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DRV8801: Fault condition, Rsense, charge pump, thermal pad

Holger Kaag
Prodigy 50 points
Other Parts Discussed in Thread: DRV8801

Hi TI-forum,

Brush-motor design using DRV8801. Prototypes, hand soldered by skilled personal. UBB=24V, Inom=480mA, Iblock=2A

If Rsense is 120mOhm and a blocked motor cause more than 500mV at SENSE pin, what will happens first, will the device damaged or will the protection stop current flow?

I see some prototypes with nFault=low but functional (UBB=ok). In datasheet block diagram there several abbreviations for fault conditions: abbreviation "STB" and "STG" are for ..?

charge pump: if UBB=24V, what voltage level should VCP and CP1/CP2 go for?

will device be functional if the thermal pad is not connected to GND (pin12/pin2)?

Thanks in common. Holger

  • 0
    Prodigy 50 points

    Hi again,

    ok. sense resistors seems not the problem. In this DRV8801RTYR design is a problem with the charge pump. The devices with nFAULT=low do have VCP close to VBB (24V), so i think fault condition is poor working charge pump.

    CP1/CP2 an reservoir capacitor are both 100nF/50V X7R located without vias as close as possible to dedicated device pins. Which design or layout miss could cause this charge-pump fault?

    Thanks, Holger

  • 0 in reply to Holger Kaag
    TI__Mastermind 20985 points

    Hi Holger,

    This device should be functional without the thermal pad connected to GND, since there are separate device pins.  However, doing that would obviously significantly hurt thermal dissipation, which limits your max operating current.

    It is possible to damage devices and break the charge pump.  So if you have some that had nFAULT low, and now VCP = VBB = 24V, I would guess the charge pump was damaged.  Do your other units still behave normally?

    Best regards,
    RE

  • 0 in reply to RE
    Prodigy 50 points

    Thank you for responding,

    I see two error cases: devices working with nFAULT=low and devices not workling with nFAULT=low. Both have VCP=VBB=24V and arround nothing at CP1/CP2. Devices show this behaviour at first bring-up with two LED's replacing the motor winding. If there is a thermal problem, only option is at soldering.

    If this behaviour is seen at devices, it is at first bring up or never, only possibilty to fix is changing the chip itself, all modification on supply, blocking capacitors or logic levels do not not have any effect. 3 out of 5 pieces show this effect at first bring up, at some devices chip need to replaced two times finding one working properly.

    are there further requirements for chip bring-up? VDD (5V) before VBB? ....dedicated logic pattern or level?

    What other reasons then over current are feasible breaking the charge pump? I would provide scm and pcb-section if this could help.

    best regards, Holger

  • 0 in reply to Holger Kaag
    TI__Mastermind 20985 points

    Hi Holger,

    If I understand this correctly, about 60% of your systems exhibit nFAULT=Low, and some of those have a nonworking charge pump.  Please make sure nSLEEP is high at that time.

    One thing you could try is to swap a nonworking DRV8801 onto a PCB that was working.  That can help determine whether the DRV8801 was damaged from an event, or if there's an issue within the rest of the system.

    Another idea is to measure the max VBB at the device pin.  Of course, Back EMF could bring this too high.

    If you'd like to send your schematic, we'll take a look.

    Best regards,
    RE

  • 0 in reply to RE
    Prodigy 50 points

    Hi,

    nSLEEP = static high (VDD)

    ... nearly .. i also see devices nFAULT=low -> measure: VCP = VBB = 24V but all fets are switching (??) -> devices would pass bring-up test if nFAULT wasn't low.

    Back EMF shouldn't a case at first bring up, it was done using (inductance less) two LED's.

    Find attached scm, i hate to confess a fault, but here i really hope you find some :-)

    Thanks, Holger

    TI_DRV8801_scm.pdf
  • 0 in reply to Holger Kaag
    Prodigy 50 points

    ... and layout screenshot for component side. There are inner planes for 0V and 24V0

  • 0 in reply to Holger Kaag
    TI__Mastermind 20985 points

    No problems stick out at me.  On a failing system you could verify the capacitors still measure good, and look for a soldering problem or short.  Sorry I don't have the answer...

    Best regards,
    RE