EDNE February 2013 - (Page 43)

designideas readerS SOLVE DESIGN PROBLEMS Linear and switcher LED supplies combine, overcome disadvantages of each topology Fabien Franc, On Semiconductor, Phoenix ↘ To control their brightness, LEDs need a constant current; this can be done with a resistor placed in series with the LED string. Both the LEDstring voltage and the supply voltage can vary, so a dedicated LED driver is a ANODE LED STRING CATHODE VOLTAGE SENSE OUT EN VIN COMPARATOR LOGIC OUT LINEAR LED DRIVER EN STEP-DOWN LED DRIVER R9 2 4.7 μF ANODE LED STRING CAT4201 D1 4.7 μF VBAT 350 mA EN R10 8.25k RSET SW GND CATHODE L 33 μH VCC R5 100k R1 22k VCC VCC R6 10k OUT VCC=5V R3 10k R4 10k R7 10k OUT LM393 Q1 2N7002 − LED CAT4101 VCC EN VCC + R2 100k R8 1600 RSET SW GND LINEAR LED DRIVER VREF=2.5V Q2 2N7002 COMPARATOR LOGIC Figure 1 The LM393 comparator monitors the LED string’s low-side voltage and enables either the buck regulator (CAT4201) or the linear regulator (CAT4101). EDNDI5312 Fig 1.eps www.edn-europe.com 45 Photo meter assesses ambient light 46 Booster circuit enables reliable solenoid operation ▶To see all of EDN’s Design Ideas, VIN 5 TO 30V STEP-DOWN LED DRIVER DIs Inside visit www.edn.com/designideas must to guarantee the current accuracy. Two solutions—each with advantages and disadvantages—are widely used: a linear constant-current LED driver or a step-down switching converter. Linear drivers are simple solutions requiring few components and are essentially noise-free, but they dissipate heat proportionally to the difference between the supply voltage and the LED forward voltage. To protect against overheating, the driver package may require an additional heatspreading area on the PCB, adding to the cost and amount of PCB real estate required and increasing the risk that the driver IC will enter thermal shutdown and turn off the LEDs. If the driver is located next to the LEDs, the additional heat can cause the LEDs to operate at an elevated temperature, shortening their lifetimes. Step-down, or buck, converters are efficient and generate little heat, but switching solutions require an inductor and a Schottky diode. The solutions also create noise, especially when the supply voltage drops and approaches the LED forward voltage. In automotive applications, RFI (radio-frequency interference) is a major concern. EMI/ RFI filters are recommended in front of the switching converters to prevent high-frequency-switching conducted noise from going back into the supply, as it may interfere with other equipment, such as the AM/FM-band radio. DIANE february 2013 | EDN europe 43 http://www.edn.com/designideas http://www.edn-europe.com

Table of Contents for the Digital Edition of EDNE February 2013

RS Components
Agilent Technologies
Maxim Integrated
Signal Integrity
Mesago SMT 2013
Test & Measurement World
Rohde & Schwarz
Mesago PCIM 2013
Mechatronics in Design
Hot technologies: trends to watch in 2013
Hot 100 products of 2012
Add LED intelligence to improve light quality, efficiency, and cost
Design Ideas
Product roundup
Tales from the Cube

EDNE February 2013