Unable to control LED current ZLED7020 driver

[made2hack]

Hello all,

I'm trying to use an external voltage source to control the current that the ZLED7020 drives in a LED application. My schematic is attached below. The datasheet example is also attached below as is the description about driving the current with an external voltage source. However, I'm not having any luck. When the switch is in position 1, the voltage on ADJ pin is 0 essentially and the LED is off. When the switch is in position 2, the ADJ pin has around 0.5V, but the LED is being driven at 1.45 Amps (how is it exceeding the max current of the ZLED chip?). When the switch is in position 3, the ADJ pin has around 1 - 1.1V, and the same current of 1.45 A. The Sense resistor is 0.09 - 0.1 Ohm, and I thought according to the datasheet that the current cannot go higher than the value set by the sense resistor, in my case around 1Amp (approx).

So a few questions:
1 - External Voltage source, does that mean a completely separate supply with a separate ground? So there is no "interference" ?
2 - Am I overcoming the ADJ pin's input impedance of 500k? as per the datasheet?
3 - Why is the current so high? It is exceeding what I thought would be the max?

ZLED7020 Datasheet

[tpowell1830]

First of all, your schematic does not match the datasheet, the ISENSE is fed to the LED load through the ISENSE resistor in the datasheet and the ADJ is listed as 0.3 VDC to 1.2 VDC, you are putting 12VDC and the equivalent voltage divider voltage (which, at a glance is over 1.2 VDC) to the ADJ.

[made2hack]

First of all, your schematic does not match the datasheet, the ISENSE is fed to the LED load through the ISENSE resistor in the datasheet

Hi tpowell1830,

Thanks for your input. You were correct, this got it working. The Sense resistor obviously has to be in the path of the current under measurement (like a shunt). I guess sometimes a fresh pair of eyes is what is needed.