Hi,

I'm working on a SMALL buck boost driver for high powered leds using the TPS63000 part found here:

http://focus.ti.com/docs/prod/folder.../tps63000.html

I want to convert this to a constant current regulator and I found this:

http://www.electro-tech-online.com/a...1&d=1157917480

Basically it uses a flating ground, sense resistor and a single op-amp to convert to a CC regulator.

My question is can I take any single op-amp and drive it in single supply mode without the -'ve and +'ve supply for this purpose?

My input voltage would be 2.5V - 4.2V so i'm looking for an op-amp that will work and is SMALL like SOT23-5 or smaller.

If anyone has any other comments about what I'm trying to achieve, please tell me.

Basically, a small small CC converter for high powered leds ~ 1A. Cheers!

Attached Images

CC_converter.GIF (9.3 KB, 23 views)

 

Look for an opamp that's specially meant for low voltage applications, and has it's negative terminal as part of its input range. I've used the TLV2370 series (http://focus.ti.com/docs/prod/folder...t/tlv2370.html) for a similar app, but it only works down to 2.7V.


BTW, if you plan on using a rechargable Lithium battery, you ought to calculate what sort of battery life you want - and how much you get at the low range of the battery's voltage. At that point the battery terminal voltage is heading south pretty quickly, and you run the risk of damaging the battery.

 

Are you aware of the considerable number of switchmode LED current regulator ICs that are available?

__________________
Ron

 

Well, there are LED drivers, then there are 1 Amp LED drivers. There are considerably fewer of those floating around.

I think I've seen a couple "flash LED" driver parts out there that do a good fraction of an amp, but I don't think they're built to do it continuously - I just saw one that wanted a 6Ohm current sense resistor...

Linear did have one part that seemed to fit well though:
http://www.linear.com/pc/productDeta...3,C1094,P15693

 

Hi, thanks for the input. What do you mean has it's negative terminal as part of its input range?

Is it okay to use a op-amp that has both -'ve and +'ve voltage supply input but tie -'ve to gnd? Thanks.

 

Yes, thank you. I am looking for one that is fully regulated, buck-boost and can handle 1A ANd is VERY SMALL.

 

I will also use that chip in another design but I want to use the TI chip for this design. Thanks!

 

No, I didn't say anything about changing the circuit. Lots of the older opamps did very strange things when you try to use voltages very close to the supply rails - in this case gnd is one of them. If you look for an opamp, what you should be looking for (in the fine text) is something that says "rail-to-rail input."

 

Basically was taught in school that op-amps work on -'ve supply and +'ve supply like -15V and +15V. Now i've read some sites on using op-amps in single supply mode like GND and +15V or something like that.

Most op-amps weren't designed for single supply so is it okay to use a dual supply op-amp and tie the -'ve supply to GND?

Hope that makes sense.

 

So let me ask you a question: If a part is attached to a +12 and -12, and suddenly the part is attached to +24 and gnd, does the part know anything changed?

The only thing that matters is how the opamp behaves close to the power supply rails.

 

Excellent. I figure if the op-amp is connected to GND and +4.2V and the input to the op-amp is around 100mV and the voltage FB is around 500mV, that should be OK right?

What should I be looking for in an op-amp that behaves nicely when operating close to the rail (GND)

Many thanks for your help.