• State
  • Locked Locked
  • Replies 1 reply
  • Subscribers 62 subscribers
  • Views 398 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.

LM2990 Negative LDO - output oscillation question

Donald Jones
Mastermind 22610 points
Other Parts Discussed in Thread: LM2990

LM2990 Negative LDO - output oscillation question

What we are seeing is ~1Vpp @ ~26KHZ oscillation at the LM2990 output.

When we remove Cout the oscillation persists, but with a different frequency.

Cin and Cout are 4.7uF X7R 25V ceramic caps.

When we replace Cout with a 10uF 10V 3.50Ω tantalum the oscillation stops.

We are confused because the ceramic cap has a lower ESR (<1Ohm ) than the tantalum (2.5Ohm). Shouldn’t the lower ESR part work fine? The customer is trying to get away from using tantalums due to their poor reliability.

Also, We would like to verify that the LM2990 has a maximum ESR of 10Ω for its output cap and not a minimum.

 

 

  • 0
    TI__Mastermind 22610 points

    The LM2990 is not stable with pure ceramic capacitance on the output, it requires Cout with some minimal ESR, such as tantalum, for stability.

    Yes, as a general rule ... lower ESR is better. However, some LDO's, like the LM2990, require some ESR in the Cout in order to maintain some minimum phase margin in the control loop for stability. 

    For LM2990 the Maximum COUT ESR is 10 Ohms.

    Minimum ESR is difficult to read off the chart in the datasheet. My guess is that the chart if specific to Cout of 10uF, and that minimum value might be 0.03 ohms

    That Cout and ESR would produce a zero in the control loop :

              fz = (1 / (2 x pi x 10uF x 0.03 ohms)) = 530kHz.

    If you need to use ceramic only for Cout, you can add a low value series resistor to Cout to simulate the ESR. The ESR requirement is not optional.