• State Resolved
  • Locked Locked
  • Replies 8 replies
  • Subscribers 62 subscribers
  • Views 710 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

bq76925: Current measurement processing time? Pin protection needed?

John M
Expert 2135 points

Other Parts Discussed in Thread: BQ76925, BQ77910A

A couple questions...

In a bq76925, is there any appreciable delay or processing time before VIOUT follows any changes in charge/discharge current levels? Other than the time that Cout would take to charge/discharge and the acquisition time by the microcontroller, that is. We have an application that would require polling the ADC measuring VIOUT at least 10x/second and we're hoping that VIOUT updates almost immediately.

We'd also like to test the bq76925 in an application requiring disconnecting/reconnecting of the battery pack (4S LiFePO4) via a single connector. Assuming that VBAT and each VCx pin is protected by a properly spec'd zener to VSS or VC0, do you recommend also adding pin-to-pin protection between VC0+VC1, VC1+VC2, etc., via an 8.2V zener (max = 8.45V) for each? Or would this interfere with use of on-board balancing and open-cell detection?

Thank you!

  • 0
    TI__Guru 54715 points

    The VIOUT begins to change almost immediately.  If you have an EVM you can inject a signal at J5 and observe the response.  A scope showed ~ 300 us rise and fall time in response to a 100mV input which seemed to dominate any delay.

    The part is generally thought of as being connected once, but if signals are kept within absolute maximum limits, it should work.  If single ended clamps and the filter caps won't do this, differential clamps would be recommended.  Remember that the zeners will have a tolerance on the breakpoint and a resistance in conduction. Placing the zeners on the VCx inputs could allow current from adjacent cells during balance if the zeners on those inputs conduct.  Placing zeners at the cell may force any inductive response of the cells in the circuit through the zener.  Placing a zener in the filter would add components, but may allow optimization of the zener.

  • 0 in reply to WM5295
    Expert 2135 points

    Perfect timing, the EVM arrived today. Great news about VIOUT's response time!

    Using 8.2V zeners (for diff. clamping), their rated max clamping voltage was 8.45V. I was hoping that since the max charge voltage for the LiFePO4 cells was 3.60V (7.20V plus imbalance max on VCx pin when balancing) that connecting directly to the VCx inputs would be the easiest solution for differential clamping? The filter's resistors would limit current flow.

    If we placed the zeners in the filter, would you recommend splitting the filter resistor into two, e.g., 24ohms-24ohms in the EVM, and placing the zener between them (and down to between the split resistor for the next lower cell)? Any other placement seemed to be outside the filter.

    Thank you for your time!

  • 0 in reply to John M
    TI__Guru 54715 points

    Yes, with LiFePO4 cells you will have more margin between the operating voltage and device limits. Remember the clamp specification of the zener is a point on the curve, be sure you are operating within its limits.  Depending on your balance algorithm, the induced voltage on the next cell may be only ~ 1.5X the cell voltage which could give you even more room.  You should be able to put a clamp at VCx. Remember that there are both differential and single ended limits for the part, your earlier post indicated you had the single ended covered.

    Placing the zeners in the "middle" of the filter resistors should limit the current into the zeners and provide some filtering and isolation between the zener clamps and the device inputs. During normal operation where the zeners do not conduct they should provide a small capacitance.  A simulation may show the effects, of course be sure to test thoroughly.

     

  • 0 in reply to WM5295
    Expert 2135 points

    I'll split the filter resistors equally and route the zeners between them (at both ends). I'll be sure to carefully check the curves for the selected zeners to make sure we never start conducting unless there's a problem.

    The design currently has TVS diodes acting as the single-end protection. is there a particular benefit to using zeners for that too in this application?

    Thank you for your help!

  • 0 in reply to John M
    TI__Guru 54715 points

    I may use the term "zener" improperly since many of the TVS products I use are zener based. A zener diode typically is meant to operate at a low current as a voltage regulator while a TVS diode is meant to protect a circuit against a voltage impulse.  The zener diode will have a continuous  power rating while the TVS will have a prominent pulse power rating and may have a continuous rating also. If you have a resistor to limit current, a zener may be most appropriate, if you are protecting against a surge possibly with little resistance, the TVS may be the better choice. Check with your preferred diode supplier for better information and recommendations.  If you have significant current surges in resistors, you may want to check with your resistor supplier for specific recommendations. 

  • 0 in reply to WM5295
    Expert 2135 points

    You read my mind...waiting for a reply from our #1 supplier now regarding protection choices.
    I inherited this project from another who has left the company. Reading his notes, I noticed that he spec'd TVS's because they were used in the bq77910A EVM board for protection of two cell-voltage pins (with empty pads to add TVS protection for other cell inputs). I checked the documentation for the bq77910A and no mention was made regarding the choice of TVS over a classic zener.

    I was going to switch to zeners just for consistency in the protection elements (cell-to-cell and cell-to-GND) since the filter resistors provide current-limiting but realized that there might be some special reason for spec'ing a TVS diode over a classic zener diode in this cell-to-GND protection application. I can see that there might be a need for protection from inductive spikes due to the bq77910A FETs turning off, but wasn't sure.

  • 0 in reply to John M
    TI__Guru 54715 points

    Your approach and comments make sense. The single ended abs max (input to ground) is different from the differential abs max (input to input), so one protection may not be sufficient. The input to ground is more restrictive, but could interfere with cell balancing (if used). If it existed, an ideal clamp on the cells within the abs max of the part should protect the part with the input filter.  Since you can't get ideal clamps, approaching your system with differential and single ended protection is good.

    The '910 has a more restrictive single ended abs max limit on the inputs than the '925 (5V vs 6V), so you may have less need and should have more margin for selecting clamps for the '925.  Also there are fewer inputs pulling on the filter on a '925 design.  The TVS parts on the '910 were particularly for the inductive response of the cells, so an impulse type device is appropriate. The one on the 4th cell both helped the filter network maintain signal levels being near the middle of the filter network and protected that more sensitive input.  With an EVM such as the 910 protector it is uncertain what it might be connected to, so the circuit may be too much for your system or too little.  Additionally components may be selected for the EVM based on low volume immediate availability rather than being optimized for a long life high volume product. Your approach of carefully considering your system and clamping needs seems good.

  • 0 in reply to WM5295
    Expert 2135 points

    That makes sense, thank you for taking the time to explain the '910 protection setup!

    I'll continue with the single-ended/diff. zener protection scheme for the first run of prototype boards using the '925 and see what some serious testing and time with a scope shows us. If the waveforms start looking nasty, we'll reconsider our options for better tailoring the protection for the specific abuses we see/measure.

    Thanks again for your time!