Using TPS54340 with external FET

takahiko yokota

Mastermind 7135 points

Other Parts Discussed in Thread: TPS54340

My customer uses TPS54340 under Vin=26V,Vout=5V,Iout=3.5A.

But it is possible fever is high, so they will use TPP54340 with external FET to suppress the fever.

Can it use this way?

If it is correct, could you tell me to use either Pch FET or Nch FET and how to connect it?

(The board space is small so buck controller don't like this application.)

Best regards,

Yokota

over 9 years ago

0 JohnTucker over 9 years ago

TI__Guru**** 191445 points

If you are going to use an external FET, you are best served to look at a controller with internal gate drivers.

0 takahiko yokota over 9 years ago in reply to JohnTucker

Mastermind 7135 points

Hi John san

I'm sorry my customer couldn't change the device to design the circuit.

Is there the method to suppress the fever?

Best regards,

Yokota

0 JohnTucker over 9 years ago in reply to takahiko yokota

TI__Guru**** 191445 points

By "suppress the fever" do you mean to reduce the IC case temperature?

0 takahiko yokota over 9 years ago in reply to JohnTucker

Mastermind 7135 points

Hi John san

I'm sorry my explanation wasn't temperature.
According to simulation by Webench, the loss was big at diode.
Is there the solvent?

Best regards,
Yokota

0 JohnTucker over 9 years ago in reply to takahiko yokota

TI__Guru**** 191445 points

I doubt you could easily substitute a low side FET for the catch diode. If you tried to drive it from the SW node, there would be no way to provide for any dead time and you would get shoot thru for sure.

I ran this in webench. For Ta = 30 C, the IC junction temp is 62 C, the diode temp is 61 C and the inductor temp is 60 C, so everything seems optimized for uniform heating. It is only 30 C temperature rise above ambient. I don't really see an issue with it. Let me know if you need further assistance.

0 JohnTucker over 9 years ago in reply to JohnTucker

TI__Guru**** 191445 points

After re-reading your post I think you are not concerned with the temperature at all only with the losses in the catch diode. That diode already has a pretty low forward voltage drop, about .447 V at your condition. Efficiency is still pretty high at >87%. Changing D1 to Vishay SL44_E3/57T lowers D1 Pd to 0.9W and increases eff. to 88.7%. You could also consider two diodes in parallel. Forward drop will be lower when they share current.

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Using TPS54340 with external FET