EDNE November 2012 - (Page 67)

designideas readerS SOLVe deSIGN PrOBLeMS Three-channel white-LED driver uses simple step-down dc/dc converter Nora Jacalan Esteves, Technor Semiconductor LLC, Pleasanton, CA DIs Inside 68 Sense automobile high-side 69 Dramatically increase the ↘ current with discrete components frequency range of RC-based voltage-controlled oscillators 71 Crystal-oscillator circuit With only a few additional components, you can use a highly efficient step-down dc/dc converter to create a constant-current, multiplechannel LED driver. It is relatively easy to drive a single-channel LED; however, it becomes more complicated when driving multiple-channel paralleled LEDs. In normal dc/dc-converter applications, the control circuitry uses a power stage comprising an inductor and a capacitor, along with feedback through a resistor-divider network, to generate a regulated constant voltage and thus a constant current through the resistor VIN 5V RF 86.6k R7 1M 1 RF 2 3 EN 4 SGND 5 PVIN C1 22 F 6 SW 7 SW 8 PGND divider. You can use an LED in place of the upper resistor of the feedbackdivider network to allow the LED to be driven with a constant regulated current. The current flowing through the LED will be equal to the reference voltage of the dc/dc converter divided by the resistance of the grounded bottom resistor. Although this method works well with one LED channel, it cannot be used to drive multiple parallel LED channels, because the mismatch in LED voltage drops would cause one LED channel to consume most of the current. As a is ultralow power ▶To see all of EDN’s Design Ideas, visit www.edn.com/designideas result, only one LED channel would be lighted. You can use the circuit in Figure 1 to drive multiple paralleled LED channels VFB 16 R3 887 COMP 15 PGOOD 14 TN1000 SVIN 13 PVIN 12 SW 11 SW 10 PGND 9 C2 22 F RPG 10k C4 2.2 nF C5 1 nF PGOOD L1 12 H C3 22 F D1 R4 15 Q1 Q3 2N3906 2N3904 R1 47 D2 D3 R2 15k Q2 2N3904 R5 47 R6 47 Figure 1 Current mirrors equalize the LED currents in each branch of this multiple-LED driver. www.edn-europe.com NOVEMBER 2012 | EDN EuropE 67 http://www.edn.com/designideas http://www.edn-europe.com

Table of Contents for the Digital Edition of EDNE November 2012

Digi-Key
Cover
Agilent Technologies
Contents
International Rectifier
Microchip
Digi-Key
Masthead
EDN comment
Microchip
Pulse
Analog Devices
Maxim Integrated
Maxim Integrated
Texas Instruments
Silicon Laboratories
Test & Measurement World
FTDI
Digi-Key
Strategies for extending product reliability
Managing the 8- to 32-bit processor migration
PCIM
Pickering Interfaces
Mechatronics in Design
Baker's Best
Digi-Key
Emerson
30 years of DSP
Hirose
Vicor
The nuances of variable-frequency drives
Digi-Key
National Instruments
Volt’s battery-stack manager aids Chevy’s drive
Digi-Key
EDNE
Design Ideas
RS Components
RS Components
RS Components
Product Roundup
Tales from the Cube
RS Components

EDNE November 2012