LM3481 Synchronization

Steve MacLeod

Other Parts Discussed in Thread: LM3481

I plan to synchronize the LM3481 with an external clock by driving the FA/SYNC/SD pin with a 300kHz clock. Per page 11 of the data sheet, the synchronization pulse needs to be greater than 300nS and less than the minimum duty cycle of the switcher. What should the polarity of the pulse be?

over 13 years ago

0 CWilson over 13 years ago

Expert 1920 points

On page 12 in the datasheet: http://www.ti.com/lit/ds/symlink/lm3481.pdf on figure 10, the gate drive is high at the same time as your sync goes high. Therefore I would assume your LM3481 triggers on pulses going high. But again... it could also be the opposite..
The point is that your gate drive is high (rising) when your sync is high (rising).

0 Giuseppe Pinto over 13 years ago

TI__Expert 3950 points

Hello Steve,

there is actually a mistake on the datasheet that we'll fix as soon as possible.

the correct sentence is:

It is also necessary to have the width of the synchronization pulse wider than the duty cycle of the converter (when DR pin is high and the switching point is low). It is also necessary to have the synchronization pulse width > 300ns

0 Steve MacLeod over 13 years ago in reply to Giuseppe Pinto

Prodigy 190 points

Thanks, Giuseppe. What is the proper polarity of the sync pulse?

0 Giuseppe Pinto over 13 years ago in reply to Steve MacLeod

TI__Expert 3950 points

Hello Steve,

the gate driver is triggered during the rising edge of the sync signal.

Attached a screen shot with :

1. switching point (purple)

2. gate driver output, DR pin (orange)

3. Sync signa (blue)

I hope that helps.

Regards,

Giuseppe

0 Steve MacLeod over 13 years ago in reply to Giuseppe Pinto

Prodigy 190 points

Thank you, again. That works!

0 aronii over 11 years ago in reply to Steve MacLeod

Expert 1140 points

Can anyone recommend a FET/MOSFET that can be used for this synchronisation?

Please recomment one that you already tested.

Thanks

0 aronii over 11 years ago

Expert 1140 points

There are 512 documents on WebBench about LM3841. Do you expect me to read all of them?

Please be more specific. A link would be most appreciated.

I'm talking about the synchronization Mosfet in Fig. 26 in the datasheet.

Thanks

0 aronii over 11 years ago

Expert 1140 points

Do you recommend a 22 Amps Mosfet for switching a 10 KiloOhms pull-up resistor to GND?

Why would anyone need such a low RDSON resistance? This MOSFET is supposed to create a square wave. Nothing more than that. The reason that a MOSFET is used is because during power-up the clock might not turn on and thus the resistor installed is taking over.

You cannot use an NPN transistor like 2N2906 for such application.

Are you sure that you work for TI?

Maybe you can forward the question to another representatives which got more time to read the questions and read the datasheet??

Is it possible to get a response from someone which used such a MOSFET in this configuration?

0 JohnTucker over 11 years ago in reply to aronii

TI__Guru**** 191445 points

I hate to jump in here but can we please keep this conversation civil? Just a friendly reminder from your forum moderators.

I think the poster is looking for a recommendation to interface to the FA/SYNC/SD pin, not an external switching FET.

0 Juan Paulo Fung over 11 years ago in reply to JohnTucker

TI__Expert 3870 points

Hello Aronii,

The 2N7002 MOSFET is a readily available and inexpensive solution. It's commonly used for quick pull-down circuits as is the case here. Since the FA/SYNC pin can only go to 6V max, an if you use the recommended 10 kOhm pull-up resistor, this FET is strong enough to pull the node down and generate your pulsed-waveform. It's available in a TO92 and a SOT23 package.

Best Regards,

-JP

0 aronii over 11 years ago in reply to Juan Paulo Fung

Expert 1140 points

Dear Juan Paulo,

Thanks for your professional answer. It makes much more sense now...

Best Regards,

Avi Aharoni

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LM3481 Synchronization