www. belling .com .cn 1 max. 3mhz, variable frequency step - up dc/dc converter BL8548 description the BL8548 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 unif orm 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 curre nt 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 maximum 2 5 0ma output current at 4 - leds application with 3v input voltage. quiescent current drawn from power source is as low as 120ua. all of these features make BL8548 be suitable for the portable devices, which are supplied by a single battery. BL8548 is available in sot23 - 6 package that is pb free. features ? up to 1 9 v 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 ? 1 9 v over output voltage protection ? compact sot23 - 6 package applications ? compact back light module ? power source for led ? constant current source typical application figure1. BL8548 typical application circuit bl8 548
www. belling .com .cn 2 BL8548 ordering information BL8548 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 BL8548 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 BL8548 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. 8 a lx pin voltage 1 9.4 v operating junction temperature ( tj) 125 ? c ambient temperature ( ta) - 40 ? c to 85 ? c power dissipation sot - 23 - 6 40 0mw storage temperature ( ts) - 40 ? c to 150 ? c lead temperature & time 260 ? c, 10sec electrical characteristics (ta=25oc vin=3v vce=3v, unless otherwise noted) symbol item conditions min. typ. max. unit vin operating input voltage 2 6 v vfb feedback voltage 0.190 0.200 0.210 v ifb fb pin bias current 10 45 100 na iq quiescent current vfb=0.3v 93 120 ua vce=0v 0.4 1.0 ua fmax maximum switching frequency vfb=0v, floating ovp pin 3 mhz ilimit switching current limit 1.8 a vcesat switching saturation voltage ilx=300ma 260 mv ilx switching pin leak current vlx=5v 0.11 ua vceh ce voltage high vce=0 to 3v 1.5 v vcel ce voltage low vce=3 to 0v 0.4 v ice ce pin bias current vin=vce=3v 18 ua vovp over voltage protection 1 9 v typical performance characteristics 0 10 20 30 40 50 60 70 80 0 1 2 3 4 5 6 ce pin bias curren(ua) ce p in voltage(v) ce pin bias current vs. en pin voltage (vin=vce) 0 10 20 30 40 50 - 50 - 25 0 25 50 75 100 ce pin current(ua) temperature ce pin current vs. temperature (vin=vce)) vce=2.5v vce=3v vce=6v
www. belling .com .cn 4 BL8548 0.0 0.2 0.4 0.6 0.8 1.0 0 1 2 3 4 5 6 quiescent current(ua) supply voltage(v) q uiescent current vs. supply voltage (vce=0v) 0 20 40 60 80 100 120 0 1 2 3 4 5 6 quiescent current(ua) supply voltage(v) quiescent current vs. supply voltage (vce=vin,vfb=0.3v) 0.12 0.14 0.16 0.18 0.20 0.22 2 3 4 5 6 fb pin voltage(v) supply voltage(v) fb pin voltage vs.supply voltage 0.12 0.14 0.16 0.18 0.20 0.22 - 50 - 25 0 25 50 75 100 fb pin voltage(v) temperature(oc) fb pin voltage vs. temperature 0 20 40 60 80 100 0 20 40 60 80 100 120 efficiency(%) output current(ma) efficiency vs. output current (4leds) vin=2.5v vin=3v vin=6v 0 100 200 300 400 500 600 4 8 12 16 20 24 maximum output currenr(ma) output voltage(v) maximum output currenr vs. output voltage vin=2.5v vin=3v vin=6v 0 1 2 3 4 2 3 4 5 6 switch frequency(mhz) supply voltage(v) switch frequency vs. supply voltage (4leds*100ma)
www. belling .com .cn 5 BL8548 application information dimming control 1. using a pwm signal to ce pin when adding the pwm signal to ce pin, the BL8548 is turned on or off by the pwm signal, 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. usin g a dc voltage to fb pin from the figure 2, we can add a dc voltage to fb pin, we adjust the led current by 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 be 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 4. pcb layout layout is critical to achieve clean and stable operation. the switch ing power stage requires particular attention. follow these guidelines for good pc board layout: 1) place decoupling capacitors as close to the ic as possible 2) connect input and output capacitors to the same power ground node with a star ground configuration t hen to ic ground. 3) keep the high - current paths as short and wide as possible. 4) if possible, connect v in, lx , and gnd separately to a large copper area to help cool the ic to further improve efficiency and long - term reliability. 5) ensure all feedback connectio ns are short and direct. place the feedback resistors as close to the ic as possible. 6) route high - speed switching nodes away from sensitive analog areas figure 4. recommended component placement led1 led2 led3 r12 r13 r1 vdc fb lc33 20 90k 5k 10 bl8 548 led1 led2 led3 r12 r13 r1 pwm fb lc33 20 90k 5k 10 r14 c12 0.1uf bl8 548
www. belling .com .cn 6 BL8548 package information package sot - 23 - 6 devices per reel 3000pcs unit mm package dimension
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