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LM317 used as a Constant Current Source

Satish Veda
Prodigy 140 points
Other Parts Discussed in Thread: LM317, TLVH431

P/N: '3' Terminal Adjustable Regulator: LM317 

Attn:- Application Specialist 

1) Is it recomended to drive the Input of a LM317 (used in a configuration; so as to emulate a constant current source) at a PWM rate of ~4kHz ?? 

2) Also do you have any reliability data for use in situation(s) where the V-Input is Pulse-Width-Modulated? 

Thanks in Advance... 

-Satish

  • 0
    TI__Guru**** 154186 points

    Satish,

    I do not see any harm. However, I have not tested the how fast the LM317 will reach and settle at the regulation current.

  • 0 in reply to Ron Michallick
    Prodigy 140 points

    >>However, I have not tested the how fast the LM317 will reach and settle at the regulation current

    Hi Ron,

               The data sheet does show i/p of 120Hz. I've used this part since the mid- 80's. But w/o "Test-Data", i.e., Efficacy & Reliability, I can't  engineer this into a Medical Device Product.

    I was looking for Objective data, for ex:-

    a) Switching Ripple

    b) MTBF numbers when used in the environment that I described (in my first post).

    Is it possible for your engineer's to give me some data? Or even an Objective Analysis would help a great deal.

    Thx.

    -SVeda

     

  • 0 in reply to Satish Veda
    TI__Guru**** 154186 points

    Satish,

    The 120Hz data sheet specification is an AC signal superimposed on the always on DC voltage. This is not same as turning input power on/off at 120Hz. Nevertheless, I tested a LM317 125mA constant current source at 4kHz, 40kHz, and 400kHz today and was impressed with the turn on time.  The MTBF number are available from the LM317 product page in the "Quality & Environmental Data" section under "DPPM / MTBF / FIT Rate". Because the LM317 under pulsed power and current limited, it should provide a longer lifetime than an always on current source application.

  • 0 in reply to Ron Michallick
    Prodigy 140 points

    >> I tested a LM317 125mA constant current source at 4kHz, 40kHz, and 400kHz today and was impressed with the turn on time.  

     

    Can you please post (or e-mail me) the turn-on-time? And associated ripple? O-Scope shots would be great.

    Also the application that I'm considering is a constant current source of >2Amps.

    Thanks!

  • 0 in reply to Satish Veda
    TI__Guru**** 154186 points

    Satish,

    The LM317 is not rated for that current.

    A circuit like this is an option. The shunt regulator is the TLVH431, the lower resistor sets current = 1.25V/R

  • 0 in reply to Ron Michallick
    Prodigy 140 points

    >> I tested a LM317 125mA constant current source at 4kHz, 40kHz, and 400kHz today and was impressed with the turn on time. 

    Ron,

           Thank you for your prompt replies. Can you please let me know (or send me O-Scope shots), for the following condition:-

    ***************Condition:- 10uS to 100uS Pulses arriving at a frequency (Pulse rep rate) of 60Hz *************

    a) Settling time

    b) On-State ripple (Pk-Pk)

    c) Required CAP(adj) to reduce On-state ripple? More than 10uF?

    Thanks!

     

     

  • 0 in reply to Satish Veda
    TI__Guru**** 154186 points

    Satish,

    I did not use a capacitor in my tests. The cap might make things worse.

    At this time my backlog is unsually high, it could be sererval days before I can run more testing.

     

  • 0 in reply to Ron Michallick
    Prodigy 140 points

    Hi Ron,

                   Please let me know if you've run the tests.

    Thx

    -Satish

  • 0 in reply to Satish Veda
    Prodigy 140 points

    Ron,

             How are you? Just checking in to see if you've run the above mentioned tests?

    Thank you and Regards,

    Satish

  • 0 in reply to Satish Veda
    TI__Guru**** 154186 points

    Satish,

    I measured the output current (across 10 ohm load resistor plus 50 ohms scope) with 10uS pulse and different input pulse voltages. Therefore the output signal, CH2, is  60mA per division (500mV/8.33ohms).

    The LM317 has a 10 ohm resistor between the output and adjust pins for a 125mA output. Channel 4 is the input voltage pulse.

    LM317R10.zip