unisonic technologies co., ltd UC3875C preliminary linear integrated circuit www.unisonic.com.tw 1 of 9 copyright ? 2015 unisonic technologies co., ltd qw-r103-075.b low cost power-saving mode pwm controller for flyback converters ? description the utc UC3875C is a high performance current mode pwm controller ideally suited for lo w standby power. the pwm switching frequency is programmable externally at normal operation. low v dd startup current make the powe r reliable on startup design and a large value resistor could be used in the startup circuit to minimize the standby power. at no load condition, the ic operates in power-saving mode for lower standby power, decreasing frequency for higher conversion efficiency at light load condition. the utc UC3875C contains protection with automatic recovery including olp (over load protection), otp (over temperature protection), ovp (over voltage protection), uvlo (v dd over voltage clamp and under vo ltage lockout).to protect the power mosfet, gate-drive output is fixed up to 15v max. the utc UC3875C contains protection ocp (cycle-by-cycle current limiting). the internal slope compensation improves system st ability at high pwm duty cycle output. leading-edge blanking on current sense input removes the signal glitch, which offering minimal external component count in the design. excellent emi performance is achieved wit h utc proprietary frequency hopping technique (zl201020615247.1) together with soft driver control. audi o noise is eliminated due to switch frequency more than 20khz during operation. the utc UC3875C has such applications as: battery charger, power adaptor, set-top box power supplies, ink jet printers, open-frame smps. ? features * utc proprietary frequency hopping technology for improved emi performance. * power-saving mode for high light-load and standby efficiency * dynamic peak current limitin g for constant output power * built-in synchronized slope compensation * otp,olp,ovp and v dd clamp for higher security * programmable pwm frequency * gate output voltage clamped at 15v * low start-up current * cycle-by-cycle current limiting * under voltage lockout (uvlo) * few external components required ? ordering information ordering number package packing UC3875Cg-s08-r sop-8 tape reel UC3875Cg-s08-r (1)packing type (2)package type (1) r: tape reel (2) s08: sop-8 (3) g: halogen free and lead free (3)green package
UC3875C preliminary linear integrated circuit unisonic technologies co., ltd 2 of 9 www.unisonic.com.tw qw-r103-075.b ? marking ? pin configuration ? pin description pin no. pin name pin type description 1 ri i a resistor connected between ri and g nd sets switching frequency. a 100k ? resistor r i results in a 65khz switching frequency. 2 comp i feedback input pin. the pwm duty cycle is determined by voltage level into this pin and sense pin input. 3 cs i current sense input pin. connected to mosfet current sensing resistor node. 4 gnd p ground. 5 out o the totem-pole output driver for driving the power mosfet. 6 v cc p power supply. 7 nc - no connection 8 hv i hv start pin, it connected the line input.
UC3875C preliminary linear integrated circuit unisonic technologies co., ltd 3 of 9 www.unisonic.com.tw qw-r103-075.b ? block diagram
UC3875C preliminary linear integrated circuit unisonic technologies co., ltd 4 of 9 www.unisonic.com.tw qw-r103-075.b ? absolute maximum ratings (t a =25c, v dd =15v, unless otherwise specified) parameter symbol ratings unit supply voltage v dd 30 v input voltage to fb pin v comp -0.3~7 v input voltage to cs pin v cs -0.3~7 v junction temperature t j +150 ? c operating temperature t opr -40~+125 c storage temperature t stg -50~+150 ? c note: absolute maximum ratings are those values beyond which the device could be permanently damaged. absolute maximum ratings are stress ratings only and functional device operation is not implied. ? operating range parameter symbol ratings unit supply voltage v dd 10~26 v ? electrical characteristics (t a =25c, v dd =15v, r i =100k ? unless otherwise specified) parameter symbol test conditions min typ max unit hv start supply current from st pin i st 2 2.8 5 ma start pin leakage current after startup i eakage 10 a supply section start up current i str v dd =v dd(on) -0.1v 0.8 20 a ic operating current i op v fb =3.5v 0.6 5 ma vdd zener clamp current i clamp v dd >v ovp 15 ma under-voltage lockout section start threshold voltage v thd(on) 11 12 13 v min. operating voltage v dd(min) 6 7 8 v control section v fb open loop voltage level v fb-open 5.1 v pwm input gain a vcs ? v fb / ? v cs 3 v/v burst-mode out fb voltage v fb(out) v sense =0 1.4 v reduce-frequency end fb voltage v fb(end) v sense =0 2.2 v burst-mode enter fb voltage v fb(in) v sense =0 1.1 v normal v fb =3.5v, ri=100k ? 