• State Resolved
  • Locked Locked
  • Replies 2 replies
  • Subscribers 63 subscribers
  • Views 318 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.

LP5900 LDO Oscillation

Dan LaChance
Expert 1770 points
Other Parts Discussed in Thread: LP5900

Customer experienced an issue with the LP5900 LDO. The spec allows 150mA output current at 100Mhz with a 300 ohm resister. Issue occurred when output current was 120mA – 130mA. At 70mA on other designs no issue. Customer first made the circuit with the ferrite on the output (0603 package) and no decoupling capacitor (right on the output of the LP5900), he experienced the oscillation. When he added the 0.1uf decoupling capacitor on the out then the oscillation went away. Customer is questioning the use of a series ferrite bead on the output and how does this affect frequency response. Lastly, has anyone else experienced a similar issue?

  • 0
    TI__Guru 71526 points

    Hi Dan,

      There may be some misunderstanding about the "bypass cap" - this means a high-frequency bypass cap, not that NO output capacitance is required.  From the Product Folder page: The device is designed to work with 0.47μF input and output ceramic capacitors. (No Bypass Capacitor is required), and from the datasheet, page 5 COUT = 0.33uF MIN to 10uF MAX.  Bottom line is that a COUT capacitor is required, and 0.1uF is below the MIN.  Also see text on pages 9 - 10 that discusses the COUT characteristics.   

      There is a Webench dialogue box on the LP5900 PF page; I ran this for VIN = 3v, VOUT = 2.5v, IOUT = 120mA, and a 1uF COUT was suggested.  Please have the engineer run a similar design based on the actual conditions.  

      As far as the ferrite bead, this is a good idea in the feed path to a sensitve device, and should be after the COUT cap, not between the VOUT pin and the COUT cap.  

    -Leonard

  • 0 in reply to LeonardEllis
    Expert 1770 points

    Thank you for the quick feedback.  Much appreciated.