• State Resolved
  • Locked Locked
  • Replies 2 replies
  • Subscribers 63 subscribers
  • Views 785 views
  • Users 0 members are here
Support feedback
Options
Options
Related

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.

Original question:

Using LM317s for bench power supplies (tracking?)

LM317: Using LM317s for bench power supplies - Preregulation

Trevor brearley
Prodigy 50 points
Part Number: LM317

In one of your data sheets  (SLVS044X –SEPTEMBER 1997 –REVISED SEPTEMBER 2016) referring to the LM317 precision voltage regulator, section 8.3.4 (figure 15) on page 12, A Tracking Preregulator Circuit is shown.

I know the LM313 is an ageing technology, but it works and it's cheap!

There is a 5v difference or 'headroom' from the output of the first regulator to the second (as I understand).

Now for a single regulator the maximum supply voltage is 40v, so does this mean for two LM317s in this configuration 'raw DC' supply can be increased to say 45v, or higher?

I'd like to build a variable regulator 0-30v but I get the feeling i could be approaching 40v maximum, which is the upper limit of just a single LM317, but I feel two LM317s in series (as per figure 15) could be supplied with greater headroom and a higher 'raw DC' supply.


Thanks for any help you may be able to provide,
Trevor Brearley

  • 0
    TI__Guru 55940 points
    Hi Trevor,

    Due to the topology of LM317 there is no GND pin. As such, there is no GND reference internal to the linear regulator unlike most modern LDOs. The advantage is that this linear regulator can be used in higher voltage applications so long as the voltage on the IN pin is never more than 40 V higher than the voltage on the OUT pin of the same regulator.

    You are correct that in the application example, R1 and R2 set the output voltage of the left LM317 at 5 V above the output of the right LM317. This allows the system Vin to be 5 V higher than the voltage on the IN pin of the right LM317. As such from the system perspective Vin can now be 45 V more than Vout:

    Vin_sys - Vout_sys = (Vin_left - Vout_left) + (Vin_right - Vout_right)
    Vin_sys - Vout_sys = 5 V + 40 V (this is the maximum differential across a single LM317)
    Vin_sys - Vout_sys = 45 V

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    Prodigy 50 points

    Hi Ryan,

    Many thanks,
    Trevor