Chris,
There are basically three ways the power in the LEDs can be adjusted. It's very important to understand the implications of each to make sure you're measuring and adjusting correctly.
1) Rset. This resistor sets the nominal current for the LEDs, i.e. the current they will run at when the IC is getting an "ON" signal on the EN/PWM pin, and Vin is higher than Vout + .5v. Basically what happens is that the IC regulates output voltage until it sees the correct current in the LED string.
2) Vin, aka the voltage supplied by the DC power supply. As long as Vin is greater than Vout (the LED voltage) plus half a volt, the IC will operate in it's normal constant current state, i.e. output voltage will be varied to maintain the target current.
3) EN/PWM pin voltage - this is a digital input, i.e. it has a high state and a low state. When you supply a PWM signal, the output voltage is rapidly switched on and off with a duty cycle matching the PWM signal. It's important to note that supplying a PWM signal does not simply shift the output to a lower current - instead, it alternates output between the nominal current set by Rset and zero. It's possible to use a simple multimeter on the output to measure an average current when feeding EN/PWM a PWM signal, but it's important to note that the LEDs ARE NOT seeing that measured current - they're seeing full current for a portion of the time, and zero current for a portion of the time. In other words, you DO NOT want to use a PWM signal to "set the current" through the LEDs.
Effectively, this is what I see as the best practice for operation:
1) Choose an Rset value to give the desired MAXIMUM current you want in your LEDs.
2) Build the rig, set your DC power supply to the lowest voltage it's trimpot will allow, and put a multimeter set to amps in series with the LEDs. Turn the rig on, and check the current you're getting. It'll likely be significantly lower than your target current. Turn the voltage on the DC power supply up, while watching the current. It'll climb as you turn voltage up. Somewhere very close to your desired nominal current, it'll plateau - adding more voltage will no longer result in more current. At this very point, the IC is operating in the most efficient constant current mode, which is the desired state of operation. You'll find that this point occurs right around half a volt higher than the Vf of the LEDs. Once you've found this point, you should no longer fiddle with the voltage adjustment on the power supply!
3) Take the multimeter out of the circuit, and operate as desired. Use the EN/PWM pin to "dim" the LEDs. Forget all about Rset and the voltage adjustment trimpot on the power supply, as you should never need to touch either again.
