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BQ51013B: What is the correct value of COMM capacitors?

Steve Rodriguez
Prodigy 150 points
Part Number: BQ51013B
Other Parts Discussed in Thread: BQ51003

Tool/software:

The datasheet clearly shows these as 47nF in Figure 8-11. The description states that when the COMM switch closes, there is 22nF between AC1 and AC2, which I assume is derived by the two 47nF in series (not exactly 22nF, but close).

However, the EVM uses 22nF on each COMM pin, and I have seen other schematic snippets showing 22nF.  This would put the capacitance between AC1 and AC2 at 11nF, which is too low, and doesn't match the figure.

Please clarify.

Thanks,

Steve

  • 0
    TI__Guru* 75866 points

    Hi Steve

    The recommended value to start with is 22nF for Comm Cap.  The value can be increased if needed to improved communications performance.

  • 0 in reply to Bill Johns
    Prodigy 150 points

    Hi Bill,

    Thanks for the reply. I am still having issues getting this to work properly. I get voltage on Vrect, and then 5V comes up for about 40msec, and then everything turns off. It repeats this every 1.25sec, which I believe is the transmitter period. I've attached two scope photos that show this. Blue is RECT, Red is OUT.

    I'm using a small coil that I was able to get working using the bq51003EVM, but it won't work on my board with the bq51013B. The coil is ~12uH, and I am tuning with Cs=220nF. Cd=2.1nF. I have a 196 Ohm resistor in parallel with a 0.1uF cap at FOD, and 50 Ohms between FOD and ILIM (1.06A Imax). I started with a 0.5A current limit, but increased it to see if that fixed anything.

    There is a thermistor connected at TS, and is approx 10K in series with 2.6K. AD is grounded. EN1, EN2, and _AD_EN are floating. I've adjusted the OUT load between open and 10 ohms, and the only difference is how fast OUT discharges after shutdown. There is a 10uF cap on the output, which I have also tried removing.

    Perusing other threads, I am assuming the receiver is not communicating with the transmitter, hence the shutdown. I have tried a couple of different COMM values (10n, 22n, 47n) and a couple different Cd values, to no avail.

    Any ideas what I'm missing?

    Thanks,

    Steve

  • 0 in reply to Steve Rodriguez
    Prodigy 150 points

    Sometimes all you need to do is ask a question, and then you figure it out minutes later! Turns out there was a capacitor at TH which I believe was screwing up the cycle between TS and CTRL. I noticed the bq51003 and bq51013B have different CTRL thresholds, which led me to look here. Long story short, I removed the capacitor between TH and GND and it started working. 

    Is a capacitor recommended here at all? I had a 1uF, but could use a smaller value if it's needed.