CD

DELAY

d

t

C

X

´

=

d

CD

DELAY

t

C

X

=

5-V LDO

and POR

CD

OUT

VDD

GND

VC1

VC2

150 nA

0.1 µF

CB_EN

+

–

Hys.

CB

Logic

CTRL

VC1_CB

CB2_EN

CB1 _EN

+

–

8

bq29200, bq29209

SLUSA52C – SEPTEMBER 2010 – REVISED MARCH 2016

www.ti.com

Product Folder Links: bq29200 bq29209

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Copyright © 2010–2016, Texas Instruments Incorporated

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

8.3 Feature Description

8.3.1 Protection (OUT) Timing

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

capacitor that is 3 s / 9 s/µF = 0.33 µF.

The delay time is calculated as follows: