UNISONIC TECHNOLOGIES CO., LTD UC3973 LINEAR INTEGRATED CIRCUIT LOW COST POWER-SAVING MODE PWM CONTROLLER FOR FLYBACK CONVERTERS DESCRIPTION The UC3973 is a high performance current mode PWM controller ideally suited for low standby power. Low VDD startup current make the power 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 UC3973 contains protection with automatic recovery including OLP (over load protection), programming OTP (over SOP-8 temperature protection), OVP (over voltage protection), UVLO (VDD over voltage clamp and under voltage lockout).To protect the power MOSFET, Gate-drive output is fixed up to 16V max. The UC3973 contains protection OCP (cycle-by-cycle current limiting). The internal slope compensation improves system stability 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. Audio noise is eliminated due to switch frequency more than 20kHz during operation. The UC3973 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 * Soft Start * Dynamic peak current limiting for constant output power * Built-in synchronized slope compensation * OLP,OVP and VDD 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 UC3973G-S08-R www.unisonic.com.tw Copyright © 2015 Unisonic Technologies Co., Ltd Package SOT-8 Packing Tape Reel 1 of 10 QW-R103-086.B UC3973 LINEAR INTEGRATED CIRCUIT MARKING PIN CONFIGURATION PIN DESCRIPTION PIN NO. 1 PIN NAME GND 2 FB 3 4 5 VIN NC RT 6 SENSE 7 8 VDD GATE PIN TYPE DESCRIPTION P Ground. Feedback input pin. The PWM duty cycle is determined by voltage level I into this pin and SENSE pin input. I Connected through a start up resistor to line voltage for VCC ON No Connection I Connected through a NTC resistor to GND for OTP. Current sense input pin. Connected to MOSFET current sensing resistor I node. P Power supply. O The totem-pole output driver for driving the power MOSFET. UNISONIC TECHNOLOGIES CO., LTD www.unisonic.com.tw 2 of 10 QW-R103-086.B UC3973 LINEAR INTEGRATED CIRCUIT BLOCK DIAGRAM UNISONIC TECHNOLOGIES CO., LTD www.unisonic.com.tw 3 of 10 QW-R103-086.B UC3973 LINEAR INTEGRATED CIRCUIT ABSOLUTE MAXIMUM RATINGS (TA=25°C, VDD =15V, unless otherwise specified) PARAMETER SYMBOL RATINGS UNIT Supply Voltage VDD 30 V Input Voltage to FB Pin VFB -0.3 ~ 7 V Input Voltage to CS Pin VSENSE -0.3 ~ 7 V Junction Temperature TJ +150 C Operating Temperature TOPR -40 ~ +125 °C Storage Temperature TSTG -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 Supply Voltage SYMBOL VDD RATINGS 9~ 24 UNIT V ELECTRICAL CHARACTERISTICS (TA=25°C, VDD=15V，unless otherwise specified) PARAMETER SYMBOL SUPPLY SECTION Start Up Current ISTR IC Operating current IOP VCC Zener Clamp Voltage VCLAMP UNDER-VOLTAGE LOCKOUT SECTION Start Threshold Voltage VTHD(ON) Min. Operating Voltage VDD(MIN) CONTROL SECTION VFB Open Loop Voltage Level VFB-OPEN PWM Input Gain AVCS Burst-Mode Out FB Voltage VFB(OUT) Reduce-Frequency end FB Voltage VFB(END) Burst-Mode Enter FB Voltage VFB(IN) Normal Switch Frequency FSW Power-Saving Duty Cycle DMAX Frequency Hopping FJ(SW) Frequency VDD Stability FDV Frequency Temperature Stability FDT Feedback short current IFB PROTECTION SECTION VCC Over Voltage Protection Threshold VOVP FB PIN Over Load Protection Threshold VOLP Over Load Protection Delay-Time TDelay Soft start time TSS CURRENT LIMITING SECTION Peak Current Flat Threshold Voltage VCS-F Peak Current Valley Threshold Voltage VCS-V Lead Edge Blanking Time TLEB DRIVER OUTPUT SECTION Output Voltage Low State VOL Output Voltage High State VOH Output Voltage Rise Time tR Output Voltage Fall Time tF RT SECTION Output current of RT pin IRT Threshold voltage for OTP VTH_OTP UNISONIC TECHNOLOGIES CO., LTD www.unisonic.com.tw TEST CONDITIONS VDD = VDD(ON)-0.5V VFB=3.5V IVCC=20mA ∆VFB/∆VCS VSENSE =0 VSENSE =0 VSENSE =0 VFB=3.5V Before enter burst mode VFB=3.5V, VSENSE=0 MIN TYP MAX UNIT 20 2.5 33 μA mA 29 2.5 1.2 31 12 7 13.5 8 15 9 4.9 5.1 3 1.6 2.5 1.5 65 22 78 ±4 60 19 70 VDD=12V~20V T=-20~100°C VFB=4.2V 1.5 240 24 100 VFB=4.2V, Duty≥60% VFB=4.2V, Duty=0% VDD=16V,IO=-20mA VDD=16V,IO= 20mA CL=1.0nF CL=1.0nF 0.85 0.55 26 4.5 125 2.5 0.95 0.65 220 70 25 85 5 5 28 150 V V/V V V V KHz KHz % % % % uA V V mS mS V V ns 0.8 V V ns ns 110 1.20 uA V 100 60 105 1.15 V V 1.05 0.75 8 100 1.10 V 4 of 10 QW-R103-086.B UC3973 LINEAR INTEGRATED CIRCUIT OPERATION DESCRIPTION The UC3973 devices integrate many useful designs into one controller for low-power switch-mode power supplies. The following descriptions highlight some of the features of the UC3973 series. Start-up Current The start-up current is only 2.5μA. 