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EM1402EVM: Switch Matrix Control

Part Number: EM1402EVM
Other Parts Discussed in Thread: BQ76PL455A

Hello, 

   I am currently using the EM1402EVM to charge and discharge various cells to a separate 12V source.  However, I sometimes hear a high pitches noise when I believe the switch matrix is not connected to a cell, leaving Vplus and Vminus floating.  I was wondering if this is normal, or if I should be worried that something is not operating properly.  I am controlling the switch matrix SPI (with a FPGA) which is separate from the UART connection (via PIC Microcontroller).  However, I am able to command both successfully.  I am able to charge/discharge a given cell and see that cell's voltage change when polling the response from the BQ76PL455A.  This leads me to think that I am correctly selecting the cell and charging/discharging from it, but after I send a "Open all switches" command to the EMB1428 connected to the charging/discharging cell, I hear a faint high pitch noise coming from the EM1402EVM.  

Additionally, when I initially set VSET via the on-board DAC through SPI, I send a command to change its internal register to a value of 155, which should make VSET about 2V given that my VIO is 3.3V (which I have probed), but instead it sets VSET a voltage of almost zero (0.03V).  Though when I send a value of 50, which should set VSET to (50/256*VIO) = 0.64V, I actually end up reading 1.48V on the DAC_OUT test point.  I also don't believe it to be a timing issue/bit shift when writing the data, because if I send a raw value of 25, so half of what I sent to get the VSET to 1.5V, VSET does change to 0.75V.  So by reducing the 8-bit value I write to the DAC's internal register by half, I see the same half reduction in the VSET.  However, no matter what I do I am unable to get VSET to 2V.  

So, I don't think these two problems are related, but I am not sure.

So,

1: Is it normal to hear a faint high pitched noise when operating the switch matrix? (either when it is connected to a cell or not)

2: Is there any additional steps I am missing that could be causing VSET to not abide by the equation given in the data sheet of (raw_value/256*VIO), or is my calculation simply incorrect?

Thank you in advance, 

Russell Okamura

  • Hi Russell,

    To answer your questions:

    1. The noise you are hearing is likely coil whine coming from the transformer. This happens when the coil vibrates at a high frequency, and is typically due to switching. While coil whine is not always sign of issues, I would suggest you measure the current passing through the coil and DC/DC converter to ensure the current peak during the "open all switches" command isn't abnormal, and doesn't exceed the transformer's max spec.

    2. For setting the VSET parameter, would you be able to test if the following formula works for your application (taken from EM1402 sample code, in file "emb1428.c", line 323):

    Setpoint = (int)(Amps/2.5/(3.2/256)) + 5;

    (This formula takes into account the specific secondary side sense resistance)

    Thanks!
    Vince
  • Thanks Vince for the response.  As for the checking of VSET, how should I be verifying that the DAC is working properly?  Should I be probing the DAC_OUT, in order to verify a certain voltage? If so, which voltage.  Say if I want 3A of balancing current.  This would mean (int)(3/2.5/(3.2/256))+5 setting the set point to 101.  In this case what voltage should I be reading on DAC_Out? 

  • Hi Russel,

    According to the DAC datasheet, the DAC under ideal circumstances outputs a voltage of Vout=VA*(D/256) where D is the decimal equivalent of the binary code that is sent to the DAC and VA is the supply voltage. So in this case, with a 3.3V supply voltage and D=101, the output would be approximately 1.3V.

    Regards!
    Vince
  • Thanks Vince! 

     This helps me out a lot.