1 tentative lc41059 current regulated white led su pply with brightness control ///// general description ///// the lc41059 is a fractional charge pump dc-dc converter with a regulator that provides four regulated current sources. it accepts an input voltage range fr om 2.7v to 5.5v and maintains a constant current determined by an external sense resistor. the lc41059 delivers up to 105ma of load current to dr ive one, two, three, or four white leds. when it drives four leds, maximum 26ma of each led is ava ilable. when it drives three leds, maximum 35ma of each led is available. current of each leds are highly matched re gardless of the input voltage range from 2.7v to 5.5v, or the led forward voltage range from 3v to 4v. it operates with 500khz fixed-frequency switching without induc tors, therefore the emi noise is ve ry limited, the pictures or tones generated in the device will not disturbed. led brightness can be controlled by both analog voltage input and pulse input. a voltage between 0v to 1.25v may be applied to the brgt pin to vary the led current linearly. alternatively, a pulse signal can be applied to the sd pin to vary the perceived led brightness. an active-low input to the sd pin will shutdown all the circuit, and the led will turned off. the lc41059 is available on wl-csp, qfn or other packages. ///// features ///// . low noise, high efficiency cmos charge pump built-in current mirror with excellent accuracy built-in constant current circuit 2.7v to 5.5v input voltage drives one, two, three or four white leds with maximum total current 105ma self-changing of charge pump mode (pass mode, 3/2 boost mode, 2 boost mode) +-0.5% current matching of any two led outputs soft start limits inrush current charge pump frequency 500khz brightness control with analog voltage input or pulse voltage input maximum 1ua shutdown current ///// applications ///// white led display backlights white led keypad backlights 1-cell li-ion battery-operated equipment including pdas, hand-held pcs, cellular phones flat panel displays ///// package ///// wl-csp, qfn or other packages are availabl e . . . . . . . . . . . . . .
2 ///// block diagrams ///// ///// pin assignment ///// sd: a low level input will disable the device, and turn off leds. a high level input (=v in ) will enable the device, and turn on leds. pulse input on the sd pin makes leds flickering in synchronization with the pulse, so the brightness can be controlled by the duty cycle. brgt: analog brightness control input. it accepts an input voltage range from 0v to 1.25v. rset resistance and brgt input voltage determine the led current. rset: connect a resistor. rset resistance and br gt input voltage determine the led current. vref: 1.2v of internally generated vref is avail able. vref can be used for a brgt input voltage. (see ?operation? chapter for more details) pin name function vin battery voltage in c1a flying capacitor (1) + c1b flying capacitor (1) - c2a flying capacitor (2) + c2b flying capacitor (2) - sd shutdown signal input pwm input brgt analog brightness control rset external resistor gnd ground vref vref output led_in1 white led input (1) led_in2 white led input (2) led_in3 white led input (3) led_in4 white led input (4) vout charge pump output tentative
3 ///// electrical characteristics ///// (unless otherwise specified, vin=3.6v, iout=80ma, ta=27c) parameter symbol condit ions min typ max unit input voltage vin 2.7 3.6 5.5 v total output current io ut vin = 2.7 to 5.5v 60~80 (20ma x4leds) (20ma x3leds) 105 (26ma x4leds) (35ma x3leds) ma charge pump frequency fclk 500k hz vcph pass mode to 3/2 boost mode 4.6 v input charge pump mode threshold vcpl 3/2 boost to 2 boost mode 3.5 v input charge pump mode hysterysis vcpy 0.1 v vin = 2.7 to 5.5v vf = 3.0 to 4.0v total output current = 80ma +-3 % vin = 3.0 to 5.5v vf = 3.0 to 4.0v total output current = 105ma +-3 % led current accuracy (led current variation caused by vin degradation or vf variation) vin = 2.7 to 3.0v vf = 3.0 to 4.0v total output current = 105ma +-5 % id-match vin = 2.7 to 5.5v vf = 3.0 to 4.0v total output current = 80ma +-0.5 % vin = 3.0 to 5.5v vf = 3.0 to 4.0v total output current = 105ma +0.5 % led current matching between any two outputs vin = 2.7 to 3.0v vf = 3.0 to 4.0v total output current = 105ma +-3 % shutdown supply current isd vin = 2.7 to 5.5v sd = 0v 1 ua tsdl shutdown response vin = 2.7 to 5.5v sd = low input 0.1 usec led current response against the sd signal input tsdh start-up response vin = 2.7 to 5.5v sd = high input 4.0 usec recommended sd input frequency (pwm) fpwm pwm input to sd pin to control brightness 100 2000 hz brgt input brgt analog input to brgt pin to control brightness 0 1.0 1.25 v tentative
4 ///// operation ///// 1. circuit description the lc41059 is a white led supply which accepts the input voltage from the li-ion battery. while the li-ion battery has the output voltage range from 2.7 to 5.5v, the forward voltage (v f ) of the white led is about 3.6v typically. so when the battery degrades the output voltage to about 3.6v, the higher voltage must be generated internally. the lc41059 employs a charge pump to step up the output voltage to 1.5 times or 2 times the input voltage. this charge pu mp selects the optimum boost mode, and changes the boost mode automatically. led current (i led ) is determined by the brgt input voltage and t he resistance of the external resistor. same voltage of brgt (v brgt ) is forced on the external resistor with high accuracy by an internal op-amp, so the current passing through the external resistor (i rset ) will be i rset = v brgt / r set . lc41059 includes the current regulator to deliver the regulated current to the leds, which are composed of current mirrors with a 40 to 1 ratio. for example, when v brgt = 1.2v, r set = 2000ohms, then i rset =0.6ma, and i led = i rset x 40= 24ma . if four leds are attached, the device drive up a to tal of 24ma x 4= 96ma through the leds. the led brightness can be controlled by both analog and/or digital methods. the digital technique uses a pwm (pulse width modulation) sign al applied to the sd pin (see 4.brightness control using pwm ). the analog technique applies an analog voltage to the brgt pin (see 5.brgt pin ). 2. soft start lc41059 includes a soft start function to reduce the in rush current. inrush current will be generated when the charge pump operates with flying capacitors di scharged. when the capacit ors are being charged with low impedance, excess current may surge into the capac itors from the battery. soft start is done to reduce stress on the battery and external components. during so ft start, the switch resistances limit the inrush current used to charge the fl ying and hold capacitors. 3. shutdown mode a shutdown pin is available to disable the lc41059 and reduce the quiescent current to 1a maximum. during normal operation mode, applying a high level input to sd pin will enable the device. pulling a low level input to sd pin will disable the device. 4. brightness control using pwm brightness control can be implemented by pulsing a sign al at the sd pin. the recommended frequency is between 100hz to 2khz. if the pwm frequency is much less than 100hz, flicker may be seen in the leds. likewise, if frequency is much higher, brightness in t he leds will not be linear. increasing and decreasing the duty cycle of the pwm signal cont rol the brightness. zero duty cycl e will turn off the led, and a 50% duty cycle waveform produces an average current of 10ma if r set is set to produce a maximum led current of 20ma. so the led current varies linearly with the duty cycle. 5. brgt pin the brgt pin can be used to smoothly vary the brig htness of the white leds. in the lc41059, this voltage is fed to the op-amp that controls the current through the mirror resistor r set . the nominal range on brgt is 0v to 1.25v. care must be taken to pr event voltages on brgt that cause led current to exceed a total of 100 ma. although this will not c ause damage to the ic, it will not meet the guaranteed specifications listed in the ?electri cal characteristics? chapter. tentative
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