Power filter requirement for 3G-SDI line driver, equalizer and reclocker

Madhu Sharma

Other Parts Discussed in Thread: LMZ31710

Hi,

In my design, a TI regulator LMZ31710RVQR is used for generating the supply voltage ( 3.3 V ) from a 5 V input. An application report AN-2145 (Power Considerations for SDI Products) tells that for 3G operation, ripple should be less than 10mV. But regulator LMZ31710RVQR has output ripple of about 15 mV. So, whether placing a PI filter near the ICs ( one filter for each driver, reclocker and equalizer) would help to reduce the ripple below 10mV? If yes, please let me know how to select the PI filter component values. I have shared more information about my design in High speed interface forum, with same subject ( Power filter requirement for 3G-SDI line driver, equalizer and reclocker).

Regards

Madhu

over 7 years ago

0 Madhu Sharma over 7 years ago

Expert 2915 points

Hi,

Can anyone please answer the above query??

Regards

Madhu

0 Frank De Stasi over 7 years ago in reply to Madhu Sharma

TI__Guru 57555 points

I would use a simple LC filter. You can set the corner frequency depending on the amount of attenuation and the switching frequency.

Be sure that it is well damped.

0 Madhu Sharma over 7 years ago in reply to Frank De Stasi

Expert 2915 points

Hi,

How about a Ferrite bead - Capacitor filter?
In most of the cases, we prefer to use this due to its low size and less components.
Also, please share if there is any doc or AN on well damped filter requirements and filter component value calculations.

Regards
Madhu

0 Arief Hernadi over 7 years ago in reply to Madhu Sharma

TI__Mastermind 23815 points

Yes you can use ferrite bead for the LC filter. You have to make sure that the ferrite bead does not saturate (current is below saturation).
You can check your ripple frequency and set the LC filter resonant frequency 1 decade below using the formula 1/sqrt(LC)

Thanks
-Arief

0 Arief Hernadi over 7 years ago in reply to Arief Hernadi

TI__Mastermind 23815 points

Hi,

Are you ok with the choosing the filter components?

Thanks
-Arief

0 Madhu Sharma over 7 years ago in reply to Arief Hernadi

Expert 2915 points

Hi Arief,

I calculated the values using below steps:

1) In LMZ31710 datasheet, for 3.3V output from 5V, the switching frequency is 750kHz ( page 11, table 4). So I considered cutoff frequency to be 100kHz.

2)By considering C= 10uF ( from my design), Using the formula 1/sqrt(LC), L will be 10uH.

My doubts are:

1) It will be an RLC filter when i choose a ferrite bead. Its cutoff frequency formula is 1/ 2 (PI) sqrt(LC) and not 1/sqrt(LC), as per some websites. Please let me know which one is correct.

2) For the ferrite bead, Both its inductance and resistance vary with frequency. So how to select ferrite bead using L value obtained in above formula ?

Regards
Madhu

0 Arief Hernadi over 7 years ago in reply to Madhu Sharma

TI__Mastermind 23815 points

Hi,

1) Yes, i make a mistake in my formula, you are correct. I am missing 2 * (PI) factor.
2) I think this article would help to explain the effect of ferrite bead as a filter
www.analog.com/.../ferrite-beads-demystified.html

Thanks
-Arief

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Power filter requirement for 3G-SDI line driver, equalizer and reclocker