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Power Supply Help

ArchiverArchiver Posts: 46,084
edited 2004-06-14 02:52 in General Discussion
I am designing a frequency to current converter that
must operate from 8 to 30Vdc. For the 4-20ma output I
am using the Analog Microelectronics AM422-1. The
AM422-1 will take a 0-5V input and convert it to a
4-20ma signal.

The desired maximum loop impedance desired is 500
ohms. In order to provide 20ma at the desired load,
the supply voltage must be 16Vdc.

I was thinking of using a LDO voltage regulator such
as the LP2951 with a 8V output. The 8 volts would then
be connected to a ICL7660 switched capacitor voltage
converter which is configured to double the input
voltage, therefore providing the required 16Vdc.

The problem that I am having is that with a total
supply current draw of 60mA, at an input voltage
greater that 26Vdc, the LP2951 goes into thermo shut
down.

I have tried to use a series resister and zener to
limit the input voltage to 20Vdc. The problem with
this is that while operation at 30Vdc is possible, it
is not possible down to 8Vdc.

Any help would be appreciated.

Thanks

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Comments

  • ArchiverArchiver Posts: 46,084
    edited 2002-11-18 00:14
    >I am designing a frequency to current converter that
    >must operate from 8 to 30Vdc. For the 4-20ma output I
    >am using the Analog Microelectronics AM422-1. The
    >AM422-1 will take a 0-5V input and convert it to a
    >4-20ma signal.
    >
    >The desired maximum loop impedance desired is 500
    >ohms. In order to provide 20ma at the desired load,
    >the supply voltage must be 16Vdc.
    >
    >I was thinking of using a LDO voltage regulator such
    >as the LP2951 with a 8V output. The 8 volts would then
    >be connected to a ICL7660 switched capacitor voltage
    >converter which is configured to double the input
    >voltage, therefore providing the required 16Vdc.
    >
    >The problem that I am having is that with a total
    >supply current draw of 60mA, at an input voltage
    >greater that 26Vdc, the LP2951 goes into thermo shut
    >down.
    >
    >I have tried to use a series resister and zener to
    >limit the input voltage to 20Vdc. The problem with
    >this is that while operation at 30Vdc is possible, it
    >is not possible down to 8Vdc.
    >
    >Any help would be appreciated.
    >
    >Thanks


    Hi John,

    I've had success with the Burr Brown 1 watt DC-DC converters in
    14-dip or 14-soic packages. E.g., 8--30 volts in to a 7805 or LM2940
    regulator, 5 volts out, that in to a DCP010515 converter, 15 volts
    out at up to 67 ma. The output is unregulated, so it will be well
    over 16 volts when the loop is drawing 20 ma. The 7805 may have to
    dissipate 1 watt worst case. The output of the DCP series is
    isolated, so you can piggy back it on top of the supply, e.g.,
    piggyback the 12v output of a DCP010512 on top of the 5 volt supply,
    and you get 17 volts at full current of 83 ma.

    Switched capacitor DC-DC chips like the ICL7660 are not good for more
    than a few ma.

    -- regards,
    Tracy Allen
    electronically monitored ecosystems
    mailto:tracy@e...
    http://www.emesystems.com
  • ArchiverArchiver Posts: 46,084
    edited 2004-06-13 18:59
    I was looking for some advice on filtering a power supply. I have a DC/DC Switching converter that has 24vdc input and 5vdc output @ 20 watts. Specs show 80mv peak to peak Max ripple and I'd like that to get down as low as possible due to using it for supply on an ADC and some other goodies. It's swithing frequency is 330khz.

    What is are good values of Caps on the inputs and outputs of this thing. Currently I'm using a 220Uf 50Volt across the input leads as well as a 220uf 50volt across the output leads. I also added a 0.1uf disc cap across the outputs as well. These seem to work but I'm just guessing on values. the 220uf 50 volt is huge but I had them laying around. Like to keep the Radial caps fairly small (Less than .500" diameter).


    I'm running 1 stamp, 1 Single board computer (Pentium), and a 12 bit adc off the same 5vdc output of the converter.

    Any suggestions.



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  • ArchiverArchiver Posts: 46,084
    edited 2004-06-13 19:11
    Sorry,
    Here are the specs of the converter in case there wasn't enough info on the previous email.

    Input Filtered to meet EN 55022, Class A and FCC Level A without external components
    Line Regulation 0.3%
    Load Regulation 0.5%
    Ripple and Noise <80 mv Pk to Pk (20 mhz BW)
    Switching Frequency (Fixed) 330khz PWM


    Richard


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  • ArchiverArchiver Posts: 46,084
    edited 2004-06-14 02:52
    You might have a look at the Calex website:

    http://www.calex.com/dcappnote.html

    They have some appnotes on applying DC-DC converters.
    Steve
    /////////////////////////////

    At 10:59 AM 6/13/04, you wrote:
    >I was looking for some advice on filtering a power supply. I have a DC/DC
    >Switching converter that has 24vdc input and 5vdc output @ 20
    >watts. Specs show 80mv peak to peak Max ripple and I'd like that to get
    >down as low as possible due to using it for supply on an ADC and some
    >other goodies. It's swithing frequency is 330khz.
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