Driving Constant Current Through 2 LEDs from 2 Batteries

Question

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:

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

_{(source: linear.com)}

Answer 1

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.

Answer 2

If you do this with a linear regulator, you have to accept the fact that as soon as the battery voltage drops below 2.75 V, you won't be able to "create" the forward voltage for the LED(s) anymore, and thus, your current will fall, inevitably.

Hence, based on non-switch mode supplies, this approach is doomed; see the typical discharge curve for an alkaline battery below:

Notice ho fast it falls below 2.75 V /2 = 1.375 V; you'd have to add another margin for the voltage drop of the linear regulator. Best LDOs I know do about ~90 mV at 50 mA, so that'd be 1.415 V as threshold voltage.

Answer 3

So let's walk through the design process:

Input voltage range

Something between 2·1.55 V = 3.10 V and what the discharge curve below tells us about what happens after drawing 50 mA for 4 h. That'd be 0.2 Ah, and we need to scale that from a 2.20 Ah battery down to the typical AAAA capacity of 0.5 Ah, so we need to look at the 0.2 Ah · 4.4 = 0.88 Ah

Discharge curve says about 1.2 V. That sounds realistic, so our overall input range, including the fact that anything has a non-100% efficiency, would be at least 2.3 V to 3.1 V.

That highlights all LDO/linear based approaches are doomed, because they can't step-up Voltage.

Design choice

As shown, we need a switch-mode power supply. Assuming we don't first want to burn energy to always work below the target voltage of 2.75V, a switch-mode supply that is able to boost and buck is necessary.

To achieve the 40mm² space restriction, we must look into highly integrated circuitry – definitely ICs that include the switch, not only the controller, but if possible even the inductor, or use switched capacitance (because our current isn't that large).

Component Choice

We visit the websites of the "usual suspects", being

• TI
• NXP
• Maxim
• Linear
• ST
• ONSemi

Buck/Boost converters

Let's assume TI is doing pretty well with their simple-switcher modules that integrate the inductor. Let's see if what we can find comes close to our space restrictions – if it doesn't, we can drop that approach (we probably won't do better than TI, will we?).

Also, let's look at ON, which are known for selling high-volume, highly integrated SMPS ICs – typically in packages that are chip-scale, and they also expert in things like mobile phone flash LED controllers.

Switched capacitance

Let's check TI's portfolio for that.

Results

1. the smallest simple-switcher module is 99mm²: http://www.ti.com/lsds/ti/power-management/buck-boost-negative-output-module-products.page Ouch. More than twice as large as allowable,
2. On has a category for LED drivers: http://www.onsemi.com/PowerSolutions/taxonomy.do?id=16200&lctn=header . The CAT3224 seems to fit our bill very well, and has a 3 mm x 3 mm = 9mm² package – leaving plenty of room for the external components necessary for the switched capacitance design. You'd need something like (very rough guess) 25mF as supercap – but I think that should work. Needs more reading of datasheet. They'll need a lot of space, though – I'd really experiment with high-capacity MLCC and see whether that works if I don't need the "flash" mode.
3. In the category of switched capacitance, an ONsemi device seems to be the most interesting for this application: the NCP1729 is an inverting driver that would fulfill your needs and is dead simple to use:
   
   but I'm not 100% certain from skimming the datasheet it works with 50mA with as little input voltage as 2.4 V. Further charge pumps should be considered.

Answer 4

Plenty of options. Simplest in mind are single chip led drivers, which often need just a diode and a small resistor-shaped inductor. The chips do come in smd packages like SOT-23, or SIP packages for low board space.

Or a current limiting diode, a one piece solution. But head voltage of your batteries may be an issue.

Answer 5

Battery selection depends with choices of chemistry and size

Design criteria for using LEDs with a battery, is based on;

• voltage, Vmin:Vmax, Vf and If for LED(s), capacity in (milli)amp-hour [mAh] or [VAh=Wh], cost, size, and/or efficiency.

Modern designed flashlights use tiny SMPS to efficiently regulate the current and reduce conductive losses using 18650 type cells to get long life and high intensity light. Often beginners want to avoid these for the sake of simplicity but should be aware that this also sacrifices performance.

However, let's analyze a low dropout (LDO) method of driving LED's from a battery to obtain the best compromise.

design attributes

1. minimize the dropout on the current sense to 50mV ( typical std. current shunt)
2. choose a "logic level" MOSFET with RdsOn near or less than ESR of battery

3. use a comparator or Op Amp that allows sensing near 0V input

    - Vout must >= Vgs spec using single supply from Vbat for low RdsOn
   

4. For 1C rates initial Voltage, Vi to final, Vf
5. Capacity C or 1C=Ah ( for h=20hour) e. the battery internal (effective series) resistance \$ESR=|\frac{\Delta V}{\Delta I}|\$

6. % Vbat unregulated range \$ \frac{\Delta V}{V_i} = \frac{V_i-V_f}{V_i}*100{\%} \$

   • % Vbat, Battery voltage drop from 100 to 0% state of charge (SoC)
     • 8% on lead acid secondary
     • 10% on Lithium 3.0V primary
     • 19% on LiPo 3.7V to 3.0V secondary
     • 33% on Alkaline 1.5V primary

Below is a sim. for 3 Alkaline cells. A better design uses 1 LiPo cell.

my Suggested java solution needs approval as below

• Note trace above for current shows excellent stability with 4.5 to 3.5V (not 3V) for 3x AAA batteries
  • Sensitivity \$ \frac{I_f}{V_{bat}} = \frac{49.0-48.96 [mA]}{4.5-3.5 [V]}*100 = 4\% \$

excuse: Falstad did not have a low RdsOn FET , so I used two.

Broad references in LED Driver design

• http://www.1000bulbs.com/pdf/understanding-led-drivers.pdf

• consumer high power low cost solutions 10A LED driver $13 cdn

• Input Voltage 8.5~48V

• Output Voltage 10~50V
• Max. Output Current 10A adjustable
• more consumer solutions
  • http://www.dx.com/s/led+drivers
  • https://www.amazon.com/s/ref=nb_sb_noss_1?url=search-alias%3Daps&field-keywords=led+drivers
  • http://www.ebay.com/sch/i.html?_from=R40&_sacat=0&_nkw=led+drivers&_sop=15

It may be pointless to reinvent the wheel, but it is useful to understand how it works.