LM22680: Self Synchronization

Zeming Kong

Part Number: LM22680
Other Parts Discussed in Thread: , LM76002

Hi there,

My customer is using several LM22680 chips and he needs to synchronize them.

The datasheet shows that a FET is needed to be connected between 2 chips, but a training video of LM22680EVAL to the same 1k resistor directly without FET. So he is wondering if he could connect his circuit in the same way without FET.

If not, why is FET necessary here, and in which condition can we connect them directly?

Best,

Zeming

over 3 years ago

0 Denislav Petkov over 3 years ago

TI__Mastermind 19771 points

Hello Zeming,

The datasheet contains the latest information.

The purpose for the FET is to first start up/initialize the device in the correct state (sync) and then self-synchronize.

Cheers,
Denislav

0 Zeming Kong over 3 years ago in reply to Denislav Petkov

TI__Expert 3515 points

Hi Denislav,

Thanks for your reply. This helped a lot.

And I think the incorrect part of that video should be deleted, some customers may be misled and make a mistake.

Best,

Zeming

0 Denislav Petkov over 3 years ago in reply to Zeming Kong

TI__Mastermind 19771 points

Yes, no problem Zeming.

0 Zeming Kong over 3 years ago in reply to Denislav Petkov

TI__Expert 3515 points

Hi Denislav,

The customer has several questions:

1. Is a external clock necessary for self sync?

2. If not, what is the operation on RT/SYNC pin when FET is turned on, can you provide a waveform?

Best,

Zeming

0 Harrison Overturf over 3 years ago in reply to Zeming Kong

TI__Genius 10685 points

Hi Zeming,

To answer #1, no an external clock is not necessary for self sync, tying each of the regulators RT/Sync pins together through the MOSFET synchronizes each device to switch with a slight phase shift to one another.

When the FET is turned ON the two parts will operate in synchronized mode, but unfortunately I do not have a waveform of this operation at this time. I will get this waveform to you, but please expect a delayed response as it will take some time to get the EVMs delivered to the lab. Please expect a reply in 1-2 business days.

Regards,

Harrison Overturf

0 Zeming Kong over 3 years ago in reply to Harrison Overturf

TI__Expert 3515 points

Hi Harrison,

Very grateful for your support.

I am looking forward to the test result.

Best,

Zeming

0 Harrison Overturf over 3 years ago in reply to Zeming Kong

TI__Genius 10685 points

Hi Zeming,

I spoke with the designer today and he told me that the RT/SYNC pin will pull high for ~100ns. The first device to pull high will clock the other one so that they will switch at the same frequency.

This is not a recommended method of frequency synchronization for newer designs however. We have other parts such as the LM76002 that can be synchronized to an external clocking signal.

Does this answer your question?

Regards,

Harrison Overturf

0 Zeming Kong over 3 years ago in reply to Harrison Overturf

TI__Expert 3515 points

Hi Harrison,

Thanks for your reply.

So you mean, when FET is on, the RT/SYNC pins will pull high and earliest one will clock other ICs?

I have promised my customer to show him the waveforms, If possible could you provide the waveforms or any materials that can explain the sequence of self sync operation?

Also, could you tell me why this method is not recommended? If external clock is better here, can LM22680 be synchronized externally instead of using a new LM76002?

Best,

Zeming

0 Denislav Petkov over 3 years ago in reply to Zeming Kong

TI__Mastermind 19771 points

Hello Zeming,

It is a bit difficult to get a waveform at the moment. We were trying some simulations but had issues with the simulator.

According to section 7.3.3 in the datasheet, the IC can be synchronized to an external clock. Here is a link.

Cheers,
Denislav

0 Zeming Kong over 3 years ago in reply to Denislav Petkov

TI__Expert 3515 points

Hi Denislav,

Understood.

Thank you very much.

Best,

Zeming

0 Denislav Petkov over 3 years ago in reply to Zeming Kong

TI__Mastermind 19771 points

You are welcome Zeming.

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LM22680: Self Synchronization