LM22672: Self synchronizing circuit for two devices

Robin Radford

Part Number: LM22672

I am refreshing a design from circa 2010. This has two switchers linked so that one syncs to the others 500k clock. This has two LM22672 with directly linked RT/SYNC pins and with a single 1K pull down to 0V. EN is not connected. This arrangement is as per the Nat Semi data sheet of that time. The new TI data sheet however shows the sync pins linked via MOSFET (switched on by one of the supplies), with two pull down resistors and also says that the startup should be controlled by the EN pin. Could someone clarify if the new scheme is necessary or can the old method still be used?. Is it essential that the EN should both be driven from a common source?

Thankyou Rob

over 8 years ago

0 Frank De Stasi over 8 years ago

TI__Guru 74005 points

In order for the devices to reliably synchronize, the MOSFET circuit should be used.

The EN should be tied together to properly initialize the regulators.

0 Robin Radford over 8 years ago in reply to Frank De Stasi

Prodigy 20 points

Thanks Frank. It looks like I will have to modify the circuit board to fit the FET. At present the EN pins are both disconnected, so they rely on the internal pull-up to Vin. As they both are connected to the same Input supply am I correct in thinking this is ok ? and that the common connection to EN shown in the data sheet isonly needed for sequencing reasons.

Rob

0 Frank De Stasi over 8 years ago in reply to Robin Radford

TI__Guru 74005 points

That should be OK

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LM22672: Self synchronizing circuit for two devices