Forum Discussion

Altera_Forum's avatar
Altera_Forum
Icon for Honored Contributor rankHonored Contributor
14 years ago

Using LM317 to power FPGA

Analysing a couple of schematics I found out some using this device. Declared input voltage is 5..9v. Research of datasheet for LM317 shows that Vin-Vout should be >3v. Thus it will not be possible to make 3v3 from 5v. Am I correct? Anyone has experience with it and can prove/disprove it?

15 Replies

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    14 years ago

    There's a dropout voltage diagram in the datasheet. It clarifies that 1.7 V differential is too low if the input voltage is as low as 5V. 6V input voltage (actual minimumm, considering voltage ripple!) would be O.K. You should use a typical LDO regulator.

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    14 years ago

    --- Quote Start ---

    There's a dropout voltage diagram in the datasheet. It clarifies that 1.7 V differential is too low if the input voltage is as low as 5V. 6V input voltage (actual minimumm, considering voltage ripple!) would be O.K. You should use a typical LDO regulator.

    --- Quote End ---

    Thank you for reply. Will LM1117-3.3 help? It seems to have 1.5v dropout. However it has 800ma...1A current rating...
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    14 years ago

    --- Quote Start ---

    Yes. Typical dropout is around 1.1 V.

    --- Quote End ---

    Cool, thanks.

    Will supplied current (1A) be enough? I was trying to run Altera's power calculation spreadsheet, but do not understand how to use it... So can not answer this question myself. Q240, in max config all pins are used bi-directionally, half of CLKs are used, reference frequency is 50MHz. Can you give any estimation on the power consumption on 3v3 power rail?
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    14 years ago

    Current consumption depends mainly on the actual amount of switching registers and logic cells. It can't be estimated without analyzing the design.

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    14 years ago

    --- Quote Start ---

    Current consumption depends mainly on the actual amount of switching registers and logic cells. It can't be estimated without analyzing the design.

    --- Quote End ---

    Scenario: 25K cells, all switching :confused:
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    14 years ago

    --- Quote Start ---

    I don't think it's a good idea to connect fpga to 317...

    --- Quote End ---

    Your reasons?
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    14 years ago

    Suppose to have a 12Vdc.

    You want to connect a fpga and all the hardware around.

    It's very easy to reach 1A (317 is limited at 1A)... However suppose to have only 700mA..

    the hardware want 3.3Vdc.

    So it drop 12-3,3 = 8.7V on 317.

    The power dissipeated by the 317 is 8.7*0.7 = 6W!!!!!

    You throw away 6W!!! And you hardware use only 2.3W

    Ever you start with 5Vdc there is 1.2W dissipated that is comparable with 2.3...
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    14 years ago

    --- Quote Start ---

    Suppose to have a 12Vdc.

    You want to connect a fpga and all the hardware around.

    It's very easy to reach 1A (317 is limited at 1A)... However suppose to have only 700mA..

    the hardware want 3.3Vdc.

    So it drop 12-3,3 = 8.7V on 317.

    The power dissipeated by the 317 is 8.7*0.7 = 6W!!!!!

    You throw away 6W!!! And you hardware use only 2.3W

    Ever you start with 5Vdc there is 1.2W dissipated that is comparable with 2.3...

    --- Quote End ---

    LM317 will not output 3v3 with input 5v as its dropout is 3v. According to datahseet D2PACK is able to feed 2.2A. LM1117 as we can see above in the previous posts is max 800ma.