LM3409: FET Bypass

Hello Ethan,

Could you be a little more specific regarding how it doesn't work when you change the sense resistor? Does it just not turn on or does it behave strangely? It is possible that it is related to the inductor. At 3A you are running the EVM inductor very close to saturation and that could cause issues. It would be easy enough to get a 22uH with a much higher current rating to test that out.

But to answer your last question the LM3409 is a great device for this type of application and you can get up to 6A continuous or pulsed without too much trouble. Using a shunt FET to bypass the LEDs is also a good option if you need really fast edges, but keep in mind you will be burning power in the shunt FET when the LEDs are off. At these frequencies EN/UVLO dimming might be perfectly suitable though and save some components. I mention UVLO because it will PWM dim (or pulse) exactly like the EN pin does except you will not go into low power shutdown mode after some time (which delays the next pulse). At 100Hz and under you would likely need to use UVLO or you will go into this mode at lower duty cycles/pulse widths, it will react much faster if the UVLO pin is used.

Regards,

Clint

Hi Clint,

I'm supporting Ethan on this issue while he's out of office. 

From your comments, it seems to be going into a shutdown mode.  I have attached some waveform showing my issue.

Pulse frequency at 1kHz is fine.

Pulse Frequency at 100Hz, the current is not the same from pulse to pulse.  For this application, we need to skip one or two pulses once in a while.  It may explain this behavior.

Pulse frequency at 50Hz, no pulse is present.  I also added the VCC voltage in channel 1.  I think it is waking up but the pulse width is not long enough to reach the full current.

So, if I understand correctly, I need to pull the UVLO pin low to stop the current and pulling the pin high will restart the pulse without the wake up time?  Will applying a 0-5V pulse on UVLO with the resistors removed work?

Thanks,

Chuchen

Hello Chuchen,

You are correct, when the frequency is low enough it goes into shutdown with the EN pin. Using UVLO will work better in this case. They act exactly the same except that when EN is pulled low VCC is allowed to discharge until you hit shutdown mode. UVLO low will keep VCC high so the response will be fast whereas you need to wait for VCC to charge if using EN. You can apply a signal directly to UVLO or you can still use resistors for a UVLO function and OR in a signal with a diode. You can look at the LM3429 datasheet for an example. In any case use UVLO rather than EN for your PWM dimming and it will work great at the lower frequencies you are using.

Regards,

Clint