LM22670 Self Synchronization

Neale Thomas

Other Parts Discussed in Thread: LM22670

Hi, the latest version of the LM22670 data sheet includes a MOSFET in the Self-Synchronization section(7.3.4).

We have implemented this in our circuit however it seems to have introduced significant noise into the power supply outputs.

Has this been noticed by anyone else?

We have found that if we reduce the 1Kohm resistors to 220R, it significantly reduces the noise on the output.

Can someone tell us if this may introduce other problems or in fact if it is a valid solution?

Many Thanks,

Neale

over 6 years ago

0 Arief Hernadi over 6 years ago

TI__Mastermind 23815 points

Hi,

What type of noise that appears at the output? Do you have a scope capture to share?

Thanks
-Arief

0 Frank De Stasi over 6 years ago in reply to Arief Hernadi

TI__Guru 58825 points

In addition to the 'scope shots we will need to see your schematic and layout.

The 100 Ohm seems a little low to me.

0 Neale Thomas over 6 years ago in reply to Frank De Stasi

Prodigy 50 points

Can you tell me how the resistor value is set, or what the range of values can be?
This is not defined at all in the data sheet.

0 Neale Thomas over 6 years ago in reply to Neale Thomas

Prodigy 50 points

Also, ... I do not know how to post schematics or scope shots in this forum.
How is this done?

0 Frank De Stasi over 6 years ago in reply to Neale Thomas

TI__Guru 58825 points

You should see the "paper clip" icon when you reply to a post.

0 Neale Thomas over 6 years ago in reply to Frank De Stasi

Prodigy 50 points

OK, ... I am just not seeing it.
Regardless, if you could find someone to answer my question on how the 1K resistor value is arrived at in the data sheet, we can probably close this off.
Cheers.

0 Frank De Stasi over 6 years ago in reply to Neale Thomas

TI__Guru 58825 points

The 1k is a compromise. If it is too low the oscilator will not be able to drive it. If it is too high it will not pull

down the RT pin and activate the sync mode.

I would use a value in the range of about 800 Ohms to 2k Ohms.

0 Neale Thomas over 6 years ago in reply to Frank De Stasi

Prodigy 50 points

Can someone confirm for me that the threshold voltage 100us after power on of the RT/SYNC pin is 0.8V as per section 7.3.3 of the LM22670 datasheet.
When I look at this pin it is in the 80mV range.

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LM22670 Self Synchronization