Electronic – Driving Constant Current Through 2 LEDs from 2 Batteries

battery-operatedconstant-currentledled-driver

I'm working on a design where space is limited. I need to drive 2 LEDs from 2 AAAA batteries and using a switching regulator requires too many components and won't fit.

EDIT: The PCB space I'm dealing with is 0.25" x 0.25" (6.35mm x 6.35mm) with an exposed pad in the center which is about 0.1" (2.54mm).

I could use a simple circuit where I just use current limiting resistors for each LED and connect it to the batteries. This is good for the space claim but I want to try to keep a steady brightness with a constant current. There are linear constant current ICs available but I'm having trouble finding one that fits my requirements. Here are the specs I'm working with:

  • LED Forward Voltage: 2.75V
  • Desired LED current (for each LED): 50mA
  • Power Source: 2 AAAA Batteries in series (3V nominal voltage)
  • EDIT: I want power the LEDs for at least 4 hours.

NXP's PSSI2021SAY is the closest part I've found but I don't think the battery voltage will be able to turn this thing on, http://www.nxp.com/documents/data_sheet/PSSI2021SAY.pdf:

PSSI2021SAY in action

EDIT: I also found this constant current circuit. I'm not sure if I'll need the negative supply though, http://www.linear.com/solutions/1562

precision const current source
(source: linear.com)

Best Answer

Think about a Joule Thief style boost converter running off a single AA battery.

I gather you spec AAAA batteries because you have limited space yet believe battery voltage must exceed LED voltage. That's not true, says the Joule Thief.

Customers hate oddball batteries. We like normal battery sizes like AA, which have much more capacity while being cheaper than AAA or AAAA. AAAA is rare and nobody will buy a product that uses them.

Customers also hate devices that don't work with NiCD or NiMH rechargeable batteries (which output 1.2 volts per cell).

More than one battery also means more than four contact surfaces to get dirty and corrode. You know those horrible LED flashlights that are everywhere? They take 3 AAA's in a "cartridge" that's about the size (but not quite) of an 18650. 4 surfaces per battery, 4 for the cartridge, 2 for the bottom cap, and 2 for the switch. No wonder they never work! Into the trash it goes.

It all says a boost converter off a single battery is the way to go.

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