201303.pdf 36567KB Mar 22 2013 09:11:22 PM

More documents

Recommendations

Info

Regulating cuRRent The LED-manufacturing process causes wide variations in brightness and color temperature from one LED to the next. As a guide to users, white-LED manufacturers allocate each manufactured unit to groups, or “bins,” of LEDs with comparable performance in terms of color, brightness, and forward voltage. But the manufacturer’s specification for each brightness and color-temperature bin is valid only under specific nominal operating conditions. This means that the LED current must be set to the nominal current stated in the data sheet in order to generate the specified brightness and color. Consequently, dimming and bright- DC/DC DC/DC DC/DC V DC-DC V LED1 V DC-DC V LED2 V DC-DC V LED3 I LED SINK V SINK1 I LED SINK V SINK2 I LED SINK V SINK3 Figure 3 A separate dc/dc converter with each LED string is an expensive option. DC/DC V DC-DC V LED1 V DC-DC V LED2 V DC-DC V LED3 EDN1303DF2 Fig 3.eps DIANE I LED SINK V SINK1 ILED SINK V SINK2 ILED SINK V SINK3 Figure 4 With one dc/dc converter serving multiple LED strings, SMPS voltage is not optimized. DC/DC DC/DC V DC-DC V LED1 V DC-DC V LED2 V DC-DC V LED3 V DC-DC V LED4 EDN1303DF2 Fig 4.eps DIANE I LED SINK V SINK1 ILED SINK V SINK2 I LED SINK V SINK3 ILED SINK V SINK4 Figure 5 A mixed architecture optimizes the balance between BOM cost and power efficiency. 34 EDN | MARCH 2013 [ www.edn.com ]
ness control can be implemented only by switching the current sent to any single LED to either on (nominal current) or off (zero current) through a digital PWM control signal. In analog dimming, the LED would be operating outside its specified nominal current, leading to unacceptable changes in color temperature and poor LED-to- LED brightness matching (Figure 6). Current-sink properties Since LEDs require a perfectly regulated constant-current power supply, it follows that the primary role of the LED driver is to set the current to the nominal value when on and to 0A when off. Therefore, the feedback loop controlling the accuracy of regulation requires an extremely precise current sink (Figure 7). While there are a variety of currentsink designs, the precision requirements of TV backlighting (current regulation better than ±0.5%) require an accurate op amp to set the I LED current independent of the I LED voltage. In backlighting driver applications, however, the task is more challenging because the accuracy of current regulation must be maintained even when the voltage at the current sink falls to very low levels. This requirement is difficult to meet, but four generations of accurate currentsink LED drivers from AMS have been designed specifically for such applications. The AS369x, AS381x, AS382x, and AS385x devices also incorporate an 250 offset-compensated op amp. Currentsink drivers require a minimum voltage at the drain (V DS(SAT) ) to ensure the full accuracy and proper operation of the sink transistor inside the saturation region. For the saturated region, the gate-source voltage primarily controls the output current. If the current sink is to operate at high efficiency, it is important that the voltage drop between V SET and V DS is low. LED drivers with op amps that 10:05 AM Your first board is ready to test. 9:00 AM Your circuit design is done and you’re ready to make a prototype. 11:48 AM Why not try a different approach before you head to lunch? Like This? For more articles like this one, go to www.edn.com/ design/LED. include built-in offset cancellation can maintain V SET at levels as low as 125 to 250 mV. Allowing an additional margin for V DS above V DS(SAT) of 150 mV, a total voltage drop at the cur- 1:03 PM Your second board is ready to test. 5:00 PM Nice work. You just shaved weeks off your development schedule. 3:14 PM After a few tweaks, you’re ready to make your finished board. 4:09 PM Your finished board is ready to go. 200 150 THEORETICAL LED LED 1 INTENSITY (MCD) 100 50 0 0 10 20 30 40 CURRENT (mA) LED 2 Figure 6 The brightness of LEDs from the same bin is guaranteed to match only at nominal current (in this case, 20 mA). ProtoMat ® Benchtop PCB Prototyping Machine What would your day look like tomorrow if you could cut yourself free from the board house and produce true, industrial quality PCBs right at your desk? LPKF’s ProtoMat benchtop prototyping systems are helping thousands of engineers around the world take their development time from days and weeks to minutes and hours. In today’s race to market, it’s like having a time machine. www.lpkfusa.com/pcb 1-800-345-LPKF “You can’t beat an LPKF system for prototyping. We do up to three iterations of a design within a day.” Leonard Weber Agilent [ www.edn.com ]
Page 1: a vs. Si vs. GaN vs. SiC vs. Si vs.
Page 4 and 5: The Trusted Leader for Any Measurem
Page 6 and 7: Promoting the real benefits of auth
Page 8 and 9: • CHiL ® Digital Control PowIRst
Page 10 and 11: Register today to attend the larges
Page 12 and 13: edn.comment By Patrick Mannion, Bra
Page 14 and 15: pulse INNOVatIONS & INNOVatORS 10 9
Page 16 and 17: pulse Dash-cam video: benefit or di
Page 18 and 19: pulse Time-of-flight proximity sens
Page 20: pulse VOICES Peter Spitzer: Motivat
Page 23 and 24: Meet Your Dream Team Four great rea
Page 25 and 26: Introducing ... SR1 Audio Analyzer
Page 27 and 28: 2500-mAh Li-ion battery. The boost-
Page 29: T-1 LED Spacer mounts ® THiNK Keys
Page 32 and 33: Highly touted organic light-emittin
Page 36 and 37: ent sink of approximately 400 mV is
Page 38 and 39: a vs. Si vs. GaN vs. SiC vs. Si vs.
Page 40 and 41: EfficiEnt PowEr convErsion Efficien
Page 42 and 43: negligible charge storage permits t
Page 44 and 45: Cree Cree is in a unique position i
Page 46 and 47: Figure 7 In this chart showing semi
Page 48 and 49: By JacoB Beningo • Beningo engine
Page 50 and 51: version of code with a description
Page 52 and 53: V IN = 48V V OUT (V) 25 20 15 10 5
Page 54 and 55: Designing a power backup system is
Page 56 and 57: designideas ANALOG SIGNALS DIGITAL
Page 58 and 59: designideas VOLTAGE (V) ISO 1 ISO 2
Page 60 and 61: designideas 350 300 250 200 150 100
Page 62 and 63: designideas CLASSICS Originally pub
Page 64 and 65: supplychain Linking design and reso
Page 66 and 67: Make an IMPACT A whole new way of d
Page 68 and 69: DDS sweep generators Model 4007B an
Page 70 and 71: Tales from The Cube KenneTh Ciszews
Page 72: Charge BIG Batteries Fast I CHARGE

201303.pdf 36567KB Mar 22 2013 09:11:22 PM

Create successful ePaper yourself

Delete template?

Save as template?