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LM74912-Q1: Part damage

Joe Miller1
Intellectual 325 points
Part Number: LM74912-Q1
Other Parts Discussed in Thread: LM74912Q1EVM,

Tool/software:

Dear Sir/Madam,

I made some modification to the LM74912Q1EVM to drive a motor.

The modifications are:
C5 changed to 100nF
C10 removed
R12 changed to 0R
J5 removed and driven with function generator 0V or 5V.
D2 fitted with CGRA4007 

VIN = 24V
Motor Speed1 start-up current 3A steady state 1.5A
Motor Speed2 start-up current 5A steady state 2A

(1) Speed1 test
Motor started and run when J5 was driven high (5V)
Motor stopped when J5 was driven high (5V)
Repeated multiple time without issue.

(2) Speed2 test
Motor did not run when J5 was driven high (5V) and fault LED D6 is illuminated.

After this event the LM74912-Q1 is damaged.
With no load J5 was driven high (5V) and fault LED D6 is illuminated.

I replaced LM74912-Q1 and repeated (1) and (2). Same result, the LM74912-Q1 is damaged.

If you place J5 on 1-2 with no load then VOUT will be 24V.

Any idea why the LM74912-Q1 gets damaged?

Regards Joe

  • 0
    TI__Mastermind 37910 points

    Hi Joe,

    How did you find out that LM74912-Q1 is damaged here?

    "If you place J5 on 1-2 with no load then VOUT will be 24V" - Is this after the device is damaged or before the motor speed2 test?

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Intellectual 325 points

    Dear Shiven,
    Thank you for the reply.
    The EVM has failed twice after trying to power the speed2 wire.
    After failure LM74912-Q1 was replaced and EN functions correctly.

    speed1 wire to GND is 1.3R
    speed2 wire to GND is 0.95R

    The FETs are good just something in the LM74912-Q1 has failed that relates to the EN input.

    I have performed the following tests:

    (1) J5 on 1-2 (all good no failures)
    Powered EVM with no load.
    Powered EVM then switched on 3A eload.
    Switched on EVM then switched on 3A eload

    (2) Signal generator connected to J5 2-3 and driven 5V.
    Fault LEDS immediately on. No VOUT.
    If you power off the EVM turn signal generator on so J5 2-3 is 5V then power on the EVM the fault LED is off and VOUT is on.

    Regards Joe

  • 0 in reply to Joe Miller1
    TI__Mastermind 37910 points

    Hi Joe,

    The fault LED will turn on only under these conditions. Do you think any of this is happening? Most likely output short.

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Intellectual 325 points

    Hi Shiven,
    speed2 caused a short circuit event as it did not have enough current to start the motor.
    After that event driving EN to 5V caused the fault LED to illuminated and VOUT to be off with or without load (eload).
    Driving EN to 5V then powering EVM allows VOUT to be on and fault LED off.
    The only difference is that when the EVM powers up with EN low taking EN high sometime later causes the fault LED to be illuminated. 
    Only changing the LM74912-Q1 allows EN to work as required until a short circuit event is triggered,
    It is the short circuit event that cause the EN pin or possibly something related to ICS/CS+ to be damaged.

    Regards Joe

  • 0 in reply to Joe Miller1
    TI__Mastermind 37910 points

    Hi Joe,

    Do you see any physical damage on the IC?

    Does this happen on multiple devices?

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Intellectual 325 points

    Hi Shiven,
    I do not see any physical damage just a change is the behaviour of the device.
    I suspect internal damage however I do not know how to prove how or why it is damaged.  

    Regards Joe

  • 0 in reply to Joe Miller1
    TI__Mastermind 37910 points

    Hi Joe,

    I am unsure why a damage like that would happen. Just the EN sequencing changed because of short circuit.

    Is this repeating with multiple devices?

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Intellectual 325 points

    Hi Shiven,

    This is repeatable on the EVM.

    Regards Joe