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LM2907-N: LM2907 Over-Speed Latch Failure

fulvio guidetti1
Prodigy 20 points
Part Number: LM2907-N

Hi TI,

 

I'm introducing LM2907 in a new project that will be tested at Nemko.

Here is the circuit:

 

When I turn off the power ,on the CP2 pin I measure :

 

Figure 1 - C1 = CP2 , C2 = Vcc

 How come CP2 generates that huge pulse?

If I turn it on again in the period of this impulse, the following happens:

The circuit goes into protection and blocks the system!

 

 

  • 0
    TI__Mastermind 21067 points

    fulvio,

    Can you re-post the schematic?  I cannot read the current version (low resolution).   Sorry for the delay!  Thanks for the help.

    Art

  • 0 in reply to Art Kay
    Prodigy 20 points

    Hi Art ,

    thanks for the answer.

    I put a zoom of the schematic. I hope is clear.

    Best regards.

    Fulvio

  • 0 in reply to fulvio guidetti1
    TI__Mastermind 21067 points

    Fulvio,

    Thanks the schematics are very clear now.  I believe the voltage transient you see on CP2 is related to the charge pump shutdown.  If you track the voltage on CP1, I think you will see voltage on CP1 after power shutdown, and it transfers to CP2 once the device power is cycled down.  It really is not unusual for many different types of devices to have some transient behavior during shutdown.  Also, it is not unusual to have a requirement after power down for device decoupling to need time to fully discharge and allow the device to be fully reset.  Unfortunately, I think for your design the requirement is that you will need to wait for all the decoupling / timing capacitors to fully discharge.

    Best regards,

    Art

  • 0 in reply to Art Kay
    Prodigy 20 points

    Art,

    one more question: to remedy the aforementioned problem, without modifying the pcb, I try to use a 'spare' circuit that I have on the pcb (see image). Q28 is active for 10mS at powerup and I force a hardware reset to the signal that joins EMIT to CP2 .
    What do you think ? Shouldn't I damage the chip?
    The trials , I've done , have given positive responses.

    Thanks.

    Best Regards,

    Fulvio

  • 0 in reply to fulvio guidetti1
    TI__Mastermind 21067 points

    Fulvio,

    This should not damage the device.  The best way to know if you are concerned about damaging a device is to look at the absolute maximum table.  For this device the input range is listed as 0V to 28V.

    In general, regarding this solution:  It seems like a reasonable approach to avoid the issue.   Generally, I try to avoid this type of approach by providing sufficient reset time, and/or power supply current capacitary.  I understand that this is not always possible, so I don't have an issue with using it.  Nice work on finding a cleaver way to overcome the transient shutdown issue.

    Best regards,

    Art