LDO current regulator for power LED

LDO current regulator for power LED

Challenge

You ' ve got a power LED? great! Build a flash light!

What does the spec say? "voltage:3.6. 3.8 V, Power 3 W. " Okay. This means it draws some. Three Mignon cells give 4.5 V for at least three hours. So we add a resistor of 1.2 ohms and we are safe. If the LED runs at 3.6 v this gives MA and 2.8 W while the resistor voltage are 0.9 v resulting in 0.675 W for the Resi Stor. So we take a resistor that can stand 1 W. simple, isn ' t it?

Yes and No. The LED in this simple circuit would glow but what happens if voltage drops because batteries go flat? If voltage drops by ten you'll get 4.05 V and current drops by some a meaning that's need rather full batteries to G ET a reasonable amount of light from your LED. First idea was to put a voltage regulator between the batteries and the circuit and adapt the resistor to its output voltag E. However, finding a suitable regulator is tricky. You need a low dropout voltage to let the LED work at full brightness when batteries voltage becomes low.

BJT Current Limiter

We can do better! To stabilize brightness we had to regulate the power consumption of the LED by controlling the current. Wikipedia [1] shows a simple circuit of the transistors and three resistors limiting current independent of supply voltage . We change the circuit by removing resistor R2 which are not really needed. What's about dropout voltage? Clearly, there is VCEsat of Q2 which can being in the area of 0.2 V at current around 1 A (for BC640, bd13x, bd23x, bd43x ...) ). However, there is the additional voltage of the emitter resistor which equals vbeon of Q1, typically some 0.7.. 0.9 V. So we get a total dropout voltage of VBEon1 + VCESAT2 resulting in some 1.0 v.

This dropout voltage are too high for three mignon cells. If You is not a keen on energy efficiency and a ready to carry four mignon cells then you is done.

Rail to Rail operational amplifier

But we can does even more better! The transistor current limiter are a control loop sensing the current by a resistor and keeping the voltage equal to VB E of a transistor. What is about reducing this voltage? There is operational amplifiers with rail to rail input and output meaning this they can handle voltages from VSS to VDD. There is numerous such parts like the MC33201.

There is a look at this current limiter circuit in Wikipedia [2]. Voltage over the sense resistor are kept equal to Voltage over Z-diode. Okay, there is no z-diodes of really low voltage. Since the amplifier have high input impedance we can use a voltage regulator and simply reduce the voltage using a voltage Divider.

One point still to notice are output current of the amplifier. If we use a BD139 transistor this would give us a current gain of all or more meaning that the amplifier have to deliver m A which is okay for the MC33201. If the transistor needs higher base current than the amplifier can deliver we have a emitter follower between Ampli Fier and transistor.

In our 3 W LED example we can use a resistor of 0.33 Ohm delivering a voltage of some 0.25 V in operation which can be Han dled by the op amp. Now add VCEsat for 0.2 V and get less than. 5 V dropout. Okay, this is a little cheating becuse VCEsat would requires-MA which won ' t be delivered by an MC33201.

dimming

Brightness can controlled by reference voltage. Simply use a potentiometer as voltage divider. or connect the amplifier input to another circuit like a multivibrator to flash the light or to some rectifier and low pas s for a light organ.

Future work

If we is controlling brightness using a mirocontroller with ADC and DAC or PWM we can even drop the Op Amp. Feedback voltage is digitized by the ADC, then a-PID program computes output voltage and DAC or PWM with RC low pass Control power transistor. This gives opportunity-additional functionality like touch pad control, remote control, or automatices.

Using a MOSFET instead of a BJT might further reduce dropout voltage. For instance, MCP87XX has very low RDS and low threshold voltage making them suitable also for the microcontroller idea R Unning the controller at 2 V or so.

Final Remarks

There is an instructable [3] showing how to build a power LED driver. This article mainly describes a discrete circuit similar to the other BJT circuit in [1] but with Q2 replaced by a power MOS Transistor. MOS Transistos can give lower dropout voltage but we need really high power high price transistor.

References

[1] Current limiting http://en.wikipedia.org/wiki/Current_limiting#Single_Power-supply_circuits
[2] op-amp current sources http://en.wikipedia.org/wiki/Current_source#Op-amp_current_sources
[3] High Power LED Driver circuits http://www.instructables.com/id/Circuits-for-using-High-Power-LED-s/step6/ the-new-stuff-constant-current-source-1/

LDO current regulator for power LED