BQ76940: Design guild-lines queries

Hi Santu,

There are several references for design including the datasheet, EVM, application reports and TI Design TIDA-00449   The bq769x0 Family Top 10 Design Considerations application report may be helpful.

1. The sense resistor should be chosen to set the dynamic range of the system current in the dynamic range of the coulomb counter for measurement and in the range of comparator trip levels for protections.   You will generally want to be sure you can pass your maximum current with sufficient margin and keep your minimum current for measurement/gauging above the noise floor of the CC.  Additionally you typically want to minimize the power dissipation of the sense resistor for low cost, low power loss, small size and low heat inside the battery pack.  A 1 mOhm resistor allows 30A with 0.9W dissipation and with the 8.44 uV lsb gives ~8.4 mV resolution, so it is a value which is convenient and allows operation in some amps.  Basically pick a value to balance all the above and meet your requirements.

2. The diodes are explained in section 10 of bq769x0 Family Top 10 Design Considerations, figure 20.  Use a conventional switching or rectifier diode so that the power pin can move slightly with supply current without the measurement input being loaded by the diode drop as might occur with a schottky diode.

3. Section 4 of the above app report describes cell balance circuits.  The filter resistors are typically large for filtering and system transients as described in section 3, and the gate resistors can be larger to similarly keep currents small. P-channel balance FETs will have an influence on cell connection as described in section 10, so N-channel may be preferred. Balancing current is set by the resistor switched across the cell. How much to balance really depends on the cell conditions, electrical load on the cells and the time between balancing - a system which operates at a constant temperature and balances on a daily cycle will need less balancing current than a system with a large thermal gradient which is cycled infrequently.

Hi WM5295,

Thank you for reply. I have gone through the documents and they are very useful to understand the concept for battery charging. I have further queries regarding the design.

1. Cell Balancing circuit
- They are various design document for BQ76940 where they have used P-channel MOSFET and other where they have used N-Channel. For example:
www.ti.com/.../slvu925b.pdf
This is EVM board BQ76940EVM for BQ76940 where P-Channel MOSFET is used for cell balancing.

Another document
www.ti.com/.../tiduar8b.pdf
This has design using N-channel MOSFET
Now I want to know the major design issue I might have if used p-channel MOSFET.

2. High side or Low side drive circuit
I have implemented High Side Drive circuit in my design. I want to know if I need to take care of any other issue that might occur or Low Side drive is better.

I really appreciate your suggestion and review. to my queries.

Thanks & Regards,
Santu Reddy

Hi Santu,

1. Either p-ch or n-ch balancing circuit will work.  N-ch FETs may be more available and will avoid the pull-up on connection described in section 10 figure 18 of the bq769x0 Family Top 10 Design Considerations application report. 

2. Since most electronics is referenced to a low side or "ground" voltage, high side protection switching may be preferred allowing a "ground" referenced communication path without isolators.  The high side FET protection drive will take some current but probably less than a communication isolator.  Each high side drive circuit technique will have its own concerns.  If you have used the bq76200, be sure to size the charge pump and PACK pin capacitors properly, consider switching losses and using the CPEN and switching the FET controls separately provides more margin for switching.  See the apnotes in the product folder.  If you have used a p-ch high side switch be sure it turns off fast enough.  If you have a another high side driver check for any special information in its data sheet or application notes.

Hello WM5295,

Thank you replying to my queries above.

I'm facing some issue with current sensing calculation.
I'm using 1 mohm in the design, and I would like to calculate the what value will the BQ76940 will read when there a current of 10 Amp follows or 20 Amps.

I would really appreciate if you could help me with understanding the current calculation.

Thanks & Regards

Santu Reddy

V_sense =1mOhm * 10A  = 10mV;

V_sense= 1mOhm * 20A = 20mV

likewise, 

2mohm @20amps = 40mV. 

The CC is 16-bit resolution; 

a step-by-step procedure is listed @ 8.2.2.1 section of the d/s

thanks

vish

Hello everyone,

Thank you Vish and WM5295 for all your suggestions, I'm in my last stage of development now.

Queries:
"The Design uses 12 cell"
1. The maximum load current ( DC Brush-less Motor, 250W ) is 10 Amp at 48 V. Now is it a good Idea to use 1mOhm sense resistor in this case because according to my calculations the sense resistor required is 4.8Ohms and when the voltage drops to 33V ( minimum volatge at each cell as 2.8V) the Load current increase to 7.5 Amp and sense resistor required in this case would be 4.4 Ohm. So should I be using 1 ohm as sense resistor?

2. In all the BQ76940 design and reference design, Low Side Drive circuit is used but in my design I have used High Side drive, now it has advantages but just wants your expertise suggestion in this.