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DRV8704: How to increase starting speed and torque

Battulga Ariunbolor
Prodigy 130 points
Part Number: DRV8704

Hello.

I have an issue with DRV8704. I designed my PCB. I made all the connections as per suggested in the DRV8704 technical document

VM = 12.6V

FET = IRF1404 (40V, 202A)

Motor = Tamiya 540 with 75:1 gearbox

Current Without Load = 1.2A

Maximum Torque = 12kg・cm (6.32A/185rpm)

The problem is the starting speed and torque are too low, the motor is not working with its full power at starting, i mean its not moving sharply, its moving gently at starting, and just giving a little pressure makes it stop and transistor gets warmer. And sometimes OCP bit set '1' and fault LED gets 'ON'.

I think its because of my configurations of DRV8704. Here is my configurations and schematic

CTRL REGISTER: DTIME = 00, ISGAIN = 11, ENBL = 1

TORQUE Register: 0xFF

OFF register: TOFF = 0x00

BLANK register: TBLANK = 0x00

DECAY register: DECMOD = 000

DRIVE register: IDRIVEP = 0x10, IDRIVEN = 0x10, TDRIVEP = 0x00, TDRIVEN = 0x00, OCPDEG = 0x11, OCPTH = 0x11 

I changed DRIVE bits, and it's still not working fine enough, Please help me on my configurations

Also its not working fine with parallel transistors, that motors couldn't move and OCP fault is detected, i don't know why? and now i'm using one H-Bridge.

What should i do? Help me please.

  • 0
    TI__Genius 14487 points
    Hi Battulga,

    I don't see any glaring issues with the schematic or your settings at this point in time. There is one detail about the transistors that may be an issue.

    The maximum gate drive current from the DRV8704 is 200 mA to turn on the FETs. The total gate charge of a single IRF1404 FET is 131 nC. This means that your turn-on time for the FET is around 655 ns. The turn-on time specified in the IRF1404 datasheet is 190 ns. Just for a single FET the turn-on time is too long. With two FETs on each gate drive output, the turn-on time will be twice as long. It is possible that the FETs are getting damaged. Also, these long turn-on times will negatively impact any PWM driving you are trying to do.

    Also, since the turn-on time is slow, the VDS of the FET will be large during this transition. The OCP measures the VDS voltage which may be why the OCP fault occurs.
  • 0 in reply to James Lockridge19
    Prodigy 130 points

    Hi James Lockridge,

    Thank you for the response

    Do you mean IRF1404 is unsuitable with DRV8704? Should i change MOSFETs?

    Is there any other way to fix that problem?

  • 0 in reply to Battulga Ariunbolor
    TI__Genius 14487 points
    Battulga,

    My previous post is an educated guess. If you can take pictures on an oscilloscope of the FET waveforms, then we can be sure that the FET turn-on time is the root cause of the problem.

    Before changing MOSFETs, you can try increasing your IDRIVE and TDRIVE settings to the maximum values. This will drive more current into the gates. Also, the current requirement for your motor seems fairly low for the FETs you are using, so you probably only need one FET for each part of the H-bridge rather than two FETs in parallel. After that, if you still do not see satisfying performance from your system, then you should look for FETs with a much lower gate charge requirement.