Using multiple LM3409's

[JMG]

Hi,

I am controlling 5x LM3409's from a pic. The ENABLE pin of each device is connected to the PIC directly, which also has a 510R resistor and LED connected to ground. This means that when that driver is enabled, the LED lights as an indicator and the ENABLE pin is energized. There are a few problems though. I seem to be getting multiple drivers turning on when only one driver is selected. I assumed that the ENABLE pin of inactive drivers was being pulled low through the 510R resistor and LED. Is that acceptable? I'm considering putting a 10K in parallel to the 510R and LED for each driver.

Any thoughts?

Thanks,

James

 

[JMG]

The above issue is solved.

I still have an analog dimming issue. I'm using a digital pot to create a potential divider that provides 0 - 1.667V into the analog adjust pin of each of the 5 drivers. They are paralleled together. The issue is that as I ENABLE more drivers, there seems to be a raise in voltage at the dim pin which obviously increases the output. This may be a issue of not enough current in the potential divider circuit to drive the dim pins correctly. Any thought on this or past experience? I am using a 10K resistor from 5V and a 5K digital pot to ground. Current is 0.3mA to the pins when the digital pot is at 5K.

 

[schmitt trigger]

The datasheet indicates:

Apply a voltage >1.74V to enable device,
voltage <0.5V for low power shutdown.

So first things first, with a scope make sure you meet those requirements. And yes, a 10k parallel resistor could help...there could be noise feed thru from the other active drivers.