• State Resolved
  • Locked Locked
  • Replies 5 replies
  • Answers 1 answer
  • Subscribers 29 subscribers
  • Views 608 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.

DRV8835: DRV8835 vs DRV8838 - different behaviour on low VM

Björn Haack
Prodigy 10 points
Part Number: DRV8835
Other Parts Discussed in Thread: DRV8838, , DRV8838EVM,

Tool/software:

Hi,

in our application we are driving a resistive load, approx. 8-25 Ohm (screenshots below are 8 Ohm). VM is adjustable between 0 and 8V.
We did tests with DRV8838 and DRV8835. DRV8838 works as expected, DRV8835 does not:

8838_ok

8835 - not ok

8835 - not OK zoom


If VM is between 0.9 V and 1.25 V, the output gets disabled. Zooming in shows short pulses every 1ms - acc. to the datasheet this could be "Overcurrent protection retry time"?!
It does not matter whether the load is connected to a single channel or to both channels in parallel. We also tested both VCC=3.3V and VCC=5V, no difference.

Since the datasheets of both devices are spec-wise nearly identical (except that DRV8835 has two channels) - why is there a different behaviour? And what happens between 0.9 V and 1.25 V in DRV8835? Is it safe to use DRV8838 for a VM down to 0V? A non-working OCP is acceptable (current is limited by the resistive load), as well as a slight degradation on rds_on, etc., because the current is low in that working area - as long as the output is not shut down.

Thanks,
Bjoern

  • 0
    TI__Mastermind 20680 points

    Hello,

    Please allow me some time to review and I'll reach out on Monday. 

    Best,

    David

  • 0
    TI__Mastermind 20680 points

    Hi Bjoern,

    This does seem like strange behavior for the DRV8835. Both of these devices don't have a undervoltage lockout (UVLO) protection on the supply voltage VM, so it seems strange that the outputs would be disabling. I also have a hard time believing the device would be tripping the overcurrent threshold (OCP) due to the small voltage supply and load (I=V/R=1.25V/8Ω=156mA). 

    How many devices have shown this behavior during your testing?

    Can you confirm the VCC voltage is not somehow dropping out?

    Is the same layout/board being used for testing between DRV8835 and DRV8838? Is the testing being done on an DRV8835EVM / DRV8838EVM or personal design?

    Yes, as long as VCC > 1.8V then VM can go down to 0V. 

    Best,

    David

  • 0 in reply to David Medis
    Prodigy 10 points

    Hi David,

    the board is a custom design. I've tested two boards, both show the same behaviour. I can check a third board tomorrow. For the DRV8838 we removed the DRV8835 from the custom board and connected a Pololu DRV8838 (www.pololu.com/.../2990).
    I don't have EVMs for either DRVs on hand. I will also check VCC tomorrow, but I doubt that there will be any noticeable drop visible, especially not below 1.8V. VCC is decoupled with 100nF right next to the DRV and buffered with 2x22uF at the linear regulator (both 3.3V and 5V).

    There are two older forum threads with similar problems, but the answers there are a bit vague and don't explain the different behaviour of DRV8838.

    https://e2e.ti.com/support/motor-drivers-group/motor-drivers/f/motor-drivers-forum/863677/drv8835-regarding-vm-and-vcc

    e2e.ti.com/.../2362723

    Regards,

    Bjoern

  • 0 in reply to Björn Haack
    TI__Mastermind 20680 points

    Hi Bjoern,

    It appears that the OCP is triggering unexpectedly. This seems to be a known issue with the DRV8835 when operating VM < 2V. The datasheet specs VM capable of going to 0V to highlight the potential without affecting the VCC (digital) power, however, the OCP circuitry begins to operate unexpectedly when VM < 2V.  

    I would suggest using the DRV8838 if needing to drop VM < 2V. 

    Best,

    David

  • 0 in reply to David Medis
    Prodigy 10 points

    Hi David,

    thanks, then I'll have to do a redesign. It would be nice to have such known issues listed in the datasheet...

    Regards,

    Bjoern