BQ7693003DBT datasheet(46/63 Pages) TI1

46 / 63 page

bq76920, bq76930, bq76940

SLUSBK2F – OCTOBER 2013 – REVISED NOVEMBER 2015

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Table 8-4. bq769x0 Design Requirements (continued)

DESIGN PARAMETER

EXAMPLE VALUE at TA = 25°C

UV Protection Delay

OCD Protection Threshold Max

15 A

OCD Protection Delay Time

320 ms

SCD Protection Threshold Max

25 A

SCD Protection Delay Time

100 µs

8.2.2

Detailed Design Procedure

To begin the design process, there are some key steps required for component selection and protection

configuration.

8.2.2.1

Step-by-Step Design Procedure

•

Determine the number of series cells.

–

This value depends on the cell chemistry and the load requirements of the system. For example, to

support a minimum battery voltage of 24 V using Li-CO2 type cells with a cell minimum voltage of

3.0 V, there needs to be at least 8-series cells.

•

Select the correct bq769x0 device.

–

For 8-series cells, the bq76930 is needed.

–

For the correct cell connections, see Table 8-2.

•

Select the correct protection FETs.

–

The bq76930 uses a low-side drive suitable for N-CH FETs.

–

These FETs should be rated for the maximum:

•

Voltage, which should be approximately 5 V (DC) 10 V (peak) per series cell: for example, 40 V.

•

Current, which should be calculated based on both the maximum DC current and the maximum

transient current with some margin: for example, 30 A.

•

Power Dissipation, which can be a factor of the RDS(ON) rating of the FET, the FET package,

and the PCB design: for example, 5 W, assuming 5 m

Ω RDS(ON).

•

Select the correct sense resistor.

–

The resistance value should be selected to maximize the input bandwidth use of the coulomb

counter range, CCRANGE, but not exceed the absolute maximum ratings.

•

Using the normal max discharge current, RSNS = 200 mV / 10.0 A = 20 m

Ω.

•

However, considering ISCD of 25 A and Abs Max SRP–SRN input of –300 mV, RSNS = 300 mV

/ 25 A = 7.5 m

•

The maximum operating current of the system should also be considered as this should be

below the maximum OCD Threshold, which with a RSNS of 7.5 m

Ω gives a max OCD current

setting of 13.3 A.

–

Further tolerance analysis (value tolerance, temperature variation, and so on) and PCB design

margin should also be considered, so RSNS of 5 m

Ω would be suitable with a 75-ppm temperature

coefficient and power rating of 5 W.

•

The bq76930 is chosen, and so the REGSRC pin needs to be powered through a source follower

circuit where the FET is used to provide current for REGSRC from the battery positive terminal while

reducing the voltage to a suitable value for the IC.

–

The FET also dissipates the power resulting from the load current and dropped voltage external to

the IC and care should be taken to ensure the correct dissipation ratings are specified by the

chosen FET.

Application and Implementation

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