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 2.5~3MΩ, 1/8W startup resistor could be used together with a VDD capacitor to provide a fast 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 voltage, which is sampled from the voltage feedback loop, is taken as the reference. Once the feedback voltage dropped below the threshold voltage, the switching frequency starts to decrease. This Power-Saving Mode function dramatically reduces power consumption under light-load conditions. The 22KHz minimum frequency control also eliminates 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 UC3973 enter burst mode at standby condition to minimize the switching loss and reduces the standby power consumption. Power supplies using the UC3973 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 POUT during IC operating. At no load or light load condition, 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 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 fSW and POUT/POUT (MAX) as followed Fig.1. Fig.1 The relation curve between fSW and relative output power POUT/ POUT (MAX) UNISONIC TECHNOLOGIES CO., LTD www.unisonic.com.tw 5 of 10 QW-R103-086.B UC3973 LINEAR INTEGRATED CIRCUIT 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 the 2nd oscillator as followed Fig.2. So the tone energy is evenly spread out, the spread spectrum minimizes the conduction band EMI and therefore eases the system design in meeting stringent EMI requirement. Fig.2 Frequency Hopping Built-in Slope Compensation Built-in slope compensation circuit greatly improves the close loop stability at CCM and prevents the sub-harmonic oscillation. Leading-Edge Blanking Each time the power MOSFET is switched on, a turn-on spike will inevitably occur at the 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 current-limit comparator is disabled and it cannot switch off the gate driver. Constant Output Power Limit When the SENSE voltage, across the sense resistor RS, reaches the threshold voltage, around 0.8V, the output GATE drive will be turned off after a small propagation delay tD. This propagation delay will introduce an additional current proportional to tD×VIN/Lp. Since the propagation delay is nearly constant regardless of the input line voltage VIN. 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 voltage. To compensate this variation for wide AC input range, the threshold voltage is adjusted by the VIN current. Since VIN pin is connected to the rectified input line voltage through a resistor RVIN, a higher line voltage will generate higher VIN current into the VIN pin. The threshold voltage is decreased if the VIN current is increased. Smaller threshold voltage, forces the output GATE drive to terminate 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 UC3973 are fixed internally at VTHD(ON)/VDD(MIN) During start-up, the hold-up capacitor must be charged to VTHD(ON) through the start-up resistor, so that the UC3973 will be enabled. The hold-up capacitor will continue to supply VDD until power can be delivered from the auxiliary winding of the main transformer. VDD must not drop below VDD(MIN) during this start-up process. This UVLO hysteresis window ensures that hold-up capacitor will be adequate to supply VDD during start-up. Gate Output The UC3973 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 achieve with this dedicated control scheme. An internal 15V clamp is added for MOSFET gate protection at higher than expected VDD input. UNISONIC TECHNOLOGIES CO., LTD www.unisonic.com.tw 6 of 10 QW-R103-086.B UC3973 LINEAR INTEGRATED CIRCUIT 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 switching of the power MOSFET whenever VDD >VOVP. The OVP event as followed Fig.3. Fig.3 OVP case Fig.4 OLP case OLP OLP will shut down driver when VFB> VOLP for continual a blanking time. The OLP event as followed Fig.4. OTP OTP will shut down driver when the NTC resistor temperature TJ>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 UC3973, and increasing the power MOS gate resistance is advised. UNISONIC TECHNOLOGIES CO., LTD www.unisonic.com.tw 7 of 10 QW-R103-086.B UC3973 LINEAR INTEGRATED CIRCUIT REFRENCE CIRCUIT (19.5V / 2.05A) BOM Reference BD1 C1 C2 C3 C6 C10 C7,C46 C11 C41, C43 C45 CX1 CY1 C40 D4 D1 D3 D5 F1 LF1, LF2 Q1 Component 2A_600V CC 10NF/1KV ELC 82μF/400V, 105C, ±20% CC 1000pF/1KV CC 1nF/50V ELC 10μF/50V, 105C, ±20% CC 0.1μF/50V CC 68pF/50V ELC 680uF/25V, 105C, ±20% CC 10NF/50V X-CAP 0.33μF/275VAC Y-CAP 1000pF/400V CC 100pF/1KV, SMD1206 1N4148 0.15A/75V 1N4007 1.0A/1000V BAV20WG 1A/200V Schottky 20A/100V Fuse 2.0A / 250V choke 10N65 10A/650V UNISONIC TECHNOLOGIES CO., LTD www.unisonic.com.tw Reference R6, R23 R7, R8 R13 R15 R16 R17 R18 R20, R21 R22 R40 R41 R42 R43 R45 R46 T1 U1 U3 U4 Component R 1.5MΩ R 47kΩ R 4.7Ω R 10Ω R 47Ω R 10kΩ R 1kΩ R 1.5Ω R 2.7Ω R 47Ω R 820Ω R 2.2kΩ R 680Ω R 68kΩ R 10kΩ RM-8 IC UTC UC3973 LTV-357-T-C TL431 8 of 10 QW-R103-086.B UC3973 LINEAR INTEGRATED CIRCUIT TYPICAL CHARACTERISTICS UNISONIC TECHNOLOGIES CO., LTD www.unisonic.com.tw 9 of 10 QW-R103-086.B UC3973 LINEAR INTEGRATED CIRCUIT 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. UNISONIC TECHNOLOGIES CO., LTD www.unisonic.com.tw 10 of 10 QW-R103-086.B