[SOLVED] Increasing required current from MC34063

[Vaughn]

Here I've attached the schematic and yhre board of my buck converter, which is using MC34063 Regulator IC to supply a 1W LED. I successfully managed to get the required voltage 3.3v but it is not sourcing the required current 350mA giving only 40mA, I am not able to figure this.

Calculation for this output from 12V says:
Ct=216 pF
Ipk=700 mA
Rsc=0.429 Ohm
Lmin=60 uH
Co=15 uF
R1=11k R2=18k (3.3V)

My configuration:
Ct = 220pf
Rsc = 0.5R (Two 1R 1/4W res in parallel)
L = I wound my self on a 6X8 drum core inductor got from junk.
Diode 1n5819
Co = 10uF
R1 and R2 through preset




What I tried
changing Rsc with 0.2Ohm 2W but current not increased
changing voltage divider with smaller value
changing out filter capcitor with bigger value
changing core size to 8X10 with thicker wire
changing Ct 133 (100 +33) for 100kHz and 220 for 60
Changing Inductor (size and number of turns both , not sure about the inductor material, wounded and measured the inductance from an arduino program)

What I am not abble to understand which component affects the output current most (Since Rsc doing nothing) Rsc or the Inductor or voltage dividers value?

Questions:
1. How to increase the current 350mA to the LED or the Output current 40mA at 3.3V is what that LED need?
2. Any DIY method to make milliohm resistors?
3. How to determine the Inductor saturation (I am dumb in Inductors)?

 

[KlausST]

Hi,

My assumption:
A LED usually needs to be current driven, not voltage driven.
--> Never connect a LED directely to a 3V3 source without current limitation circuit.
A resistor may be sufficient.

Let´s say you want the LED to be driven with 350mA@3.3V, then add a 1 Ohms resistor in series with the LED and adjust the output voltage to 3.3V + (0.35A x 1Ohms) = 3.65V.

Klaus

 

[FvM]

You should absolutely add an input filter capacitor. Depending on the source impedance and cable length, the input voltage may collapse during current pulse.

 

[Vaughn]

Suppose I don't add any resistor with LED in series then I should not worry about the voltage across it, if the current is 350mA in my case..?

 

[Vaughn]

This circuit is working fine, there is nothing wrong with the buck converter but the problem was with the way I was driving the LED and was assuming that it should draw 350mA @ 3V3 from this converter and this is where I was wrong. I reached to this conclusion by testing some other loads to it rather then LED and is sourcing upto 378mA without damaging the converter, not 350mA because some refine works are required in Inductor I think. Well this was my first buck converter and it's working.

And for that LED Current limiter is required as KlausST you said.. and I added input filter capacitor to it FvM and its doing better.