unisonic technologies co., ltd uc3873 linear integrated circuit www.unisonic.com.tw 1 of 10 copyright ? 2013 unisonic technologies co., ltd qw-r103-068. d low cost power-saving mode pwm controller for flyback converters ? description the uc3873 is a high performance current mode pwm controller ideally suited for low standby power. low v dd startup current make the power reliable on startup design and a large value resistor could be used in t he startup circuit to minimize the standby power. at no load c ondition, the ic operates in power-saving mode for lower standby power, decreasing frequency for higher conversion efficiency at light load condition. the uc3873 contains protection with automatic recovery including olp (over load protection), programming otp (over temperature protection), ovp (ove r voltage protection), uvlo (v dd over voltage clamp and under voltage lockout).to protect the power mosfet, gate-d rive output is fixed up to 16v max. the uc3873 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 with 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 uc3873 has such applications as: battery charger, powe r 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 * soft start * dynamic peak current limitin g for constant output power * built-in synchronized slope compensation * olp,ovp and v dd clamp for higher security * programming otp for higher security * fixed switch frequency 65khz * 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 lead free halogen free uc3873l-ag6-r UC3873G-AG6-R sot-26 tape reel
uc3873 linear integrated circuit unisonic technologies co., ltd 2 of 10 www.unisonic.com.tw qw-r103-068. d ? marking information ? pin configuration ? pin description pin no. pin name pin type description 1 gnd p ground. 2 fb i feedback input pin. the pwm duty cycle is determined by voltage level into this pin and sense pin input. 3 rt i connected through a ntc resistor to gnd for otp. 4 sense i current sense input pin. connected to mosfet current sensing resistor node. 5 v dd p power supply. 6 gate o the totem-pole output driver for driving the power mosfet.
uc3873 linear integrated circuit unisonic technologies co., ltd 3 of 10 www.unisonic.com.tw qw-r103-068. d ? block diagram
uc3873 linear integrated circuit unisonic technologies co., ltd 4 of 10 www.unisonic.com.tw qw-r103-068. d ? 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 fb -0.3 ~ 7 v input voltage to cs pin v sense -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 oper ation is not implied. ? operating range parameter symbol ratings unit supply voltage v dd 10 ~ 24 v ? electrical characteristics (t a =25c, v dd =15v unless otherwise specified) parameter symbol test conditions min typ max unit supply section start up current i str v dd = v dd ( on ) -0.1v 2.5 20 a ic operating current i op v fb =3.5v 1.5 2.5 ma v dd zener clamp current i clamp v dd >v ovp 15 ma under-voltage lockout section start threshold voltage v thd ( on ) 12.5 13.8 14.5 v min. operating voltage v dd ( min ) 7.5 8.2 9.5 v hysteresis v dd ( hy ) 5.6 v control section v fb open loop voltage level v fb-open 5 v pwm input gain a vcs ? v fb / ? v cs 3 v/v burst-mode out fb voltage v fb ( out ) v sense =0 1.6 v reduce-frequency end fb voltage v fb ( end ) v sense =0 2.4 v burst-mode enter fb voltage v fb ( in ) v sense =0 1.5 v switch frequency normal f sw v fb =3.5v 60 65 70 khz power-saving before enter burst mode 22 khz duty cycle d max v fb =3.5v, v sense =0 76 % 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 % feedback resistor r fb 25 k ? protection section v cc over voltage protection threshold v ovp v fb =4.2v 28 v fb pin over load protection threshold v olp 4.4 v over load protection delay-time t dela y 55 ms soft start time t ss 3 ms current limiting section peak current flat threshold voltage v cs-f v fb =4.2v, duty 60% 0.92 v peak current valley threshold voltage v cs-v v fb =4.2v, duty=0% 0.80 v lead edge blanking time t leb 300 ns driver output section output voltage low state v ol v dd =15v,i o =-20ma 0.8 v output voltage high state v oh v dd =12v,i o = 20ma 8 v output voltage rise time t r c l =1.0nf 500 ns output voltage fall time t f c l =1.0nf 160 ns rt section output current of rt pin i rt 95 100 105 ua threshold voltage for otp v th _ otp 0.95 1 1.05 v
uc3873 linear integrated circuit unisonic technologies co., ltd 5 of 10 www.unisonic.com.tw qw-r103-068. d ? operation description the uc3873 devices integrate many useful designs into one controller for low-power switch-mode power supplies. the following descriptions high light some of the features of the uc3873 series. start-up current the start-up current is only 2.5 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 uc3873 enter burst mode at standby condition to mi nimize the switching loss and reduces the standby power consumption. power supplies using the uc3873 can easily meet even the strictest regulations regarding standby power consumption. switch frequency set the maximum switch frequency is fixed to 65khz. switch frequency is modulated by output power p out during ic operating. at no load or light load cond ition, most 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 sw itching 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 m ode to reducing frequency mode. the relation curve between f sw and p out /p out (max) as followed fig.1. fig.1 the relation curve between f sw and relative output power p out / p out (max)
uc3873 linear integrated circuit unisonic technologies co., ltd 6 of 10 www.unisonic.com.tw qw-r103-068. d ? 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 0.8v, 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 uc3873 are fixed internally at v thd(on) /v dd(min) during start-up, the hold-up capacitor must be charged to v thd(on) through the start-up re sistor, so that the uc3873 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 v dd(min) during this 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 uc3873 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 achieved through dead time control. the low idle loss and good emi system design is easier to achiev e with this dedicated control scheme. an internal 15v clamp is added for mosfet gate prot ection at higher than expected v dd input.
uc3873 linear integrated circuit unisonic technologies co., ltd 7 of 10 www.unisonic.com.tw qw-r103-068. d ? 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 the ntc resistor 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 uc3873 , and increasing the power mos gate resistance is advised.
uc3873 linear integrated circuit unisonic technologies co., ltd 8 of 10 www.unisonic.com.tw qw-r103-068. d ? refrence circuit 12w (5v/2.4a) bom reference component reference component bd1 bd 1a/600v l2 1mh 68mm c2 ec 22 f/400v 105c q1 4n60_to-220f c1 ec 4.7 f/400v 105c r1,r2 r 1.5m ? 1206 c3 cc 0.01 f/1000v r3 r 200k ? 1206 c4 ec 10 f/50v r4 r 0 ? 0805 c5,c13 cc 104p/25v 0805 r5 r 10 ? 0805 c8 cc 102p/25v 0805 r6 r 43 ? 1206 c9 cc 222p/100v 1206 r7, r7a r 2 ? 1206 c10, c11 ec 470u/16v 105c r8 r 51k ? 1206 c12 cc 222p/16v 0805 r9 ntc r 12k ? 0805 d2 diode fr107 r10 r 22 ? 1206 d3 diode fr102 r12 r 820 ? 0805 d4 diode 1n4148_sod-123 r13 r 2.2k ? 0805 d5 diode sb2040_to-220f r14 r 1k ? 0805 f1 fuse 2a/250vac r15, r16 r 22k ? 0805 1% l1 10 h 6mm r17 r 330k ? 0805 1% r18 r 220 ? 1206 t1 ee-22 u1 ic uc3873 u2 tl431 u3 pc817b
uc3873 linear integrated circuit unisonic technologies co., ltd 9 of 10 www.unisonic.com.tw qw-r103-068. d ? typical characteristics v thd(on) (v) v dd(min) (v) i str (a) f sw (khz) f sw-power-saving (khz) d max (%)
uc3873 linear integrated circuit unisonic technologies co., ltd 10 of 10 www.unisonic.com.tw qw-r103-068. d ? typical characteristics (cont.) 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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