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TPS735: Noise issue with LDO

Vignesh M26
Intellectual 590 points
Part Number: TPS735


Hi TI Team,

We are using the TPS73501DRVR regulator in our Image sensor's 2.9V analog domain. Attached regulator section for reference (Fig.1). We found lines(noise) in the image sensor output as shown below (Fig.2). We have measured the ripples.

Input = 3.3V

Output=2.9V


2.9V LDO Input (3.3V) ripple = 103mV

2.9V LDO Output ripple =43mV

We have tried the below cases and found the lines in the image sensor.

  1. Added 2x10uF cap in C5(input of LDO) -> 3.3V ripple voltage reduced to 60mV from 103mV & 2.9V ripple reduced from 43mV to 28.4mV.

  2. Instead of 3.3V input, we provided 5V from Regulated Power Supply -> 2.9V ripple reduced to 21.4mV

  3. Added 22uF cap in C6 (output of LDO) So now total 22+1 = 23uF  -> 2.9V ripple reduced to 16.6mV

  4. Removed R51 and the LDO and sourced 2.9V from the Regulated Power supply. We found no line issue on the camera (Fig.3). The ripple current measured is 15.8mV.

  5. We thought that due to the drop out voltage is 400mV (3.3 – 2.9 =0.4) So we increased 3.3V input to 5V. We also get same result. Ripple voltage is 21.4mV and found image line issue.



Please suggest the solution to this issue. What is the problem with this LDO ?

Fig1) 

Fig.2)

Fig.3)

Thanks,

Vignesh.M

  • 0
    TI__Mastermind 37025 points

    Hi Vignesh,

    The TPS735 needs at least 2uF of output capacitance to maintain stability.  To account for tolerances and environmental conditions like temperature, you will probably want a minimum of 4.7uF on the output.

    You might want to try increasing C60 to 1nF or 10nF to help reduce noise even further.

    If the input supply to the LDO is a switching converter, and the ripple frequency you are measuring is the same as the switching frequency of the switching converter, then we want the PSRR of the LDO to reject the switching frequency.  By increasing the output capacitors of the LDO the PSRR is improved at lower frequencies, which may be lining up with the switching frequency of the input supply. 

    Thanks,

    - Stephen