SLUSA52C September   2010  – March 2016


  1. Features
  2. Applications
  3. Description
  4. Revision History
  5. Device Options
  6. Pin Configuration and Functions
  7. Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 ESD Ratings
    3. 7.3 Recommended Operating Conditions
    4. 7.4 Thermal Information
    5. 7.5 Electrical Characteristics
    6. 7.6 Recommended Cell Balancing Configurations
    7. 7.7 Typical Characteristics
  8. Detailed Description
    1. 8.1 Overview
      1. 8.1.1 Voltage Protection
      2. 8.1.2 Cell Balancing
    2. 8.2 Functional Block Diagram
    3. 8.3 Feature Description
      1. 8.3.1 Protection (OUT) Timing
      2. 8.3.2 Cell Voltage > VPROTECT
      3. 8.3.3 Cell Connection Sequence
      4. 8.3.4 Cell Balance Enable Control
      5. 8.3.5 Cell Balance Configuration
      6. 8.3.6 Cell Imbalance Auto-Detection (Via Cell Voltage)
      7. 8.3.7 Customer Test Mode
      8. 8.3.8 Test Conditions
    4. 8.4 Device Functional Modes
      1. 8.4.1 NORMAL Mode
      2. 8.4.2 PROTECTION Mode
  9. Application and Implementation
    1. 9.1 Application Information
    2. 9.2 Typical Applications
      1. 9.2.1 Battery Connection
        1. Design Requirements
        2. Detailed Design Procedure
        3. Application Curve
    3. 9.3 System Example
      1. 9.3.1 External Cell Balancing
  10. 10Power Supply Recommendations
  11. 11Layout
    1. 11.1 Layout Guidelines
    2. 11.2 Layout Example
  12. 12Device and Documentation Support
    1. 12.1 Related Links
    2. 12.2 Community Resources
    3. 12.3 Trademarks
    4. 12.4 Electrostatic Discharge Caution
    5. 12.5 Glossary
  13. 13Mechanical, Packaging, and Orderable Information

Package Options

Refer to the PDF data sheet for device specific package drawings

Mechanical Data (Package|Pins)
  • DRB|8
Thermal pad, mechanical data (Package|Pins)
Orderable Information

8 Detailed Description

8.1 Overview

The bq2920x provides overvoltage protection and cell balancing for 2-series cell lithium-ion battery packs.

8.1.1 Voltage Protection

Each cell voltage is continuously compared to a factory configured internal reference threshold. If either cell reaches an overvoltage condition, the bq2920x device starts a timer that provides a delay proportional to the capacitance on the CD pin. Upon expiration of the internal timer, the OUT pin changes from a low to high state.

8.1.2 Cell Balancing

If enabled, the bq2920x performs automatic cell-balance correction where the two cells are automatically corrected for voltage imbalance by loading the cell with the higher voltage with a small balancing current. When the cells are measured to be equal within nominally 0 mV, the load current is removed. It will be re-applied if the imbalance exceeds nominally 30 mV. The cell mismatch correction circuitry is enabled by pulling the CB_EN pin low, and disabled when CB_EN is pulled to greater than 2.2 V, for example, VDD.

If the internal cell balancing current of up to 15 mA is insufficient, the bq2920x may be configured via external circuitry to support much higher external cell balancing current.

8.2 Functional Block Diagram

bq29200 bq29209 fbd_lusa52.gif

8.3 Feature Description

8.3.1 Protection (OUT) Timing

Sizing the external capacitor is based on the desired delay time as follows:

bq29200 bq29209 TimeDelay.gif

Where td is the desired delay time and XDELAY is the overvoltage delay time scale factor, expressed in seconds per microFarad. XDELAY is nominally 9 s/µF. For example, if a nominal delay of 3 seconds is desired, use a CCD capacitor that is 3 s / 9 s/µF = 0.33 µF.

The delay time is calculated as follows:

bq29200 bq29209 TimeDelay2.gif

If the cell overvoltage condition is removed before the external capacitor reaches the reference voltage, the internal current source is disabled and an internal discharge block is employed to discharge the external capacitor down to 0 V. In this instance, the OUT pin remains in a low state.

