This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

Variable Input Variable Output

Other Parts Discussed in Thread: LM317, TPS75105

Hi folks,

Can anyone please suggest, what type of power supply architecture should be used for variable input and variable output power supply. It should be a type of voltage follower with current regulating capability of 45mA. I tried using LM317 but since it s a regulator, it will not support my cause.

Thanks in advance!!

Regards

-Anand

  • Hi Anand,

    I am not sure I fully understand what it is that you are attempting to do.  It sounds like you are looking for a current regulator.  Please take a look at TPS75105 and see if this will work in your application.  You are able to set the current using a resistor and tie multiple outputs together in order to have a higher current (this would be required to reach 45mA).

    Very Respectfully,

    Ryan

  • Hi Ryan,

    Thanks for your reply, what I am trying to implement is a very nice question. I feel sorry for not being very clear in my previous post.

    Below is image from LM317 what I was trying to use  

    My expectation was to get the same output voltage, what I am feeding at the input voltage, but the voltage collapsed every time I load LM317 with some load say of 100 ohm to 150 ohm. Just to make clear, I am not using LM317 for some battery charging as shown in the above diagram, I am willing to drive some ASIC using LM317. 

    Secondly my requirement is to implement a ratio metric  power supply, where I give voltage between 10V to 12.5V and expect a lower voltage of say 5.0V to 6.25V, which is exactly 0.5 times of the input voltage, and should be able to provide the enough current of 45mA to my ASIC every time voltage changes. That's why I feel it be variable input variable output voltage regulator.

    Regards,

    -Anand

  • Hi Anand,

    To quickly explain the LM317, the diagram above is the LM317 configured in a constant current mode. This works by having a 1.25V reference across the resistor between Adjust and Output. You can think of it as though the LM317 is a black box which forces 1.25V accross the resistor. Therefore, the current through the resistor is 1.25V / 24ohm = ~52mA. Since this will be connected in series to your end device, the current throughout is the same.

     

    Therefore, this will set 52mA through your load. If your load is 100ohms (max) then this means your load will see 5.2V (100ohms * 52mA). Next, the LM317 is consuming the excess power.

     

    So if current in = current out, then power into LM317 = 12.5V (max) Vin @ 50mA = 625mW and power out = I^2 R = 0.052^2 * 124 = 335mW. Therefore, the LM317 is consuming 0.625W - 0.335W = 290mW.

     

    Hopefully this helps provide some understanding of the LM317. This is how some basic linear regulators work (and in this case, it is being used to regulate current).

    For your application, the question is do you need it to be ratio metric, or do you simply need 5V with 45mA of current? If this is for a simple project where you just need a very easy to use 5V output and you do not care about size or efficiency, perhaps the LM7805 device may be desired:

    http://www.ti.com/product/lm7805c

     

    This is another linear regulator but is fixed at 5V output and is not adjustable. This device simply puts out 5V with an input voltage from 8V to 30V. It is an old, popular device which has been around for many years. Very straightfoward to use.

    If you need 45mA at your load, then the requirement if using the LM7805 is >8V and >45mA available at input.

    Goodluck with your project!

    Alex

     

  • Hi Alex,

    You got me correct. As explained in my previous post I was looking for ratio metric. My requirement is to implement a ratio metric  power supply, where I give voltage between 10V to 12.5V and expect a lower voltage of say 5.0V to 6.25V, which is exactly 0.5 times of the input voltage, and should be able to provide the enough current of 45mA to my ASIC every time voltage changes.

    And, thanks for the detailed and insight explanation of LM317.

    Regards

    -Anand