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Charge Pump Issue, TRS3223

John Connors
Prodigy 70 points

Other Parts Discussed in Thread: TRS3223

I am using two TRS3223 RS-232 transceivers.  We have built several boards and they seemed to work fine except for one.  On this particular board, both chips show saw tooth noise on the negative side of the charge pump.  Has anyone seen anything similar or now how to fix it?  I thought perhaps the cap were wrong, but they seem to be fine.  Again, the other board work fine, so I do not believe there is an error in the design, but perhaps assembly.

 

Thanks for your help

 

  • 0
    TI__Guru* 76130 points

    Hello John,

     

    As you said, perhaps the problem is with the assembly or with the PCB, Can you share your schematics for review.

    Regards.

  • 0
    TI__Guru**** 171346 points

    Hello John,

    Most important is the value of the capacitors and VCC level.
    Does the V- pin sawtooth have a quick negative edge and a slow positive slope or is it reversed?

    During testing was there a load (RS232 port connected to a remote RS232 port)?

    Regards,
    Ron Michallick

     

  • 0 in reply to Ron Michallick
    Prodigy 70 points

    Hi Ron,

      Sounds like you may have seen this before.  Any help would be appreciated.

     

    The VCC is +5V and the caps (ceramic) were chosen based on the table in the datasheet. Off the top of my head, I want to say 330nF and 47nF.  I suspected the, so I put a DCR on them and they read similarly to the caps on a board that works fine, and seemed right for the VCC level.

     

    The V- and V+ have noise on them.  The V- has the quick edge from approx. -4V to -6V and a linear ramp back to -4V.  The period is about 37ms.  The V+ has a quick edge from +4v to +6V (synchronously with the quick edge of the V-) but falls back to +4v very quickly.

     

    This is without a load.  I have notice that without a load, the output pin seems ok, but with a load, the amplitude starts to sag.

  • 0 in reply to John Connors
    TI__Mastermind 28265 points

    what type of load current are you pulling?

    the charge pumps only supply a few milli amps (say 2 to 3 mA).

    Thanks,

    Thomas (Senior Apps guy for RS485 but also interested in 232) 

  • 0 in reply to John Connors
    TI__Guru**** 171346 points

    John,

    It is normal to have some sawtooth ripple on V+ and V- and this ripple will be larger at 5V than 3.3V VCC. This is the charge pump being active only a small percentage of the time when the DOUT pins have no load. (the capacitive charge pump does overshoots its target voltage a little in a single cycle of boost)

    However, your sawtooth amplitude, V+&V- minimum magnitudes, and sawtooth period are not normal. I believe the devices are damaged, possibly by ESD.
    Try replacing the chips.

    Regards,
    Ron

     

     

     

  • 0 in reply to Thomas Kugelstadt
    TI__Guru**** 171346 points

    Thomas,

    I was referring to load on the DOUT pins, not directly from V+ or V-.

     

  • 0 in reply to Ron Michallick
    Prodigy 70 points

    Hi Ron,

      I bought the bullet and swapped one of the parts.  Charge pumps looked clean and the corruption on the port was gone, so I went ahead and swapped the other as well.  Not sure how they got damaged, and it's odd that both were, but they're up and running now.

     

    Thanks for your help.  I'll keep an eye out future problems in case there's a process or assembly change that could help.

     

    Thanks!

    -John