Home      Projects     Experiments     Circuits     Theory     BLOG     PIC Tutorials     Time for Science

 High Efficiency High Current LED Buck Driver using the A6210 AuthorGiorgos LazaridisMarch 23, 2012

PAGE 1 of 2 - High Efficiency High Current LED Buck Driver using the A6210

The circuit on a breadboard for test
Some time ago i published a theory page regarding the LED driving and controlling methods. These methods were all linear regulators, very simple to make but very inefficient -in terms of power consumption- for high current applications. The idea was to use this theory page as an entrance level for the SMPS LED drivers.

The first SMPS (Switching Mode Power Supply) LED driver that i made is a Buck-Regulating LED Driver using a chip from Allegro Microsystems, the A6210. I was provided some samples from Farnell for testing and prototyping, along with some other cool staff. Do not forget to pay a visit to Farnell on-line store and Element14 website.

The A6210 can drive up to 3A load with constant current, with switching frequencies up to 2 MHz and supply voltage from 9 to 46 volts. It has additionally an optional PWM input to control the brightness of the LED. The sense voltage is limited to 0.18 volts for higher efficiency, since the power dissipation on this sense resistor is minimal. I will be using a 10-12V 1A 10 Watt LED, powered from 24 VDC supply.

Doing something new? Prototyping on a breadboard saves time
I thought that i was experienced enough to get a chip - any chip - and start making PCBs and staff. The A6210 comes only in one package, a QFN 16 with external dimensions 4 by 4 mm. That is small, ridiculously small, and makes prototyping even harder.

When i saw the chip, i decided to immediately design a tiny PCB to test it, but this was really a bad idea. I should either have make a larger PCB so that i could easier switch parts, or mount the chip on a QFN to DIP socket and prototype on a breadboard. The result from my previous decision was many hours lost on PCB making and a fried A6210 chip:

 How small is the A6210? It is ridiculously small! I had to design two completely different PCBs and fail before i decide to prototype on a breadboard So i ordered some QFN16 to DIP sockets I should have done this from the beginning. Now i can put the QFN on a breadboard for test.

A PCB would be of course more elegant than a socket on a breadboard, but at least now i can change the biasing parts very easily:

The circuit
The circuit is not complicated at all, after all, the chip itself has all the necessary parts integrated and only a few external components are needed. Inside the datasheet of the A6210 there are sample circuits that may fit your needs. If not, you can very easily change them:

 Click to enlarge

 Resistors R1 Resistor 220 KOhm 1/4 Watt 5% Carbon Film R2 Resistor 130 mOhm 1/4 Watt 5% Carbon Film Capacitors C1 Electrolytic Capacitor 1 uF 50 Volts C2 Ceramic Capacitor 22 nF 50 Volts ICs IC1 A6210 3A 2MHz Buck-Regulating LED Driver Diodes D1 SB140 1A Schottky Barrier Rectifier Misc L1 Power Inductor 22uH 2.8A UNI-PAC(TM) Surface Mount

Choosing the right parts can be somewhat tricky. There is a complete How-To in the chip's datasheet, but i totally suggest that you use the A6210 Design Tool excel l sheet from Allegro Microsystems:

Here is how it works. Open the spread sheet and go to the second sheet, named "Component Calculator". Then fill the grey fields with the values for your circuit:

• Input supply voltage - The supply voltage e.g. 24V
• Total LED span voltage - Add up the forward voltages of your LEDs that are connected in series. If only one LED is used, then use the forward voltage of this LED.
• Average LED current - The current that will flow through the LED(s)
• Target LED ripple current - The A6210 does not need an external capacitor to reduce the output ripple, because it can be adjusted to have minimum current ripple. Keep this value as low as possible, but remember that low ripple values will require larger coil inductance and thus larger coils in size. A 10-20% of the average current is a good beginning.
• Forward voltage drop of schottky diode - Typically this value is 350 to 400mV for schottky diodes
• Valley voltage (typ. 183 mV) - Just keep this 183 mV
• Select switching frequency - This value cannot exceed the "Maximum possible switching frequency" proposed on the cell above it.

• After putting these values, you'll get some suggesting values for R2, R1 and L. Bellow each suggestion there is a blue field. Your next step is to select a part value for these three parts (R2, R1 and L) and fill the blue cells. So, if for example you get the value 198 mOhm for R2 in the cell D21, then use the most close number to this which is 200 mOhm in cell D23. If your calculations lead to a problem, you will get a message in red fonts in cell A33 which goes like this:

Ripple voltage too low. Select an inductor that produces a voltage greater than 20mV

This means that you either need to change your coil size (D39), or increase the ripple current (D12). Finally, you will receive the actual performance summary of your circuit in the green cells at the botton, calculated for the parts selected in the blue cells.

You may wanna then go to the fourth sheet of the excel named "Efficiency Estimator". There are some interesting values for your circuit, such as the overall system efficiency and the temperature that the A6210 will reach during operation.

Remember that all the above calculations are strictly theoretical. When it comes to design, the size of the PCB wires, the pads and all the other design characteristics will play an essential role in the final performance. Especially when it comes to R2 since its value is in the scale of milli-ohms.