august 2002 1 application note 40 application note 40 micrel application note 40 leds vs. ccfl final general description leds vs ccfl white leds (light emitting diodes) are becoming more popular choice for backlighting color lcds (liquid crystal displays) in cellular phones, pdas (personal digital assis- tants), digital cameras and other handheld, portable devices. blue leds are emerging as a popular backlight for mono- chrome lcds. the advantages of white and blue leds over other light sources like ccfls (cold cathode fluorescent lamps) are that the power source is simple to implement, the life span is longer, the size is smaller and the cost is lower. ccfl requires an ac power source ranging from 2kvac at 30khz-40khz for operation. this requires dc-ac power source conversion for portable devices, which translates to bulky transformer-based circuits. a single white or blue led requires a 3.6v to 4.2v dc power source. this means simple inductor-based circuits can be used to power the leds. because of the low voltage power, the white and blue leds are ideal for li-ion battery applications. the life span of the ccfl is around 30khrs to 35khrs. for the leds, the life span is over 50khrs. the only advantage to using the ccfl over the white and blue leds is that the ccfl is brighter but much larger and more expensive to implement. micrel, inc. ?1849 fortune drive ?san jose, ca 95131 ?usa ?tel + 1 (408) 944-0800 ?fax + 1 (408) 944-0970 ?http://www.mic rel.com overview this application note describes and shows various applica- tion circuits to power white and blue leds using the mic2142 and mic2145. the mic2142 is a micropower, boost-switch- ing regulator housed in a sot-23-5 package. the input voltage range is between 2.2v to 16v with an adjustable output voltage up to 22v. in addition, the 330khz operation allows small surface mount external components to be used. the mic2145 is an msop-8 boost-switching regulator with programmable peak switch current to allow the user to set the maximum efficiency to occur in the applications. the input voltage range for the mic2145 is 2.4v to 16v with an adjustable output voltage up to 16v. series configuration figure 1 shows one of the most common applications for the white and blue leds, which is to configure them in series. in this particular circuit, three white leds are used. the advantage of this configuration is that the feedback voltage of the mic2142, which is a tightly controlled parameter, and r1 set the led's current. equal amount of current goes through the series string of white leds. because the led? current is proportional to its brightness, the result is uniform bright- ness for all the leds. a zener diode in parallel with the leds is recommended to protect the mic2142 if one of the white leds opens for any reason, causing the switch voltage to increase above the maximum rating of the switch pin. the challenge of the series configuration is that the mic2142 has to boost the output voltage up to the sum of all the forward drop of the white leds plus the feedback voltage. l1 22 h murata lqh32cn220k1 r3 1k r2 1m u1 mic2142 en sw vcc gnd fb 1 2 3 4 5 d1 mbr0530 r1 86.6 ? c3 10 f/16v murata grm42-6x5r106k16 c2 1 f 25v +vin 3.0v to 4.2v li-ion batt. c1 10 f/6.3v taiyo yuden jmk212bj106mg gnd shdn z1 16v white leds figure 1. constant-current led driver
application note 40 micrel application note 40 2 august 2002 80 81 82 83 84 85 0 5 10 15 20 efficiency (%) led current ( ma ) np04s b220m clq4d10-220 a914byw-220m ldr655312t-220w lqh32cn220k1 v in = 3.0v 3 white leds l = 22 h figure 2. typical efficiency vs. led current figure 2 shows the efficiency of the constant-current led driver for various inductor manufacturers. with a constant- current led driver, efficiencies above 80% can be achieved with low profile inductor and capacitors. parallel configuration figure 3 shows the parallel led configuration with the mic2145 as the led driver. similar to the series configura- tion, the feedback voltage of the mic2145 and r3 set the current for one of the white leds. by using the same value resistor for each of the white leds, the rest of the white leds should have similar current. the current for each led will not match perfectly due to the resistor tolerance, so there will be a tiny variation. this variation may cause the led's bright- ness to be uneven as compared to the series configuration. in shutdown mode, due to the nature of the boost architec- ture, the output voltage will equal the input voltage minus the voltage drop of the schottky diode. this will cause the white leds to slightly turn on if the voltage of the battery is high enough. a small switch between the battery and vin can be used to disconnect the white leds from the battery. the advantage of the parallel configuration is that the mic2145 only has to boost the output voltage up to one forward drop plus the feedback voltage. this allows the mic2145 to drive at least 10 white leds in parallel with efficiency above 80% using low profile components. brightness control the brightness of the leds can be easily controlled by injecting a pwm (pulse width modulation) signal into the enable pin of the regulator, using a logic control, or applying a dc voltage from the dac. figure 4 shows a 400hz pwm signal can be used to control the leds current by varying the duty cycle of the pwm signal. figure 5 shows a logic signal and a transistor can be used to switch the leds current from 8ma to 1ma and vice versa. figure 6 shows a dac can be used to modulate the led's current. l1 10 h sumida cr43-100 mic2145 d1 on semiconductor mbr0530t1 rset en vdd sw r3 v out 5v/150ma pgnd fb sgnd r1 100k gnd jp1 +v in 3.0v to 4.2v ss c2 0.01 f c1 10 f/6.3v murata grm40 x5r 106k 6.3 r2 10k r3 c3 10 f/6.3v murata grm40 x5r 106k 6.3 led1 led10 figure 3. parallel configuration l1 22 h murata lqh32cn220k1 u1 mic2142 en sw vcc gnd fb 1 2 3 4 5 d1 mbr0530 r1 86.6 ? c3 10 f/16v murata grm42-6x5r106k16 c2 1 f 25v +vin 3.0v to 4.2v c1 10 f/6.3v taiyo yuden jmk212bj106mg gnd pwm z1 16v blue leds figure 4. series white led driver with pwm brightness control
august 2002 3 application note 40 application note 40 micrel l1 47 h murata lqh32c470k34 r3 1k u1 mic2142 en sw vcc gnd fb 1 2 3 4 5 d1 mbr0530 r1 1.27k c3 10 f/16v murata grm42-6x5r106k16 r2 1m c2 1 f 25v c1 10 f/6.3v taiyo yuden jmk212bj106mg gnd +vin 3.0v to 4.2v shdn z1 16v white leds 174 ? figure 5. dual mode l1 22 h murata lqh32cn220k1 r3 1k r2 1m u1 mic2142 en sw vcc gnd fb 1 2 3 4 5 d1 mbr0530 r1 86.6 ? c3 10 f/16v murata grm42-6x5r106k16 c2 1 f 25v c1 10 f/6.3v taiyo yuden jmk212bj106mg gnd dac +vin 3.0v to 4.2v shdn z1 16v white leds r4 r5 figure 6. series white led driver with analog dimming control summary the demand of white and blue leds will dramatically in- crease over the year due to the their low voltage, small size, simplicity and long life-span. micrel s led drivers will allow designer to easily power these white and blue leds with high efficiency, low profile components, and minimum component counts. micrel, inc. 1849 fortune drive san jose, ca 95131 usa tel + 1 (408) 944-0800 fax + 1 (408) 944-0970 web http://www.micrel.com this information is believed to be accurate and reliable, however no responsibility is assumed by micrel for its use nor for an y infringement of patents or other rights of third parties resulting from its use. no license is granted by implication or otherwise under any patent or pat ent right of micrel, inc. ? 2002 micrel, incorporated
|
