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PTN78020A - Output Noise

jerome tual
Prodigy 140 points
Other Parts Discussed in Thread: PTN78020A

HI,

On my analog receiver, I use the PTN78020A to generate a -12V supply The noise level of my product must be less than -110dBv in the frequency band 30 to 60Khz.

But, I measure some noises between 35Khz and 45Khz and I am sure that this noise comes from PTN78020A :

1. when it is OFF, I have not noise,

2. the noise level changes with the load on the -12V supply (0 to 50mA).

3. if I increase the capacitor value of the output PI filter, the noise level decreases.

Before the regulator module, there are one 100uF electrolytic capacitor and two 10uF ceramic capcitor and on the ouput, there are five 22uF ceramic capacitors near the pin. In order to reduce ouput noise, there is a PI filter : L = 1.5uH and C= 44uF.

I do not understand where the noise it may come? (the switching frequency of PTN78020A is equal to 550Khz?)

Regards,

Jerome.

  • 0
    TI__Mastermind 38230 points

    Jerome:

    The PTN78020x EMC low frequency low sub harmonic noise can be present below 550KHz switching frequency.

    I recommend you increase the output  PI filter ceramic capacitance on both input and output PI terminations  by 47uF plus  add a 100uF polarized capacitor close to t he output power pins, pin1/7.  

    A PI filter on the input similiar to the present output PI filer with 10-22uF ceramic capacitors will  improve the input reflected ripple .

    The EMC data is below on PTN78020A. There are switching sub harmonics  present at every 100kHZ below 550kHz.

    Tom