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LM3409: LM3409 and LM3409HV PMOS selection

Morten Hjerde
Prodigy 200 points
Part Number: LM3409

Hi

We ship a lot of products with LM3409 and with LM3409HV.

The only PMOS suggestion in webbench is FDC5614P. This is a 60V PMOS in a Super SOT-6 package and it performs admirably.

The switching frequency is around 500kHz. We use the LM3409 in several different configurations from 20V to 50V and 0.9 to 4A.

The problem is getting hold of enough FDC5614P. It is a constant struggle and we desperately need one or more alternatives. We have tried to calculate the mosfet losses (conduction loss + switching loss + gate loss) in an effort to find a good alternative. But we have not been able to find a PMOS that comes close to the FDC5614P neither on paper or in practical tests.

Our calculations may be wrong of course. 

Could you suggest or point to a method or formula to correctly calculate the power losses in this PMOS. And are there any additional things to keep in mind regarding the LM3409 driver except to minimise Qg?

/M

  • 0
    TI__Mastermind 28655 points

    Hello,

    It's not just gate charge to look at.  There is RDSon, the gate threshold at the current you are running as well as installing and testing the part on the board since two parts with similar specifications can behave quite different.  I quick look came up with FDC5661N_F085 as something to compare and test FDC5612 would be another to look at (not recommending, just two parts that appear to be possible replacements that need to be verified and tested).  One thing to watch is the performance of the part.  A newer part can be overall better specification wise but can cause problems in a circuit because it can switch faster this is why just looking at parameters is not enough, it should be tested as well.

    A crude but simple way to get an idea is to measure the temperature rise of the part compared to the original one.  Looking at waveforms also can help determine if the replacement is similar or works well enough.

    Best Regards,

  • 0 in reply to Irwin Nederbragt
    Prodigy 200 points

    Thank you Irwin

    Seems like calculations/simulations may not get us there so we have ordered a number of potential replacements and will test them one by one 

    /Morten