8.3.2 Cell Voltage > VPROTECT

When one or both of the cell voltages rises above VPROTECT, the internal comparator is tripped, and the delay begins to count to td. If the input remains above VPROTECT for the duration of td, the bq2920x output changes from a low to a high state, by means of an internal pull-up network, to a regulated voltage of no more than 9.5 V when IOH = 0 mA.

The external delay capacitor should charge up to no more than the internal LDO voltage (approximately 5 V typically), and will fully discharge in approximately under 100 ms when the overvoltage condition is removed.

bq29200 bq29209 ovr_vol_sen_lusa52.gif Figure 4. Timing for Overvoltage Sensing

8.3.3 Cell Connection Sequence


Before connecting the cells, populate the overvoltage delay timing capacitor, CCD.

The recommended cell connection sequence begins from the bottom of the stack, as follows:

  1. GND
  2. VC1
  3. VC2

While not advised, connecting the cells in a sequence other than that described above does not result in errant activity on the OUT pin. For example:

  1. GND
  2. VC2 or VC1
  3. Remaining VCx pin

8.3.4 Cell Balance Enable Control

To avoid prematurely discharging the cells, it is recommended to turn off (pull high) the active-low Cell Balance Enable Control pin at lower State of Charge (SOC) levels.

8.3.5 Cell Balance Configuration

The cell balancing current may be calculated as follows:

For Cell 1 (VC1–GND) balancing current, ICB1:

Equation 1. bq29200 bq29209 Cell1_Equation.gif

For Cell 2 (VC2–VC1) balancing current, ICB2:

Equation 2. bq29200 bq29209 Cell2_Equation.gif


RCB = resistor connected between the top of Cell 1 and the VC1_CB

RCB1 = resistor connected between the top of Cell 1 and the VC1

RCB2 = resistor connected between the top of Cell 2 and the VC2

RVD = resistor connected between the top of Cell 2 and the VDD

8.3.6 Cell Imbalance Auto-Detection (Via Cell Voltage)

The VMM_DET_ON and VMM_DET_OFF specifications are calibrated where VDD = VC2 = 7.6 V and VC1 = 3.8 V. The recommended range of cell balancing is VC2 and VDD between 6.0 V and 8.4 V, and VC1 between 3 V and 4.2 V. Below VDD = 6 V, it is recommended to pull CB_EN high to disable the cell balancing function.

bq29200 bq29209 Threshold_Change.gif Figure 5. VMM_DET_ON and VMM_DET_OFF Threshold

8.3.7 Customer Test Mode

Customer Test Mode (CTM) helps to greatly reduce the overvoltage detection delay time and enable quicker customer production testing. This mode is intended for quick-pass board-level verification tests, and, as such, individual cell overvoltage levels may deviate slightly from the specifications (VPROTECT, VOA). If accurate overvoltage thresholds are to be tested, use the standard delay settings that are intended for normal use.

To enter CTM, VDD should be set to approximately 9.5 V higher than VC2. When CTM is entered, the device switches from the normal overvoltage delay time scale factor, XDELAY, to a significantly reduced factor of approximately 0.08, thereby reducing the delay time during an overvoltage condition.


Avoid exceeding any Absolute Maximum Voltages on any pins when placing the part into CTM. Also, avoid exceeding absolute maximum voltages for the individual cell voltages (VC1–GND) and (VC2–VC1). Stressing the pins beyond the rated limits may cause permanent damage to the device.

To exit CTM, power off the device and then power it back on.

bq29200 bq29209 v_limits_tst_lusa52.gif Figure 6. Voltage Test Limits

8.3.8 Test Conditions

bq29200 bq29209 icc_neas_lusa52.gif Figure 7. ICC, IIN Measurement
bq29200 bq29209 icb_meas_lusa52.gif Figure 8. ICB Measurement

8.4 Device Functional Modes

This device monitors the voltage of the cells connected to the VCx pins and depending on these voltages and the overall battery voltage at VDD the device enters different operating modes.

8.4.1 NORMAL Mode

The device is operating in NORMAL mode when the cell voltage range is between the over-charge detection threshold (VPROTECT) and the minimum supply voltage.

If this condition is satisfied, the device turns OFF the OUT pin.


The device is operating in PROTECTION mode when the cell over voltage protection feature has been triggered. See Cell Voltage > VPROTECT for more details on this feature.

If this condition is satisfied, the device turns ON the OUT pin.