60 65 70 khz switch frequency power-saving f sw before enter burst mode 22 khz duty cycle d max v fb =3.5v, v sense =0 75 % frequency hopping f j(sw) -4 +4 % frequency vdd stability f dv v dd =12v~20v 5 % frequency temperature stability f dt t=-20~100c 1.5 5 % ri resistor value range r i 50 100 150 k ? feedback resistor r fb 25 k ? protection section vcc over voltage protection threshold v ovp v fb =4.2v 27 v fb pin over load protection threshold v olp 4 4.2 4.4 v over load protection delay-time t delay 75 ms otp threshold t (thr) v fb =4.2v 160 c current limiting section over current flat threshold voltage v cs v fb =4.2v 0.6 0.7 0.8 v lead edge blanking time t leb 350 ns driver output section output voltage low state v ol v dd =16,i o =-20ma 0.8 v output voltage high state v oh v dd =16,i o =20ma 11 v output voltage rise time t r c l =1.0nf 100 ns output voltage fall time t f c l =1.0nf 60 ns
UC3875C preliminary linear integrated circuit unisonic technologies co., ltd 5 of 9 www.unisonic.com.tw qw-r103-075.b ? operation description the utc UC3875C devices integrate many useful designs into one controller for low-power switch-mode power supplies. the following descriptions highlig ht some of the f eatures of the utc UC3875C series. start-up current the start-up current is only 5 a. low start-up current allows a start- up resistor with a high resistance and a low-wattage to supply the start-up power for the controller. for ac/dc adaptor with universal input range design, a 2.5~3m ? , 1/8w startup resistor c ould be used together with a v dd capacitor to provide a fa st startup and low power dissipation solution. power-saving mode operation the proprietary power-saving mode function provides linearly decreasing the switching frequency under light-load conditions for higher efficiency. the feedback volt age, which is sampled from the voltage feedback loop, is taken as the reference. once the feedback voltage dropped below the thresh old voltage, the switching frequency starts to decrease. this power-savin g mode function dramatically reduce s power consumption under light-load conditions. the 22khz minimum frequency control also elim inates the audio noise at any loading conditions. at zero load condition, the magnitude of power loss is in proportion to the number of switching events within a fixed period of time. reducing switching events leads to the reduction on the power loss and thus conserves the energy. the utc UC3875C enter burst mode at standby condition to mi nimize the switching loss and reduces the standby power consumption. power supplies using the utc UC3875C can easily meet even t he strictest regulations regarding standby power consumption. switch frequency set the maximum switch frequency is set through the 100k ? ri-pin resistor to 65khz. switch frequency is modulated by output power p out during ic operating. at no load or light load condition, mo st of the power dissipation in a switching mode power supply is from switching loss on the mosfet transistor, the core loss of the transformer and the loss on the snubber circuit. the magnitude of power loss is in proportion to the number of switching events within a fixed period of time. so lower switch frequency at lower load, which more and more improve ic?s efficiency at light load. at from no load to light load condition, the ic will operate at from burst mode to reducing frequency mode. the relation curve between f sw and p out /p out (max) as followed fig.1. the maximum switch frequency is set through the ri-pin resistor ri: f sw =6500/ri (k ? ) khz. fig.1 the relation curve between f sw and relative output power p out / p out (max)
UC3875C preliminary linear integrated circuit unisonic technologies co., ltd 6 of 9 www.unisonic.com.tw qw-r103-075.b ? operation description (cont.) frequency hopping for emi improvement the frequency hopping is implemented in the ic; there are two oscillators built-in the ic. the first oscillator is to set the normal switching frequency; the switching frequency is modulated with a period signal generated by the 2nd oscillator. the relation between the first oscillator and th e 2nd oscillator as followed fig.2. so the tone energy is evenly spread out, the spread spectrum minimizes the conduction band emi and therefor e eases the system design in meeting stringent emi requirement. fig.2 frequency hopping built-in slope compensation built-in slope compensation circuit greatly improv es the close loop stabilit y at ccm and prevents the sub-harmonic oscillation. leading-edge blanking each time the power mosfet is switched on, a turn-on sp ike will inevitably occur at t he sense-resistor. to avoid premature termination of the switching pulse, a 400ns leading-edge blanking time is built in. conventional rc filtering can therefore be omitted. during this blanking period, the curr ent-limit comparator is disabled and it cannot switch off the gate driver. constant output power limit when the sense voltage, acro ss the sense resistor r s , reaches the threshold volt age, around 0.7v, the output