problem with mc34063

hi
i use a mc34063 for drive 4 power led.led's are 1 watt.
i use 47uH inductor and 1500pF capacitor and a 10 ohm multi turn for limit pin.from this site **broken link removed** (vin=14.8/i out=350ma/f=24kh/vout=13/ripple=30)
i need 13 volt and 350 mA in output for drive 4 serie power led.
output voltage of circuit is 13 volt but when i connect led's to circuit voltage drop to 11.8 volt and even by turning limit resistor output voltage note chenge.
input voltage was from a 0-18 volt source and i change it from 15 to 18 and output was 11.8 in that range and current was 240 mA.
why voltage drop about 1.2 voltage and current wasn't 350 mA even by changing limit resistor ?

Assuming it is built correctly, the most likely reason is that it has gone into current limit. All the switching current passes through the current limiting resistor so it may need to be a power rated device, if you are using a small multi-turn potentimeter it probably isn't going down to a low enough resistance, momentarily try shorting it out to see if it raises the output voltage. The potentiometer has to pass about 700mA so a small one is not suitable.

Also note that you are using a drop-down configuration with only abut 0.5V safety margin at the input. If your input voltage, while under load, drops below about 14.5V it will stop working properly.

I use a MC34063 in a similar system but wire the LEDs in two parallel pairs with a 0.5 Ohm resistor in each of the parallel paths to help balance the current. The regulator then has to generate only 2 * LED Vf. I run them from a 12V SLA battery with the current limit set to 700mA.

Brian.