LM27222, What to do with LEN?

Jeramy Hughes

Other Parts Discussed in Thread: LM27222

The data sheet for the LM27222 is not clear on what to do with LEN pin. It shows it going the the PWM "controller". Can I just tie this to +5V? That is how I have it now and it kinda works, but the MOSFETS get warm. Thoughts?

over 8 years ago

0 Jeramy Hughes over 8 years ago

Prodigy 55 points

Is there any other way to get technical support from ti?

0 Peter Meaney over 8 years ago in reply to Jeramy Hughes

TI__Mastermind 31580 points

Hi Jeremy,

Let me find the correct apps engineer to support your question.

Regards

Peter

0 Jeramy Hughes over 8 years ago in reply to Peter Meaney

Prodigy 55 points

Peter, any luck tracking down the right apps engineer? I'm starting to think I should just use a different chip because I can't seem to get an answer.

0 Peter Meaney over 8 years ago in reply to Jeramy Hughes

TI__Mastermind 31580 points

Hi Jeramy,

Sorry no feedback yet. Looking at the datasheet the LEN should be pulled high and then the mosfets will follow the IN signal. Depending on the power been processed it is normal for the fets to get warm.

Take a look at the gate drive signal for the low side fet and the drain of the low side fet. they should be in sync.

Regards

Peter

0 Kaisar Ali over 8 years ago

TI__Intellectual 2010 points

Hi Jeramy,

Sorry about the delayed response. This post slipped through the crack.

Both LEN and IN are active high and are internally pulled down to gnd. The LEN is intended to avoid false LS switch turn ON. Fig 4 shows how the LEN can force the LG OFF when intended. LEN is part of shoot-though protection that LM27222 offers which can hold the LG off for 170ns, beginning from the instant when IN (HG) goes high. Keeping the LEN floating or high at all time is possible depending upon the application.

If you are keeping the LEN at 5V all the time, it shoudn't cause FET over heating. How much current are you drawing through the FETs? Can you share a schematic or gate switching waveforms?

0 Jeramy Hughes over 8 years ago in reply to Kaisar Ali

Prodigy 55 points

Hi Kaisar,
Is it possible to contact you offline to discuss this?
Thanks,
Jeramy

0 Kaisar Ali over 8 years ago in reply to Jeramy Hughes

TI__Intellectual 2010 points

Sure Jeramy. I can PM you my contact information.

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LM27222, What to do with LEN?