Simple and easy - Constant current source

[xbrite]

I know you guys are eager to get a very reliable constant current source to drive LED's. I found an ideal device to make a simple constant current source easily programmable while requiring very few external components.

1) The device is ST890 from ST Microelectronics. You can find the datasheet from www.st.com. Click products, key-in 'st890' at P/N search, click PDF file to see the sheets.

2) This device is a linear device. The input voltage is 2.7V-5.5V, current adjustable upto 1.2A.

3) This device is suitable for 3cell NiCad/Ni-MH or 1 cell Lithium Ion battery. It can be used with 3 cell Alkaline batteries but I recommend to use Ni-MH or Li-Ion as the voltage stays at 3.6V most of the running period and so the power loss is minimal. Not for 3V Lithium battery(CR-123)!

4) You just select a resistor to fix the current required. Easy!

5) This can be used to run upto 3 1watt Luxeon LED's in parallel. It may require small resistor in series to balance the current thru individual LED's which may have slightly different Vf.

Cheers...

 

[dano]

Cool, and moved to Modified...

 

[georges80]

And the Maxim Cross reference part is MAX890L. At least you can easily get samples from Maxim.
george.

 

[UK Owl]

XBRITE - Do you work for the marketing department at ST.COM ?

8 solder connections on the chip alone !

If I wanted to do that muck soldering I would build a transistor based regulator of my own !

This looks far more complext to use than simply adding a resistor to an LM317 so it runs in constant current mode.

 

[xbrite]

Dear UK Owl,

I don't have any connection with ST. I just found the device and made one experiment and it worked great.

You mentioned LM317 with a current setting resistor but this requires 1.2V drop voltage so the supply voltage should be higher than 4.5V which also reduces effieciency very much. If you consider the number of compenets to be used, this is fairly nice comparing to the step down or boost regulator for sure. In addition, this circuit gives a very satble constant current thruout the discharging period so that you have very stable light output. With 3 cell Nicad or NiMH, the efficiency is approx 88% which is not bad at all.

In addition, you can make the circuit on a very small real estate or footprint so that this can be easily installed to any of the modifications.

Cheers..

 

[jeff1500]

xbrite,

What did you solder the chip to?

Did you etch a circuit board?

 

[xbrite]

No, I didn't etch a circuit board but used a universal PCB with many holes/round patterns. As the device is in SO-8 package, you can solder the external components with care. You can see the picture in my other post re the same subject. The pins are assigned conveniently for soldering. The 1-2 pins are soldered together, 3-4 and 6-7 pins also to be soldered together. Open for Pin 8.

I recommend you to use 1uF Multi Layer cermaic cap for input filter soldered between pin 1-2 and 3-4 and 0.1uF for output filter soldered between pin 3-4 and 6-7. I used a Surface mounting 1uF for input filter and leaded 0.1uF for output filter. a current setting resistor for 1W LED, 4.7 Kohm was located between pin 5 and 3-4 at the other side of PCB as I didn't have surface mount device. Connect B+ at Pin 1-2 and B- at Pin 3-4. Put Cathode of the LED at pin 6-7 and anode at pin 3-4 and this is it. You may need to cut out some of the round copper patterns at pin 5 and 8 to prevent shorts in pin 6-7.

I was able to put all these on 10mm by 8mm PCB and could be even smaller if SMD is used.

Cheers..

 

[Doug Owen]

LM317 a simple but poor choice?

[ QUOTE ]
xbrite said:

You mentioned LM317 with a current setting resistor but this requires 1.2V drop voltage so the supply voltage should be higher than 4.5V which also reduces efficiency very much.

[/ QUOTE ]

Actually it's worse than that. Check the specs, there's a minimum "headspace" of about 1.5 volts (minimum voltage between input and output). This gets added to the 1.2 Volts for a total of over 2.7 Volts 'wasted'. This raises our absolute minimum battery voltage to 6.1 Volts for a 3.4 Vf LED. Using NiMH cells this means at least 6 cells (five for a total of 6.0 won't cut it), *twice* the number needed for a low drop out regulator. Even using alkalines, it's probably necessary to use 5 cells for margins.

This is for sure an easy solution if you have 'power to burn' (like say in a automotive usage), but getting even 50% efficiency with batteries isn't at all easy.

Or so I see it.

Cheers.

Doug Owen

 

[jeff1500]

Re: LM317 a simple but poor choice?

Have you ever seen a preprinted circuit board that would fit a micromax-8 chip?

I used a SurfBoard 8 pin SIP adapter, but there's only room for just the chip.

 

[ElektroLumens]

Re: LM317 a simple but poor choice?

Just checked out the specs on both these circuits. The MAX890L has much better documentation. Price is a little over $1. Circuit design is about as simple as it gets.

Instead of the 100k ohm resistor, a trim pot could be used, for variable current control. Nice. /ubbthreads/images/graemlins/grin.gif

Nice circuit, I'll have to try it out. I wonder if Digikey or Pioneer carry it?

Wayne j

 

[xbrite]

Re: 890

Hello Wayne,

You don't need 100KOhm at Fault pin. You need approx 4.7KOhm as a current setting resistor for 1W LED with 3 cells of Ni-MH or 1 cell Li-Ion. You can put a 100KOhm trim pot in series to this resistor which enables dimming.

Even for 5W LED, this device from ST works though the spec says Max supply voltage of 6V. I applied approx 8V and 2.3KOhm for current setting, and it worked fine with around 700mA to the 5W LED. I think this works fine with 6cell Ni-MH or 2 cell Li-Ion. If the input voltage is too high, it starts to blink as the internal thermal shut down occurs. I do not see any problem with these batteries but if you happen to have blinking problem, this can be resolved by inserting a general purpose diode to the B+ supply to reduce the applied B+ to this device.

See the photo below. The current setting resistor is at rear side of the PCB.

Cheers..

 

[ElektroLumens]

Re: 890

xbrite,

Where did you purchase the chip?

Wayne j

 

[xbrite]

Re: 890

Dear Wayne,

I got it through a ST distributor in Korea.

Cheers..