gate drive will be turned off after a small propagation delay t d . this propagation delay will introduce an additional current proportional to t d v in /lp. since the propagation delay is nearly co nstant regardless of the input line voltage v in . higher input line voltage will result in a larger additional current and hence the output power limit is also higher than that under low input line vo ltage. to compensate this variation for wi de ac input range, the threshold voltage is adjusted by the v in current. since v in pin is connected to the rectified i nput line voltage through a resistor r vin , a higher line voltage will generate higher v in current into the v in pin. the threshold volt age is decreased if the v in current is increased. smaller threshold voltage, forces the output gate drive to termi nate earlier, thus reduce the total pwm turn-on time and make the output power equal to that of low line input. this proprietary internal compensation ensures a constant output power limit for wide ac input voltage from 90vac to 264vac. under voltage lockout (uvlo) the turn-on and turn-off thresholds of the utc UC3875C are fixed internally at 13v/6v. during start-up, the hold-up capacitor must be charged to 15.8v thro ugh the start-up resist or, so that the utc UC3875C will be enabled. the hold-up capacitor will continue to supply v dd until power can be delivered from the auxiliary winding of the main transformer. v dd must not drop below 10v during th is start-up process. this uvlo hysteresis window ensures that hold-up capacitor will be adequate to supply v dd during start-up. gate output the utc UC3875C output stage is a fast totem pole gate driver. cross conduction has been avoided to minimize heat dissipation, increase efficiency, and enhance reliability. a good tradeoff is ac hieved through dead time control. the low idle loss and good emi system design is easier to ac hieve with this dedicated control scheme. an internal 15v clamp is added for mosfet gate protection at higher than expected v dd input.
UC3875C preliminary linear integrated circuit unisonic technologies co., ltd 7 of 9 www.unisonic.com.tw qw-r103-075.b ? operation description (cont.) protection controls the ic takes on more protection functions such as ovp, olp and otp etc. in case of those failure modes for continual blanking time, the driver is shut down . driver is reset after failure is eliminated. ovp the ovp will shut down the switchin g of the power mosfet whenever v dd >v ovp . the ovp event as followed fig.3. fig.3 ovp case fig.4 olp case olp olp will shut down driver when v fb > v olp for continual a blanking time. the olp event as followed fig.4. otp otp will shut down driver when junction temperature t j >t (thr) . pcb layout note noise from the current sense or the control signal can cause significant pulse width jitter in continuous-conduction mode, and slope compensation helps alleviate these problems. good placement and layout practices should be followed. avoiding long pcb traces and component leads, locating compensation and filter components near the utc UC3875C , and increasing the power mos gate resistance is advised.
UC3875C preliminary linear integrated circuit unisonic technologies co., ltd 8 of 9 www.unisonic.com.tw qw-r103-075.b ? refrence circuit (12v/1.5a) bom reference component reference component bd1 1n40074 q1 4n65k cx1 (optional) nc r4 (optional) 0 ? cy1 (optional) yc 1nf/400v (y1) r1,r2 r 4.7m ? 1206 c2 ec 33 f/400v 105c r3 r 200k c1 ec 6.8 f/400v 105c r5 r 10 ? c3 cc 1nf/1000v r6 r 22 ? 1206 c4 ec 10 f/50v r7 r 0.66 ? 1w c5,c13 (optional) cc 100nf/50v r8 r 10k ? 0603 c8 cc 1nf/25v r9 r 100k ? 0805 c9 nc r10 nc c10, c11 ec 470uf/25v r12 r 220 ? 0805 c12 cc 0.1 f/50v 0805 r13 r 1k ? 0805 c14 100pf/25v r14 r 680 ? 0805 d2 diode 1n4007 r15 r 39k ? 0805 d3 rs1d r16 r 10k ? 0805 d4 (optional) 1n4148 r17 200k ? 1206 d5 mbr10100 r18 1k ? 0603 f1 2a/250v t1 ee-219 l1 nc (short) u1 ic utc UC3875C l2 33 h 68mm u2 tl431 zd1 nc u3 pc817
UC3875C preliminary linear integrated circuit unisonic technologies co., ltd 9 of 9 www.unisonic.com.tw qw-r103-075.b utc assumes no responsibility for equipment failures that result from using products at values that exceed, even momentarily, rated values (such as maximum ratings, operating condition ranges, or other parameters) listed in products specifications of any and all utc products described or contained herein. utc products are not designed for use in life support appliances, devices or systems where malfunction of these products can be reasonably expected to result in personal injury. reproduction in whole or in part is prohibited without the prior written consent of the copyright owner. the information presented in this document does not form part of any quotation or contract, is believed to be accurate and reliable and may be changed without notice.
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