LM3409HV: Why is the current limited to 4A

Hello,

The LM3409 is not limited to 4A. You are correct in that it depends on the FET and the inductor, not really the output capacitor though. The inductor is simple, you just need to have a saturation current rating greater than the peak current set with the LM3409. But the FET requires a little more consideration. The power dissipation, both from Rds(on) and the AC losses are a concern. But as far as whether the 09 can supply the current you need to consider the FET gate charge and the switching frequency you want to use. The most common limiter of current is this combination. The VCC current has a limit specified in the datasheet. The current required to drive the external FET is the gate charge times the switching frequency (Qg*Fsw) and that is usually the limiting factor. If you make sure Qg*Fsw is less than the minimum current limit then there really isn't a limit to your maximum current. I have seen the LM3409 used for currents in excess of 10A. If you could provide your operating specs (Vin, Vout, Fsw) and a BOM I would have a better idea. But there is for sure no limit at 4A for the device.

Regards,

Clint

Thank you Clint. That helps heaps!

Clinton Jensen said:

The current required to drive the external FET is the gate charge times the switching frequency (Qg*Fsw) and that is usually the limiting factor. If you make sure Qg*Fsw is less than the minimum current limit then there really isn't a limit to your maximum current.

What is the minimum current limit? What is it referred to as in the datasheet? I've listed here a design spec and I did a quick calculation of the BOM which is listed here as well. I had an earlier design from WEBENCH and the Ruv2 and Ruv1 values are from there. I'd really appreciate it if you could let me know if this is a viable design.

Specifications:

Fsw = 500KHz
Vin(nominal) = 12V
Vin(max) = 36V
Vout = 5.2V
Iled = 10A
delta-Il-pp = 500mA
Vturn-on = 5V
Vhys - 1.1V

BOM:

Coff = 560pF
Roff = 6.81K
L = 12uH
Rsns = 20mR
Cin1 = Cin2 = 15uF

PFET = see the options listed below:
IRF9Z34PBF    60V    18A    140mR    34nC
FQP17P10    100V    16.5A    190mR    39nC
FQP17P06    60V    17A    120mR    27nC
IRF9540NPBF    100V    23A    117mR    97nC
FQP27P06    60V    19A    70mR    43nC
2SJ600-AZ    60V    25A    79mR    38nC
IRF5210PBF    100V    40A    60mR    180nC

Diode = MBRB10100CT or SDM8M100P5
Ruv2 = 49.9K
Ruv1 = 16.5K

Thanks.

Hi,

I finally built this design and it works, however the MOSFET gets really hot - to the tune of 95 deg C or so. If I were to add multiple MOSFETs (with the same part numbers) in parallel, would that work?