Voltage Regulator and Resistors for LED Circuit

Question

I'm trying to design a circuit to power 17 LEDs. The LEDs I'm using are AA2810AQBS/D Blue SMD LEDs with 3.3v Forward Voltage at 20mA.

I would like to power this using EITHER 2xAA Alkaline batteries OR a 5V USB battery pack.

I was looking at the AAT1217-3.3, 3.3V Step Up Converter but I'm not sure if 3.3V is enough and it's unclear to me if this can accept the voltage range that I need for 2xAA's or 5V battery pack (potential battery voltage range of 2.4V to 6V?)

Can I power these LEDs with 3.3V and 1 ohm current limiting resistors?

Any suggestions on a low cost regulator and LED current limiting resistors for my needs?

Answer 1

Lowest cost solution from a battery.

• Load Power = 3.3V*20mA*17=1.1W @340mA , which would be short operating time for AA's and 5V USB pack has no tolerance specs, cost or other obvious assumptions were given.

• until then, this is the cheapest.

^{simulate this circuit – Schematic created using CircuitLab}

If Vf@20mA is matched with 1% ( expected from same batch) and Board has good copper heat radiation on pads for cooling, you can eliminate all the R's and drive with 3.3V Buck regulator rated for >350mA.

Or use a 3.3~3.7V LiPo cell and 3.3 Buck regulator with 90% efficiency max or TPS736-3.3 LDO (0.4A) for $0.75 but same efficiency as 17 * 15ohm R's but depends on 1% matching but gives constant If output to Vbat=3.4.

For a Boost Regulator with 20mA output, you need an array of 17S1P with 17*3.3V = 56.1V.

The problem

You cannot trust any EBAY specs for Ah when there is no datasheet to prove how they perform when loaded faster than 20h discharge rates are used. Anything must be proven with a datasheet and supported by supplier. Not all 18650 cells or USB packs are the same!! Buyer beware!

Answer 2

Running 17 LEDS in parallel across a battery wont work very well since forward voltage variance will be large compared to the nominal voltage drop across the resistors resulting in a lot of brightness variance and loss of light as the battery dies.

You could use a buck-boost voltage regulator to get you more headroom to play with but LEDs really want to be driven with a constant current not a voltage so you are kind of going down the wrong road.

A better solution is to arrange your LEDs in series strings and drive them with a proper current regulator such as an NJU6050. NOTE: This is just an example, other devices are available from other manufacturers and vendors.

When configured correctly this device will take a variety of input voltages, 2.5V to 6.5V, which is in your battery range and deliver the 20mA you need through the string of LEDs.

There is a limit to how many LEDs the thing will drive, dependent on the max forward voltage of each LED, so you will need to double or triple up the circuits to power all 17 LEDs.

Answer 3

Microchip has some very efficient LED drivers made especially for battery powered. This would work great with a 3.6V (3.0–4.2V) Lithium Cobalt Oxide battery. Recommended: Panasonic NCR18650B

$1.50 Solution (0.50 x 3) in single quantities.

About 98.8% Efficiency.

Very small foot print 2mm x 2mm + two 0603 resistors

The MIC2843A is a high efficiency linear White LED (WLED) [NOTE: White LEDs are actually Blue] driver designed to drive up to six WLEDs, greatly extending battery life for portable display backlighting keypad backlighting, and camera flash in mobile devices. The MIC2843A provides the highest possible efficiency as this architecture has no switching losses present in traditional charge pumps or inductive boost circuits. The MIC2843A provides six linear drivers which maintain constant current for up to six WLEDs. It features a typical dropout of 40mV at 20mA. This allows the WLEDs to be driven directly from the battery eliminating switching noise/losses present with the use of boost circuitry.

MIC2843A Datasheet

Other similar drivers: Microchip LED drivers