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Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | SubscribeqMqM | Next PageqqM qMMQmagsTHE WORLD’S NEWSSTAND ®design forum | DIMMING PORTABLE DEVICESAnalog technique simplifies LEDdimming in portable applicationsProducts ranging from mobile handsets to LED fl ashlights require SSL dimming control, explainsCHRISTOPHER JAMES GLASER, and analog implementations can offer better effi ciency than PWMdimming in such products.Portable equipment that uses LEDbasedsolid-state lighting (SSL)requires an efficient drive circuitthat prolongs battery life, as well as dimmingoptions to adjust the light output forthe ambient lighting conditions. LED dimmingis necessary in applications such asthe backlight for smart phones or portableGPS navigation systems to ensure an easilyreadable display in both bright sunlightand the dark of night. In flashlights, a usermay deem longer battery life more importantthan delivering the most light possible.Either analog dimming or pulse-widthmodulation(PWM) dimming can be usedin such applications. By using an innovativeapproach to creating a reference voltage, ananalog design can prove more efficient thanPWM-based designs.Both analog and PWM dimming techniquescontrol the LED drive current, whichis proportional to the light output. Analogdimming is simple, requires the least controloverhead, and generally is more efficientthan PWM dimming due to the lowerforward voltage of the LEDs at lower drivecurrents.Analog dimming, however, requires ananalog voltage to be generated by a separatevoltage reference – perhaps using theoutput of an RC filter on a square waveinput signal, or from an expensive digitalto-analogconverter (DAC). The circuit inFig. 1 eliminates the complexity of thesetechniques to provide simple, cost-effectiveanalog dimming based on varying aCHRISTOPHER JAMES GLASER is anapplications engineer in TI’s battery powergroup (www.ti.com).VIN4V to 17V 11PVIN12PVIN4.7μF10AVIN13ENR1100k9SS/TRDEF8TPS62150RGTFSW PGND PGND7 16 15potentiometer. The total solution is an efficient,low-cost, low-component-count LEDdriver for a single high-current LED, suchas Osram’s Golden Dragon, for use in small,battery-powered devices.Circuit operationThe circuit requires a voltage-regulating,synchronous, buck converter that providesoutput currents of up to 1A from up to a17V source, such as the TPS62150. In Fig.1, this buck converter regulates the currentin the LED by using the feedback (FB)pin to control the voltage across the senseresistor R2. The FB voltage is controlled bya combination of a precision internal referencevoltage, which typically is 0.8V, andan external input on the SS/TR (slow startand tracking) pin.When the voltage on the SS/TR pin isbelow 1.25V, the FB pin voltage tracks theSS/TR pin voltage by a factor of 0.64 asexpressed by this equation –V FB = 0.64 *SWSWSWV OS12314FB5PG4AGND6ETPac172.2 μH LED current:1A maxFIG. 1. LED driver with analog dimming enabled by the potentiometer R1.R20.1522μFV SS/TR . By controlling the FB voltage andtherefore the voltage across R2, the IC variesthe current that drives the LED.The SS/TR pin has a built-in currentsource of typically 2.5 μA. This is commonlyused to charge a capacitor and createa smooth, linear ramp-up of the SS/TRpin voltage. In a typical buck converter, thisthen creates a linear and well-controlledramp-up of the output voltage while reducinginrush current from the input supply. Forthis design, a resistor to ground produces aconstant voltage at the SS/TR pin instead.A potentiometer is placed on the SS/TRpin to keep the voltage at that pin between250 mV (potentiometer = 100 kΩ) and 0V(potentiometer = 0Ω). Recalling the equationabove, that means the voltage on theFB pin varies between 160 mV and 0V. Witha 0.15Ω resistor for R2, the LED current variesbetween 1.07A and 0A. Since the FB pinvoltage is linearly related to the SS/TR pinvoltage, the potentiometer provides linear78 NOVEMBER/DECEMBER 2011 LEDsmagazine.comPrevious Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | SubscribeqMqM qMM MQmags| Next Page q qTHE WORLD’S NEWSSTAND ®
Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | SubscribeqMqM | Next PageqqM qMMQmagsTHE WORLD’S NEWSSTAND ®design forum | DIMMING PORTABLE DEVICESanalog dimming as shown in Fig. 2.This circuit has very good efficiency,since the FB pin voltage is a relatively lowvalue. That low voltage reduces the lossesin the sense resistor, R2. In addition, theTPS62150 employs a power-save mode atlight load currents to keep the efficiencyhigh for the majority of the load range.Fig. 3 shows the efficiency of the circuitin Fig. 1 operating from a 12V input, andusing TDK’s VLF3012ST-2R2 inductor in theswitching output.The efficiency of this circuit can beimproved, but at the cost of circuit size.For example you could connect the FSW(switching frequency) control pin to theoutput voltage, which reduces the operatingfrequency, and/or select a differentinductor with lower DCR (DC resistance)and/or better AC loss characteristics. Efficienciesin excess of 90 percent are possible,although these two methods wouldlikely take more circuit board area to realize.The design presented in Fig. 1 deliversa small solution size with good, albeit notthe greatest, efficiency.Circuit limitationsSince this circuit uses an imprecise analoginput (a manually adjustable potentiometer)for regulating the LED current, the tolerancesof the sense resistor, potentiometerI out (A)1.21.00.80.60.40.20.00.000.05Dimming linearity0.10 0.15V SS/TR (V)resistance, and SS/TR pin current and howthey affect the LED brightness do not matter.If the LED is too bright, the user simplyturns the potentiometer resistance down.If it is too dim, simply turn the potentiometerresistance up. With a multi-turnpotentiometer, theLED brightness canbe sufficiently controlledfor ordinaryapplications, suchas flashlights andbacklights.One drawback ofthis design is the offsetbetween the SS/TR pin and FB pinvoltages. When theSS/TR pin is pulledall the way down to0V, by decreasing0.20FIG. 2. Dimming linearity of the circuit in Fig. 1, in whichdimming is achieved using a potentiometer.0.25Efficiency (%)10095908580750.00.1the potentiometer resistance, there is stillabout 50 mA of current flowing throughthe LED. Thus, the LED cannot be turnedoff completely unless you add a switch toground with a pull-up resistor connectedto the EN (enable) pin.Other analog dimming methodsThe advantage of using the potentiometercircuit described in this article is its simplicityand cost-effectiveness. The requiredanalog voltage for analog dimming is generatedby a precise current source alreadyin the IC which is then translated to a correspondinglight output by a user-adjustableresistor. Besides this potentiometer,no other components are required. Theinput for the dimming,the potentiometer,is the onlyneeded component.If this precise currentsource were notpresent, other methodsof generating therequired analog voltagefor analog dimmingwould have tobe considered. Sometraditional optionsinclude: using a separatereference voltageIC that createsa precise analog voltage; varying the dutycycle of the PWM output of a microcontrollerthrough an RC filter to generate theprecise analog voltage; or using a microcontrollerwith a DAC to generate the preciseanalog voltage.0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9LED current (A)FIG. 3. Effi ciency of the circuit in Fig. 1 over the dimming range.1.0Each of these three options requires aninput from the user to vary the light. Withthe reference voltage IC, a potentiometer isstill required as an input to the IC to adjustthe voltage and control light output. Thereference IC alternative is more costly thanthe simple option focused on in this article.The final two alternatives require amicrocontroller and again add cost. Whilesmart phones and GPS systems include amicrocontroller, the typical flashlight doesnot. The decision on pursuing such methodsdepends on the application at hand assome products would benefit from a moreelegant user interface, perhaps with touchscreencontrol.Option three replaces the potentiometerwith a larger and more expensive DAC. TheDAC could offer better granularity in theoutput analog voltage, and therefore morepreciselight control than does the potentiometer.The application should dictatewhether the expense is justified.Using a potentiometer on the SS/TR pinof the buck converter is a simple, small, andlow-cost method to provide linear analogdimming to a high-current LED in applicationssuch as backlights and flashlights.With analog dimming, the efficiency overmost of the dimming range remains near 85percent from a 12V input supply. The completecircuit requires only six components,plus the high-power LED.MORE: Visit the Drivers & Control section ofthe LEDs Magazine website for news, featuresand product information - www.ledsmagazine.com/Drivers.LEDsmagazine.com NOVEMBER/DECEMBER 2011 79Previous Page | Contents | Zoom in | Zoom out | Front Cover | Search Issue | SubscribeqMqM qMM MQmags| Next Page q qTHE WORLD’S NEWSSTAND ®
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