LM3424BKBSTEVAL: IS and GATE waveform when R6 is removed

Gary Jiang

Part Number: LM3424BKBSTEVAL
Other Parts Discussed in Thread: LM3424

Hi team,

I did an experiment using LM3424 buck-boost EVM that I removed R6 and then test the GATE and IS waveform. Power on and I found that IS and GATE has the same peak value(nearly 6.9V, equal to VCC) and frequency, their waveform are basically the same.

What I want to know is: Why the IS voltage will be the same with GATE voltage when the NMOS is turned on, rather than VIN after I remove R6?

Thank you

over 4 years ago

0 user5988235 over 4 years ago

Prodigy 95 points

why do you think MOSFET is turned on after you removed the R6?

0 Gary Jiang over 4 years ago in reply to user5988235

TI__Intellectual 980 points

If Q1 is not turned on, IS should remain steady no matter the GATE voltage, assuming there is no direct effect between IS and GATE.

Besides, I measured the TP1 voltage which is changing as the GATE changes. If MOSFET is not turned, TP1 should remain steady to.

0 user5988235 over 4 years ago in reply to Gary Jiang

Prodigy 95 points

I think you maybe need to consider the parasitic params of the MOS

because the IS will be floating

0 Tuan Tran74 over 4 years ago in reply to user5988235

TI__Mastermind 18890 points

Hi Taoran,

When you remove the resistor the source is floating and what you see is the AC coupling mechanism of the FET. Please don't remove the RIS.

Thanks Tuan

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LM3424BKBSTEVAL: IS and GATE waveform when R6 is removed