Path: EDN Asia >> Design Ideas >> Power/Smart Energy >> WLED driver operates down to 1.2V supply voltage
Power/Smart Energy Share print

WLED driver operates down to 1.2V supply voltage

13 May 2016  | Dave Wuchinich

Share this page with your friends

Many LED drivers, using both charge pumps and inductors, are available to boost the 1.2 to 2.4V available from single- and dual-cell NiMH (nickel-metal-hydride) batteries to the 3.6V that white LEDs require. However, most of these circuits, such as the Maxim MAX1595, require a minimum input voltage of approximately 2.5V to operate properly. The MAX1595 works with an input voltage of 2.4V but does not ensure an adequate output until the input voltage reaches approximately 3V. Furthermore, as the battery voltage decreases to the threshold level, the output becomes erratic. The circuit in the figure uses a flip-flop to generate flux in an inductor, which then charges a capacitor in the common boost configuration. US Patent 4,068,149 describes the flip-flop's operation in an application for operating an incandescent safety lamp's flasher (Reference 1).


Figure: In this circuit, transistors Q1 and Q2 form a flip-flop that toggles at 60kHz, providing a drive current for the output LED down to the 1V battery voltage.


In figure, R1 provides a path for starting current through the base-emitter junctions of Q1 and Q2. Q2 thus turns on and, in so doing, turns on Q1, rapidly forcing both transistors into saturation. However, C1 charges through R2 to the battery voltage minus the base-emitter drop of Q1 and the saturated collector-emitter voltage of Q2, eventually causing Q1 to turn off and thereby also turning off Q2. C1 then discharges through R1 and R2 and the forward-biased base-collector junction of Q2. The R2C1 time constant determines the turn-on time, and (R1+R2)(C2) determines the turn-off time. C2 acts as the capacitive input filter for the current flowing from L1 when Q2 is off and provides a substantially constant voltage to power D2, a standard white LED. The output voltage is proportional to the battery voltage.

With the component values in figure and with L1, a Coilcraft MSS7341-104MLB, the operating frequency is approximately 60kHz. With a battery voltage of 2.36V from two NiMH cells, approximately 20 mA of current flows through the LED. In tests simultaneously driving two LEDs, each with its own current-limiting resistor, R3, the energy-conversion efficiency of the circuit at this battery voltage is approximately 80%. Operation continues with battery voltages of slightly more than 1V, and the delivered current diminishes but still provides usable illumination.
Reference
Wuchinich, David G, "Flasher circuit with low power drain," US Patent 4,068,149, Oct 28, 1975.


About the author
Dave Wuchinich contributed this article.


This article is a Design Idea selected for re-publication by the editors. It was first published on September 3, 2007 in EDN.com.




Want to more of this to be delivered to you for FREE?

Subscribe to EDN Asia alerts and receive the latest design ideas and product news in your inbox.

Got to make sure you're not a robot. Please enter the code displayed on the right.

Time to activate your subscription - it's easy!

We have sent an activate request to your registerd e-email. Simply click on the link to activate your subscription.

We're doing this to protect your privacy and ensure you successfully receive your e-mail alerts.


Add New Comment
Visitor (To avoid code verification, simply login or register with us. It is fast and free!)
*Verify code:
Tech Impact

Regional Roundup
Control this smart glass with the blink of an eye
K-Glass 2 detects users' eye movements to point the cursor to recognise computer icons or objects in the Internet, and uses winks for commands. The researchers call this interface the "i-Mouse."

GlobalFoundries extends grants to Singapore students
ARM, Tencent Games team up to improve mobile gaming


News | Products | Design Features | Regional Roundup | Tech Impact