Due to their light output and long life, high-brightness LEDs are well-suited for use in flashlights. Typically, the LEDs are driven with a constant current. So when the battery voltage drops, the flashlight just stops working. In some situations, this could be dangerous. It would be nice to have a flashlight that would automatically dim when the battery voltage drops. Just like a traditional flashlight, it would indicate a low-battery condition and would last longer.

Typically, the forward voltage of a white LED is 3 to 5 V. Therefore, if the input voltage drops below 3 V, a boost converter can drive the LED. Figure 1 shows such a boost converter with automatic dimming based on battery voltage.

The L6920D is a high efficiency step-up controller requiring very few external components to convert the battery voltage to the selected output voltage or current. The device will start up at 1 V and operate down to 0.6 V. Its quiescent current is only 10 µA. The device also includes an internal synchronous rectifier implemented with a 120-m? P-channel MOSFET, which replaces the conventional boost diode, to improve the efficiency.

In this circuit, the LED current (I_LED) is a function of the battery voltage (V_FB). When the battery voltage drops, the LED current drops. In this way, the battery can last longer, and the flashlight will offer dimming characteristics. Assuming V_FB = 1.23 V, determined by the L6920D:

If R5 >> R4, then R5||R4 ˜ R4, then:

Finally:

Figure 2 shows the resulting LED current calculated using the resistance values in the example circuit.

We compared a constant LED flashlight and this auto-dimming version, using two AAA batteries. Running times were 50 and 90 minutes, respectively. The auto-dimming function nearly doubled the battery life and provided a warning when the battery voltage dropped.

Reprints

Printer-Friendly

Email this Article

RSS

Submit PlanetEE del.icio.us Digg Reddit Newsvine StumbleUpon Netscape Yahoo MyWeb Live Bookmarks Fark  Submit

Font Size

What's This?

Edge-Pad Conditioners Advance 32-nm Fabrication LDO Driver Targets High-Power LED Lighting Programmable Controller Manages Midsize Machines MIL-SPEC SMPS Cap Family Adds 500V Members White LED Flash Drivers Offer Alternative To Xenon Lamps Spacers/Nuts Install Like SMT Components

Cadence Comes At Power From Two Perspectives Collaboration Results In First IEEE 1149.7 cJTAG Semiconductor IP Core Yield Enhancement Software To Aid Solar Cell Fabs Audio Engine Codec Library Expands With Dolby Pro Logic Additions Accellera Rolls New Version of Analog, Mixed-Signal Standard 45-nm Via-Programmable ASICs Add High-Speed I/O Transceivers To Feature Mix Verification Evolves Into Lean, Mean Bug-Stomping Machines User Advisory Group To Guide Open Verification Methodology’s Evolution

1) Build A Smart Battery Charger Using A Single-Transistor Circuit (304 views today) 2) Use Inexpensive Components To Create An Infrared Close-Object Alert Circuit (205 views today) 3) Easily Convert Decimal Numbers To Their Binary And BCD Formats (157 views today) 4) Narrow-Shift Digital Discriminator Detects Differential GPS Corrections (155 views today) 5) Embedded: FPGA Toolkit Supports Hard And Soft Processor Designs (140 views today) ALL TOP 20

Reader Comments Interesting circuit. Thanks for sharing. I'm also designing an LED flashlight using the NCP3065. It has an external current limit resistor that can be used to dim the LED when the input voltage starts to fall so you get this feature for FREE. Anonymous -August 06, 2008 Could this circuit be adapted for a single NiMH cell? If so, how? Anonymous -August 06, 2008
POST YOUR COMMENTS HERE Name: Email:
Your Comments: Enter the text from the image below Please refresh the page if you have trouble reading this text.