peculiar problem with input current

Discussion in 'General Electronics Chat' started by abq_guy, Feb 2, 2009.

  1. abq_guy

    Thread Starter New Member

    Feb 2, 2009
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    Hello,

    I am using a DC-DC converter from DATEL ( BWR-15/670-D12A). The inputs are +15V and the outputs are +15V and -15V. The converter can give upto 670mA of output current on both +15V and -15V individually.

    I am using these output voltages to power 2 Linear LDO regulators (LT1962 and LT1964). These regulators take in the +15V and the -15V from the DC-DC converter and convert them to +14.85V and -14.85V respectively.

    I use these +14.85V and -14.85V to power 5 LTC2704 (DAC's ), 5 OP400 op-amps, and 5 LT1885 op-amps.

    When I turn the power On, the DC-DC converter starts taking in about 0.375A of current, but it rises to 1.0142A, at which point the +14.85V goes down to 1.62V and the input current drops to 0.250A.

    Why is this happening ? Any explanations.

    Thanks,

    abq_guy
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    Please post a schematic of how you have things connected, along with links to the datasheets.
     
  3. Audioguru

    New Member

    Dec 20, 2007
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    Like I said a few minutes ago on the other website, I think the low dropout regulators are oscillating maybe because the output capacitors are not correct.
     
  4. abq_guy

    Thread Starter New Member

    Feb 2, 2009
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    Hello Audioguru,

    I checked the outputs of the LDO's . They look stable. WHat I mean by stable is that I don't see any ripple or oscillations on the oscilloscope. I see a clean +15V straight line and a clean -15V straight line. Should I be measuring it differently ?

    Thanks,

    abq_guy
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    I went to the trouble of downloading some of the datasheets.
    The negative regulator has a current limit of 200mA; the positive regulator a limit of 300mA.

    You have a total potential load of 740mA in your circuit as it has thus far been described (basic components only), depending upon how things are connected; this is assuming that the opamps' outputs are shorted to one rail or another.

    You need to either provide more information in the way of a schematic, or just start unplugging things until the problem goes away. Right now, I'll suggest that you're way over your power budget, and overly optimistic of the capabilities of your LDO regulators.

    After all, they can't generate power themselves.
     
  6. abq_guy

    Thread Starter New Member

    Feb 2, 2009
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    Hello SgtWookie,


    I am attaching the schematic with this post. You are right about the LDo current capabilities. But how did you get the 740mA ?

    According to my calculations:

    OP400: 3mA/Chip So total is 5*3mA = 15mA for 5 chips

    LT1885: 4mA/chip So total is 10*4mA = 40mA for 10 chips

    LTC2704-16: 20mA/ V+ So total is 20mA*5 =100mA for 5 chips


    Total Current is 15+40+100 = 165mA

    Please let me know how you calculated it.

    Thanks,

    abq_guy
     
  7. abq_guy

    Thread Starter New Member

    Feb 2, 2009
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    Any inputs??

    Thanks,

    abq_guy
     
  8. abq_guy

    Thread Starter New Member

    Feb 2, 2009
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    0
    Some Updates on the problem

    I noticed that when all the 4 DAC's were turned ON, and the total current drawn by the DATEL converter from the OUTSIDE power supply was about 1.1A

    I then connected the LDO's back but removed the output filter caps on the DATEL, and this time the total current drawn by the DATEL from the OUTSIDE power supply was about 0.485A.

    All 4 DAC's were set to +10V during these tests. I don't understand, why the Input current would change with respect to the LDO's being there or not being there.

    Thanks,

    abq_guy
     
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