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Thread: What exactly does drop out voltage mean?

  1. 5th January 2004 06:20 PM #1
    Gandledorf
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    Default What exactly does drop out voltage mean?

    I'm looking over 5V some switching regulators, and have a question: Does drop out voltage mean the point at which it can no longer provide a regulated 5V at XmA? So if for the chart of drop out voltage vs. load, at 500mA the drop out voltage is slightly < .15V (lets call it .15V for simplicity), does this mean that as long as my battery pack is supplying .15V or more, it can still produce 5V @ 500mA?
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  2. 5th January 2004 07:14 PM #2
    Nigel Goodwin
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    Default Re: What exactly does drop out voltage mean?

    Quote Originally Posted by Gandledorf
    I'm looking over 5V some switching regulators, and have a question: Does drop out voltage mean the point at which it can no longer provide a regulated 5V at XmA? So if for the chart of drop out voltage vs. load, at 500mA the drop out voltage is slightly < .15V (lets call it .15V for simplicity), does this mean that as long as my battery pack is supplying .15V or more, it can still produce 5V @ 500mA?
    Not quite, it means that the incoming supply has to be at least 0.15V higher than the output for the device to work. This would be classed as a 'low drop out - LDO' regulator, a standard analogue 7805 probably requires 3V or so.
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  3. 5th January 2004 07:21 PM #3
    Gandledorf
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    How would one figure out the working voltage range then, when one could reliably draw 5V @500mA? The general data sheet for this device is here: http://focus.ti.com/lit/ds/symlink/tps7150.pdf, I'm just a little unclear as to what I am looking for to determine this. I'm interested in the TPS7150 series.

    EDIT: On page 16 there is a graph of input vs. output which I missed, do I correctly assume that I can expect 5V regulated output @ 500mA until my pack hits 5V?
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  4. 6th January 2004 02:08 AM #4
    Roff
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    Quote Originally Posted by Gandledorf
    How would one figure out the working voltage range then, when one could reliably draw 5V @500mA? The general data sheet for this device is here: http://focus.ti.com/lit/ds/symlink/tps7150.pdf, I'm just a little unclear as to what I am looking for to determine this. I'm interested in the TPS7150 series.

    EDIT: On page 16 there is a graph of input vs. output which I missed, do I correctly assume that I can expect 5V regulated output @ 500mA until my pack hits 5V?
    If you zoom in on that graph, you'll see that when your input gets down to 5v, your output will be about 4.9v. Figure 8 on p.15 shows that the dropout voltage at 500ma is 130mv, so your output will start to drop when your input reaches 5.13v. These numbers are typical, not guaranteed. Keep in mind that your circuit will probably work down to at least 4.5v, possibly lower.

    The TPS7150 is not a switching regulator. If you want maximum battery life, you should probably use a switcher.
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  5. 6th January 2004 03:02 AM #5
    Gandledorf
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    Quote Originally Posted by Ron H
    Quote Originally Posted by Gandledorf
    How would one figure out the working voltage range then, when one could reliably draw 5V @500mA? The general data sheet for this device is here: http://focus.ti.com/lit/ds/symlink/tps7150.pdf, I'm just a little unclear as to what I am looking for to determine this. I'm interested in the TPS7150 series.

    EDIT: On page 16 there is a graph of input vs. output which I missed, do I correctly assume that I can expect 5V regulated output @ 500mA until my pack hits 5V?
    If you zoom in on that graph, you'll see that when your input gets down to 5v, your output will be about 4.9v. Figure 8 on p.15 shows that the dropout voltage at 500ma is 130mv, so your output will start to drop when your input reaches 5.13v. These numbers are typical, not guaranteed. Keep in mind that your circuit will probably work down to at least 4.5v, possibly lower.

    The TPS7150 is not a switching regulator. If you want maximum battery life, you should probably use a switcher.
    Thanks for the explanation, it was really helpful.

    I was originally looking to user the MAX603 switching regulator, but couldn't find it at all, or any equivalent switching regulator, any suggestions?
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