MC34063A dc convertor

[Joe G]

I'm using a auto-usb convertor MC34063A to power 2) NCP1117 voltage regulator set for current regulation. ( using a 4.7 1% resistor) to supply two 3 watt white leds at approx 266mA. I adjusted to smps to put out 6.2V by changing R2. What my question is: when the supply to the MC34063A is about 9V +/-, I get the required amp output to the leds. When the supply is turned up to the approx voltage of a vehicle @14.4Volts, the amps to the leds drops down to approx 120mA's ??? I've tried using different auto-usb convertors and have found out the same reaction. What gives because this has me really stumped also, the voltage output stays spot-on @ 6.2V
thanks.

 

[ChrisP58]

It would be a lot easier to help you find the problem if you post a schematic of your circuit.

 

[rumpfy]

Joe,
These things are really simple to get going but they have some limitations when you push them.
The inductor needs to be sized reasonably closely to the calculated correct value otherwise the switch current gets to its maximum value too soon.
There is an excellent Application note from Onsemi/motorola "M34063A-D".
Just go through this AN and see if you can identify your problem.
I have done some of the things you seem to be trying to do, and generally I have to fiddle with the timing capacitor.
hope this helps.

 

[skimask87]

Almost sounds like he might be driving the '1117 towards thermal overload.
But, probably never know without some sort of schematic...

 

[Joe G]

well, I found out my dilemma, all the usb convertors are indeed correct, as is the current from both 1117 reg. and going into the leds.....The problem was in my current meter that is on my psu...Like the old saying goes.
You are only as good as your tools