LM3409: EN pin PWM issue

Hello Michael,

Using PWM2 also uses the Shunt MOSFET. Enable dimming at your frequency should be very possible.

What is J1 state in, how do you have it configured?
What are you driving EN with?
Have you looking at your 36V supply to make sure it is not dropping out when there is a current demand?
How is the EVM connected to 36V?
Can you look at VCC? Pulling EN below 0.5V puts the device into low power shutdown.

Best Regards,

Hi Irwin,

Using PWM2 the dimming is working according to spec.

* J1 state is open (no jumper) when I use EN pin PWM (according to datasheet)
* I use external 5V power source with optocoupler and pulldown resistor on EN pin (the blue line on the picture is the EN pin)
* Didn't look on the power supply line - I can check but it works with PWM2 so I don't think its the problem . Measured the Vin, its steady, no drops
* I use 220-36V, 4A DC power supply and I connect the EVM with banana jack (50cm wires)
* I will take a look on VCC - Vcc is same as Vin when EN is grounded

***update***

at 40Hz (50% duty cycle) PWM on EN the the issue is resolved, there is a clean signal.

But the system has to work at 20-60Hz

Thank you,

Hello Michael,

Vcc cannot be the same as Vin, Vcc should be 6V. In low power shut-down the VCC will drop. From the datasheet:
"8.4.1 Low-Power Shutdown
The LM3409/09HV can be placed into a low-power shutdown (typically 110 μA) by grounding the EN terminal
(any voltage below 0.5 V) until VCC drops below the VCC UVLO threshold (typically 3.73 V). During normal
operation this terminal should be tied to a voltage above 1.74 V and below absolute maximum input voltage
rating."

If Vcc is dropping and going into low power perhaps a large VCC capacitor will allow a longer PWM low time.

Best Regards,

Hi Irwin,

I measured Vcc relative to ground, going over the datasheet the Vcc is referenced to Vin so I do get 0v when EN is low and around 6V when EN is high.

I also saw that the rise time is effected by OFF time, when OFF time is lower than 10ms the rise time is OK.

I read on the forum the the sleep time of EN is 10ms, and its explain the high rise time.

I'll try to add larger Vcc capacitor, also I read that I can use UVLO to do PWM - how do I do it?

Thanks,

Hi,

Can you please show a drawing\schematic how to do UVLO PWM?

Thanks,