www. belling .com.cn 1 max. 3mhz, variable frequency step - up dc/dc converter b l8545 description the BL8545 variable frequency step - up converter drives white leds with a constant current to provide backlight in cell phones, pdas, and other hand - held devices. it features allowing series connection of the white leds so that the led currents are identical for uniform brightness. an enable input can be pulsed repeatedly to adjust leds brightness. the fast 3mhz operation frequency allows for smaller capacitor and inductor. fault condition protection uses cycle - by cycle current limiting to sense maximum inductor current and over - voltage protection. the 0.2v low reference voltage minimized the power loss across the current sense resistor. the converter can operate from 2v to 6v, and capable of delivering max imum 200ma output current at 4 - leds appl ication with 3v input voltage. quiescent current drawn from power source is as low as 120ua. all of these features make BL8545 be suitable for the portable devices, which are supplied by a single battery. BL8545 is available in sot23 - 6 package that is pb free . features ? up to 24v output voltage ? wide operation range: 2v to 6v ? maximum 3mhz operating frequency ? pwm dimming control ? shutdown current <1ua ? current limit cycle - by - cycle ? low current sense threshold: 200mv ? 24v over output voltage protection ? compact so t23 - 6 package applications ? compact back light module ? power source for led ? constant current source typical application figure1. BL8545 typical application circuit BL8545 l1 10uh led1 led2 led3 led4 led17 led18 led19 led20 r1 r5 d1 lx gnd fb ce ovp vin on off vin ss14 4.7uf 4.7uf cin cout lc3300
www. belling .com.cn 2 b l8545 ordering information BL8545 1 2 3 code description 1 temperature & rohs: c: - 40~85c, pb free rohs std. 2 package type: b6: sot - 23 - 6 3 packing type: tr: tape & reel (standard) marking information product classification BL8545 cb6tr marking lezz le: product code zz: date code pin description block diagram pin no. symbol description 1 lx switching node 2 gnd ground 3 fb pin for feedback voltage 4 ce chip enable pin (active with h ), connect to vin if not used. 5 ovp over voltage protection 6 vin power supply lezz 1 2 3 4 5 6
www. belling .com.cn 3 b l8545 absolute maximum rating parameter value max input voltage - 0.3v to 8v ce pin voltage - 0.3v to (vin+0.3v) lx pin output current 1.6a lx pin voltage 24v operating junction temperature ( t j ) 125 ? c ambient temperature ( t a ) - 40 ? c to 85 ? c power dissipation sot - 23 - 6 250mw storage temperature ( ts) - 40 ? c to 150 ? c lead temperature & time 260 ? c , 10sec electrical characteristics (ta=25 oc vin=3v vce=3v, unless otherwise noted) symbol item conditions min . typ. max. unit v in operating input voltage 2 6 v v fb feedback voltage 0.190 0.200 0.210 v i fb fb pin bias current 10 45 100 na i q quiescent current v fb =0.3v 93 120 ua v ce =0v 0.4 1.0 ua f max maximum switching frequency v fb =0v, floating ovp p in 3 mhz i limit switching current limit 1.6 a v cesat switching saturation voltage i lx =300ma 260 mv i lx switching pin leak current v lx =5v 0.11 ua v ceh ce voltage high v ce =0 to 3v 1.5 v v cel ce voltage low v ce =3 to 0v 0.4 v i ce ce pin bias current vin=vce=3v 18 ua v ovp over voltage protection 24 v typical performance characteristics ce pin bias current vs. en pin voltage (vin=vce) 0 10 20 30 40 50 60 70 80 0 1 2 3 4 5 6 ce p in voltage(v) ce pin bias curren(ua) ce pin current vs. temperature (vin=vce)) 0 10 20 30 40 50 -50 -25 0 25 50 75 100 temperature ce pin current(ua) vce=2.5v vce=3v vce=6v
www. belling .com.cn 4 b l8545 q uiescent current vs. supply voltage (vce=0v) 0.0 0.2 0.4 0.6 0.8 1.0 0 1 2 3 4 5 6 supply voltage(v) quiescent current(ua) quiescent current vs. supply voltage (vce=vin,vfb=0.3v) 0 20 40 60 80 100 120 0 1 2 3 4 5 6 supply current(v) quiescent current(ua) fb pin voltage vs.supply voltage 0.120 0.140 0.160 0.180 0.200 0.220 2 3 4 5 6 supply voltage(v) fb pin voltage(v) fb pin voltage vs. temperature 0.120 0.140 0.160 0.180 0.200 0.220 -50 -25 0 25 50 75 100 temperature(oc) fb pin voltage(v) efficiency vs. output current (4leds) 0 20 40 60 80 100 0 20 40 60 80 100 120 output current(ma) efficiency(%) vin=2.5v vin=3v vin=6v maximum output currenr vs. output voltage 0 100 200 300 400 500 600 4 8 12 16 20 24 output voltage(v) maximum output currenr(ma) vin=2.5v vin=3v vin=6v switch frequency vs. supply voltage (4leds*100ma) 0 1 2 3 4 2 3 4 5 6 supply voltage(v) switch frequency(mhz)
www. belling .com.cn 5 b l8545 application information dimming control 1. using a pwm signal to ce pin when adding the pwm signal to ce pin, the BL8545 is turned on or off by the pwm si gnal, so the leds operate at either zero or full current. the average led current increase proportionally with the duty cycle of the pwm signal. 2. using a dc voltage to fb pin from the figure 2, we can add a dc voltage to fb pin, we adjust the led current b y changing the dc voltage, which control the brightness, dc voltage range is from 0v to 2v. figure 2. dimming control using a dc voltage 3. using a filtered pwm signal to fb pin t he filtered pwm signal can be considered as an adjustable dc voltage. i t can b e used to replace the variable dc voltage source in dimming control. t he circuit is shown in figure 3. figure 3. dimming control using a filtered pwm signal BL8545 BL8545 led1 led2 led3 r12 r13 r1 vdc fb lc3300 90k 5k 10 led1 led2 led3 r12 r13 r1 pwm fb lc3300 90k 5k 10 r14 c12 0.1uf
www. belling .com.cn 6 b l8545 package information
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