LED driver with cheap UC3843

uc3843

Hey guys,

i've got some problems with the UC3843
Normally i am using Simple Switcher like LM2575 for driving LED (1W/3/5W) but now i tested a circuit with 6 3W LEDs in series, powered by 24V an the LM2575 get very hot because of its internal darlington
even others of the Simple Switcher family with MOSFETS are having a real worse RSon for my application - so they will have the same problem and getting very hot.

since long time, i want to build an constant current led driver with an UC3843 and an IRF7240 P-channel MOSFET. But after i read many datasheets (from TI, On Semi, Motorola,...) i found may applications for boost and flyback circuits, but not for buck switchers. i even could not find out, how to control the feedback (an LM257x constant current is easily build able with an shunt and a opamp to control the current).
is it possible to build a constant current LED driver with UC3843 without an transformer ( i don't need galvanic isolation and my power supply has 24V/20A).

is there anybody who could help me with an circuit or a description about it?

thanks
and sorry for my worse english

uc3843 application note

Hi! I apologize myself for the quality of my English...
see if this serves you

uc3843 application

Images

lm2575 led driver

maybe this will also work: (note: I didn't tested in actual yet. )

it's a (modified) BUCK configuration.
you can simulate this using Simetrix (they have simulation models for UC384x PSU controllers).

lm2575 led

PCB

uc3843 led driver

current table

uc3843 led

thanks largosoft

but this design is based of an MC34063. Ok it is very cheap, too - but its frequency is very low - so a gigantically inductor is needed (as you can see on the PCB).
Have you tried this driver? Is the PNP getting very warm at the maximum current?
I think i will need a P-FET like IRF7240 because at my application are 6 Cree LEDs powered by an 24V open frame power supply at a current with more that 1 A and it should not be warmer than 50°C!

Has anybody an design with an UC3843 as an constant current source?

Can't use the UC3843 for what you're doing, which seems to be stepping down. UC3843 is geared towards boost and flyback. However, you could use it in conjunction with a SEPIC converter, but this involves using a coupled inductor. A simple IC to use would be a TL494 in voltage mode, but still able to control the current. There are also loads of other ICs that will do stepping down using external switch.