BQ2961_17 Datasheet, PDF (19/33 Pages) Texas Instruments – Overvoltage Protection for 2-Series, 3-Series, and 4-Series Cell Li-Ion Batteries with Regulated Output Supply

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9.2.2 Detailed Design Procedure
bq2961, bq2962
SLUSBU5D – NOVEMBER 2013 – REVISED JANUARY 2017
NOTE
The device VSS must be connected first during PCB test or cell attachment. Failure to do
so can damage the REG pin.
1. If VSS pin cannot be connected first, it is required to add a resistor of a minimum of 5 ohms to a maximum of
10 ohms (a 5-ohm resistor is used in the reference schematic, Figure 17) in series with the REG capacitor.
When VSS is floating, the REG capacitor always charges up to the VDD voltage. When VSS is finally
connected, the REG capacitor will be discharged. Adding a small resistor in series reduces the current
strength and avoids any potential damage to the REG pin. The 5-ohm resistor can be placed in series with
the REG connect circuit (as shown in Figure 17) or in series of the REG capacitor (as shown in Figure 18).
Placing the resistor in series with the REG circuit results in a small drop of VREG (for example: max loading
of 1 mA with a 5-ohm resistor will drop 5 mV on VREG), but such a connection can protect again rush
current discharge from both REG capacitor or external filter capacitor connected to the REG pin. Placing the
resistor in series with the REG capacitor is an alternative to avoid additional drop in VREG if the filer
capacitor used by the external circuit is much smaller than the REG capacitor.
2. After VSS is connected, the device allows random cell connect to the Vx pin.
3. Cell should be connected to the lower V_n pin; the unused V_n pin should be shorted to the (V_n-1) pin. See
Figure 17 for details.
100 Ÿ
Protector
FETs
Pack +
VCELL3
VCELL2
VCELL1
1 NŸ
0.1 µF
1 NŸ
0.1 µF
1 NŸ
0.1 µF
VDD
V4 _bq2961
V3 _bq2962
V2
PWPD
OUT
REG
VSS
V1
5 Ÿ (*)
0.1 µF 0.47 µF
External
Circuit
e.g., RTC
Pack ±
Copyright © 2017, Texas Instruments Incorporated
Figure 17. 3-Series bq2961 and bq2962 Schematic
4. A zener diode can be added to the REG pin to VSS as shown in Figure 18. This is recommended to protect
the circuit connected to the REG pin if floating VSS in the field is a risk concern. When VSS is floating
(during cell connection when VSS is not connected first or in system fault with BAT– wire broken), the REG
voltage always pulls up to VDD. In a 4-S configuration, the REG voltage can reach approximately 16 V with
VSS floating. Adding a zener diode clamps the REG voltage to a safe level for the external circuits
connected to the REG pin. Having the zener diode can also protect the external circuits if REG pin is shorted
to OUT pin or any other high-voltage output terminal. If a zener diode is used, TI recommends putting the
diode on the battery side with the bq296xxx device to allow protection on both the REG pin as well as the
circuit connected to REG under floating VSS condition. The resistor in series with the REG pin is not required
in this case.
Copyright © 2013–2017, Texas Instruments Incorporated
Product Folder Links: bq2961 bq2962
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