LM3409: why LM3409 used PFET

Hello Park,

There are a variety of reasons the LM3409 uses a PFET instead of a NFET for the top FET.

1)  It simplifies the driver.  It no longer needs a boot strap capacitor attached to the SW node and diode for a floating driver.  It also means no level shifter for the controller to the drive of the NFET.  If you were to use an NFET you could probably have to use a large boot strap capacitor to make sure you can control drive the NFET under all circumstances including widely ranging frequencies and widely varying duty cycles and current ranges due to shunt FET dimming to ensure the boot cap is always charged and able to be turned on.  A PFET makes this much easier because it can be turned on at any time and is not dependent on the boot cap.

2)  Using a PFET controller allows you to control the LED current at very high duty cycles as it can go into a linear mode and not switch but operate in the linear mode to provide the controlled current going into the LEDs.  You can't do that with an NFET driver because you need to pull the SW node to ground to charge the boot strap cap or the NFET can't be turned on.  

Let me know if you have other questions.  

-Francis Houde