is restored to normal status from charge overcurrent status when the voltage at the V– pin returns to charge overcurrent detection voltage or higher by removing the charger from the system.
The charge overcurrent detection feature does not work in the over-discharge status.
The resistance RV–D between V– and BAT and the current sink IV–S is not connected in the charge overcurrent status.
9.4.6 0-V Charging Function (Available)
This feature enables recharging a connected battery that has very low voltage due to self-discharge. When the 0-V battery charge starting charger voltage V0CHG or higher voltage is applied to Pack+ and Pack– connections by the charger, the COUT pin gate drive is fixed by the BAT pin voltage.
Once the voltage between the gate and the source of the charging FET becomes equal to or greater than the turn ON voltage due to the charger voltage, the charging FET is ON and the battery is charged with current flow through the charging FET and the internal parasitic diode of the discharging FET. Once the battery voltage is equal to or higher than the over-discharge release voltage, the device enters normal status.
- Some battery providers do not recommend charging a depleted (self-discharged) battery. Consult the battery supplier to determine whether to have the 0-V battery charger function.
- The 0-V battery charge feature has a higher priority than the charge overcurrent detection function. In this case, the 0-V charging will be allowed and the battery charges forcibly, which results in charge overcurrent detection being disabled if the battery voltage is lower than the over-discharge detection voltage.
9.4.7 0-V Charging Function (Unavailable)
This feature inhibits recharging a battery that has an internal short circuit of a 0-V battery. If the battery voltage is below the charge inhibit voltage V0INH or lower, the charge FET control gate is fixed to the Pack– voltage to inhibit charging. When battery is equal to V0INH or higher, charging can be performed.
Some battery providers do not recommend charging a depleted (self-discharged) battery. Consult the battery supplier to determine whether to enable or inhibit the 0-V battery charger function.
9.4.8 Delay Circuit
The detection delay timers are based from an internal clock with a frequency of 10 kHz.
If the over-discharge current is detected, but remains below the over-discharge short circuit detection threshold, the over-discharge detection conditions must be valid for a time greater than or equal to over-discharge current delay tOCCD time before the DOUT goes low to turn OFF the discharge FET. However, during any time the discharge overcurrent detection exceeds the short circuit detection threshold for a time greater than or equal to load circuit detection delay tSCCD, the DOUT pin goes low in a faster delay for protection.
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