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Problem with DRV8402

xforus
Intellectual 345 points

Hello, 

I'm designing an emmiter antena (20kHz) with DRV8402.

I have to send a data frame, for example:"10101010". 

It's ok when I send "1"(sending a 20khz frequency), When OUT A is "1" OUTB is "0". And when OUTA is "0" OUTB is "1".

But when I have to send a "0" I have to make something like a "Brake" with a motor, but in this case is an antenna. So OUTA and OUTB have to be "1".

Here is my problem. In the middle of "0" the signal comes down and comes up very slowly in 8ms more or less. 

I want OUTA and OUTB high all the time of bit rate.

Can you help me?Thank you.

  • 0
    TI__Genius 11830 points

    Hey, your pictures didnt come through.  Can you edit your post and under the "options" tab attach them?

    Thanks!

  • 0 in reply to Foe
    Intellectual 345 points

    Hello Foe,  these are the pictures:

  • 0
    TI__Guru**** 189401 points

    Hello,

    Are you using your own PCB, or the EVM for this device?

    What is the load current? 

    -d2

  • 0 in reply to Don Dapkus
    Intellectual 345 points

    Hello, I'm using my own PCB and the load is an inductor of 161uH with a capacitor in series of 330nF. 

    The problem apears when the power supply is higher than 3,5v. 

  • 0 in reply to xforus
    TI__Guru**** 189401 points

    Our device is an all n-mos output stage. We make use of a bootstrap circuit, which has some duty cycle limitation.

    It seems like your timing should be okay, but it looks like the bootstrap capacitor is discharging. This makes sense as you raise the PVDD, the problem gets worse.

    I suggest you scope the BST_x terminals, these are the reservoir caps for the bootstrap. They get charged when the low-side is turned on. If you see the voltage dropping on them (it should be PVDD), then this is the problem. You will need to use a floating DMM or differential measurement since this voltage is not ground-referenced.

    If this is the problem, then an easy way to fix it is to change the logic such that a 'zero' in your system is both outputs low.

    If that is not possible, you could try doubling the value of the bootstrap capacitors to 100 nF.

    You should also have VDD and GVDD tied directly together (some of our early EVMs had these isolated by a 10 ohm resistor).

    The bootstrap capacitor gets charged when the OUT_x is tied to ground. Make sure these nodes are getting grounded to insure proper bootstrap charging.

    Make sure you use Y5V ceramic caps with 2x your PVDD voltage rating for BST_x.

    -d2

  • 0 in reply to Don Dapkus
    Intellectual 345 points

    Hello Don you were right, I fix it putting 300nF bootstrap capacitors.

    Thankyou very much.