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Help with power issues

Discussion in 'Electronic Design' started by linnix, Mar 31, 2009.

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  1. linnix

    linnix Guest

    I am runing a sensor (1.5mA),
    op amp lm/lmv/lpv358 (1/0.2/0.1mA)
    and µC (0.5mA) on two button cells

    The batteries range from 4V to 6V.
    The sensor needs at least 4V.
    The µC can have at most 3.3V.
    The op amp is a sigma-delta A2D from sensor to µC.

    So, I am thinking about a 4V zener to the sensor,
    minus diode drop to the µC.

    Should I power the op amp before or after the diode?
    Also, 4V zener is hard to get.
    Should I use 5V plus two more diodes?

    I have to commit to PCB in a few days,
    no time for more prototype/testing.
     
  2. linnix

    linnix Guest

    Op amp analog input from sensor and digital output to µC. I am just
    worrying about peak output voltage hitting the op amp or the µC.
    At 4V, the batteries are dead anyway. I should have said operating
    range 4V somthing to 6V.
    But a 4V LDO is expensive, compared to the others.

    4V LDO: 0.30
    Zenar: 0.05
    Diode: 0.01
    OK, will try that.
     
  3. linnix

    linnix Guest

    OK, OK. I will have both on the PCB and decide later.
    A thousand jumpers by yours truly.
     
  4. Eeyore

    Eeyore Guest

    Suggest using micropower voltage regulators. Zeners are horrible.

    Graham
     
  5. linnix

    linnix Guest

    Yes, but standby power is less than 100µA, with everything else off
    except the µC. Active power is only for several minutes once in
    several days or weeks. Standby power needs to be several months on
    two CR2032.
     
  6. linnix

    linnix Guest

    Here is the simplified schematic:

    http://linnix.com/SigmaDeltaEpilson.gif

    Actually, we have considered a custom made 4V LDO with 3.5µA standby
    current. But we are trying to avoid the $3000 minimum order for the
    moment, until necessary, we will just stuff it with zeners or even 6
    regular diodes.
     
  7. linnix

    linnix Guest

    Yes, perhaps I need one (10K) for the µC and another (1K) for the
    rest. Two zener is still cheaper than the LDO. I know LDO is the
    right solution, but we need something to bridge us there.
     
  8. linnix

    linnix Guest

    never mind. 10K won't run the µC
     
  9. James Arthur

    James Arthur Guest

    Seiko Instruments has a number of uA-quiescent regulators.

    http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=728-1023-1-ND

    datasheet: http://datasheet.sii-ic.com/en/voltage_regulator/S812C_E.pdf

    About $0.30 @ qty 3,000, as you said.

    James Arthur
     
  10. Eeyore

    Eeyore Guest

    I though Vin was variable so you'd have to use them in shunt mode and that
    *gobbles* current plus zeners aren't specced at currents that low. A
    bandgap reference would be better.

    Graham
     
  11. Eeyore

    Eeyore Guest

    Isn't that always the problem with shunt regulation ?

    Graham
     
  12. linnix

    linnix Guest

  13. Eeyore

    Eeyore Guest

  14. James Arthur

    James Arthur Guest

    Does that matter? The CR2032 is 98% utterly dead and gone at 2.5v,
    or 5v for two in series. So you've got lots of headroom.

    Extending the discharge cutoff to 2.0v/cell gets you 2% extra
    battery lifespan, maybe.


    The problem with your zener shunt regulator idea is that it
    won't work.

    Cheers,
    James Arthur
     
  15. linnix

    linnix Guest

    Yes, either one will work. But the customer has already committed to
    the other version, we don't want to change plan and layout again. The
    PCB is already set for SC-70 package with the custom pinout.
    Unfortunately, the check has been in the mail for weeks. I am just
    going to jumper a 1k R, zener and a power switch (for standby), as
    well as a $3000 ransom note to remove them.
     
  16. linnix

    linnix Guest

    But seriously, I would probably jumper the different pinouts until the
    custom version is ready.

    Back to the main issue. I am just wanting to make sure that the op
    amp output won't go beyond Vcc2, since it is running at higher Vcc1.
    In theory, the output should be 1.5V less than Vcc1.
     
  17. Eeyore

    Eeyore Guest

    You got it. They are the lousiest most power hungry 'regulators' in the world.
    They have better uses.

    Graham
     
  18. Eeyore

    Eeyore Guest

    That's another story altogether. What are the devices ? Many are tolerant of such
    issues now.

    Graham
     
  19. linnix

    linnix Guest

    It's an LCD ASIC. Supply beyond 3.3V mess up the LCD, with all
    segments lighting up. It was not a problem with the AVR, but a
    problem with the ASIC. I guess the AVR clams down on the LCD drivers,
    but not the ASIC.
     
  20. linnix

    linnix Guest

    We have also considered the TL431. 0.10 @ 10,000 is acceptable, but
    the standby current could be higher.
     
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