Led driver low current, little area, price critical

#1

About two years I am searching Led Driver.

Project is too easy and this is it problem.

  • Supply voltage of the circuit: 10…40VDC (Mainly circuit supply is 12-24 VAC from transformer…)
  • Output 20 mA 1 led, it is, circuit drive ONLY one standard white “20 mA, 3.6V bright led”
  • Linear devices not work as you know, 40 VDC supply and one led output- even with 20 mA heat is big problem
  • Price critical, but it is is quite flexible: total circuit price max 2 eur, eg. 5 eur, it explode. 3 eur… hmm… bad, but acceptable. (2 eur = Led driver circuit, coil and schottky.)
  • Size is critical also. Complete circuit board size is 16x16 mm, and it include 8-pole connector, switch, led, rectifier (supply is AC or DC), rectifier capacitor. AND with all this led driver…
  • I does not like too cute case, eg. BGA is rejected. How I say… “using microscope, non-ramble-hand and good soldering iron it is possible make prototype, but if I must take special soldering oven, X-ray-devices, we make something which is not electronic”. So… ugly spider is ok… but bga not…
  • Looks on my PCB there is space for this circuit… looks… hmmm… LED-driver looks there is space about SOT-23, switcher coil looks 1206 (maybe bigger is possible)… schottcy look there is space for 0805…

So. Mainly problem is: I need large voltage area and most difficult is input voltage 40 VDC, output 1 led 20 mA. Reason of large waste energy it must be switcher. And, as you see, I have very little PCB: I cannot put “many led series, I need only one and no space”. Only way is find any very little led driver circuit which accept little coil…

Any idea?

#2

Have you looked at something like CL520K4-GCT-ND. It is a TO252 so the package is a bit larger. It will take a 4.75-90V and is a constant 20mA out.

#3

Hmm… DPAK/TO252. DPAK take about 4x more space when is available. Also… linear and extremely prohibited.

Linear circuits are ok, but if need large voltage drop, result is much power. So, linear circuits need much space: Large case (as DPAK) or heatsink. In this case, my PCB is 16x16 mm and both sides are filled with components: if use DPAK there is no space for other electronics.

So, switcher is only possible, but looks there is no any switcher. All switchers are high-current (tenth to hundreds milliamps) and if try adjust it to 20 mA circuits goes out of specs: it is, must calculate special coil and make many jinx exceed frequencies (eg. 250 kHz must overclock to megaherz etc…)…

Maybe any way is try find any microcontroller, fet and build any 5x5 mm special switcher… but is is quite bad idea… or order any special ad-hoc-circuit… but it is really poor idea… I only try drive one led…

Hmm… maybe any “normal” switcher, from 10-40 supply to 5 VDC and little resistor…

So, looks I am right? No any manufacturer make little size, low current led driver SWITCHER…? Maybe not? So, it is really no any led switcher…

#4

These are some of the smaller switch mode LED drivers carreid by Digi-Key that will tolerate 40+ volts at the input. I like the LT3590 for situations where space is critical because of the integrated diode–it’s one less part that must be placed on the board.

#5

LT3590, interesting. But, maybe not good: “I-led = (Vin - Vout)/Rsense”. It is, if input Voltage is 12, current is not same as Input Voltage = 40V. So… if use this circuit, it need input regulator and then we are in start point again :).

BUT: problem solved. MCP16331 is quite simple and universal. It is: Output voltage is fixed, but eg. 5V out and 70 ohm resistor is just ok result. Coil is quite big, but maybe 20 mA output current need quite little coil. MCP is quite little: eg. TDFN is 2x3 mm, coil is maybe 2x3 mm. Also other large input area/5V output regulators are suitable.

So: Looks led drivers are quite special and sensitive area. In this case is better idea use voltage regulator…

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#6

The LT3590 datasheet could be written better; that equation on page 6 does seem to indicate that LED current changes with Vin, but that is not correct; the chip adjusts the duty cycle to keep (Vin-Vled) constant at 200mV. Studying figure 1 in the datasheet closely will help explain this.

General-purpose DC-DC converter ICs like the MCP16331 can also be adapted for service as LED drivers, as you have suggested. Doing so as below could save a few resistors compared with making a fixed 5v and then driving an LED through a resistor with that. Still, this approach requires 2 external diodes that an LT3590 solution would not.

One common difference between ICs marketed as “LED drivers” as opposed to “voltage regulators” is the feedback reference voltage; it tends to be much lower for LED drivers because the entire load current usually flows through it. Especially for higher LED currents, this makes a big difference in the amount of power that the current sense resistor must dissipate.

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#7

In my opinion this MCP16331 solution is good. Of course it need much external components, but it is quite little, price is ok. And: It is quite universal use, I can use it in many other system. Eg. my other topic, “3v3 speech amplifier”, in this case I can use this as regulator. Also I have many, many circuits I can use this MCP. I have also evalboard of the MCP16331, ordered from Digikey so I can make tests :).

In this case, this “max 50 V in” is very good. I have only one question. In normal use coil is this 15 uH. Now our output voltage is about it 3,6V and output current only 20 mA, not 500 mA. 15 uH coil is quite big. Any recommendation of suitable coil? 1 uH? 0,1 uH? 0,01 uH? Any digikey reference? Then I can quite easy test this all reason I have it eval board…