UC3845A

UC3845A High Performance Current Mode Controller The UC3845A series of high performance fixed frequency current mode controllers are specifically designed for off–line and dc–to–dc converter applications offering the designer a cost effective solution with minimal external components. This integrated circuit features an oscillator, a temperature compensated reference, high gain error amplifier, current sensing comparator, and a high current totem pole output ideally suited for driving a power MOSFET. Also included are protective features consisting of input and reference undervoltage lockouts each with hysteresis, cycle–by–cycle current limiting, a latch for single pulse metering, and a flip-flop which blanks the output off every other oscillator cycle, allowing output deadtimes to be programmed for 50% to 70%. This device is available in an 8–pin dual–in– line plastic package as well as the 14–pin plastic surface mount (SOP–14). The SOP–14 package has separate power and ground pins for the totem pole output stage. The UC3845A is designedfor lower voltage applications having UVLO thresholds of 8.5V (on) and 7.6V (off). l l l l l l l l Output Deadtime Adjustable from 50% to 70% Current Mode Operation to 500 kHz Automatic Feed Forward Compensation Latching PWM for Cycle–By–Cycle Current Limiting Internally Trimmed Reference with Undervoltage Lockout High Current Totem Pole Output Undervoltage Lockout with Hysteresis Low Startup and Operating Current CD SUFFIX PLASTIC PACKAGE 8 DIP D8 SUFFIX PLASTIC PACKAGE 8 SOP CS SUFFIX PLASTIC PACKAGE SOP-14 SIMPLIFIED BLOCK DIAGRAM PIN CONNECTIONS Pin numbers adjacent to terminals are for the D suffix 8-DIP package. Pin numbers in parenthesis are for the S suffix SOP–14 package. NOTES: 1. Maximum Package power dissipation limits must be observed. 2. Adjust VCC above the Startup threshold before setting to 15 V. 3. Low duty cycle pulse techniques are used during test to maintain junction temperature as close to ambient as possible Tlow = 0°C, Thigh = +70°C. 4. This parameter is measured at the latch trip point with VFB = 0V. 5. Comparator gain is defined as: AV ∆V Output Compensation ∆V Current Sense Input 4-30 UC3845A High Performance Current Mode Controller ABSOLUTE MAXIMUM RATINGS Item Total Power Supply and Zener Current Output Current, Source or Sink (Note 1) Output Energy (Capacitive Load per Cycle) Current Sense and Voltage Feedback Inputs Error Amp Output Sink Current Power Dissipation and Thermal Characteristics CS, D8 Suffix, SOP-14, SOP-8 Package Maximum Power Dissipation Thermal Resistance, Junction to Air CD Suffix, 8-DIP Package Maximum Power Dissipation Thermal Resistance, Junction to Air Operating Ambient Temperature Range Operating Junction Temperature Storage Temperature Range Symbol (ICC +IZ) IO W Vin IO Rating 30 1.0 5.0 -0.3 to +5.5 10 Unit mA A µJ V mA PD RθJA PD RθJA TA TJ TS 862 145 1.25 100 0 to 70 150 -65 to 150 o mW C/W W C/W o o o o C C C ELECTRICAL CHARACTERISTICS VCC = 15V (Note 2), RT = 10k, CT = 3.3nF, TA = 0 to 70oC (Note 3) unless otherwise noted. REFERENCE SECTION Item Symbol VREF Reference Output Voltage (IO = 1.0mA, TJ = 25oC) Line Regulation (VCC = 12V to 25V) Regline Load Regulation (IO = 1.0mA to 20mA) Regload Temperature Stability TS Total Output Variation over Line, Load, Temp. VREF Output Noise Voltage (f = 10Hz to 10kHz, TJ = 25oC) Vn Long Term Stability (TA = 125oC for 1000 Hours) S Output Short Circuit Current ISC OSCILLATOR SECTION Frequency fOSC TJ = 25oC TA = 0 to 70oC Frequency Change with Voltage (VCC = 12V to 25V) ∆fOSC/∆V Frequency Change with Temperature ∆fOSC/∆T Oscillator Voltage Swing (Peak-to-Peak) VOSC Discharge Current (VOSC = 2.0V) Idischg TJ = 25oC Min Typ 4.9 5.0 --2.0 --3.0 --0.2 4.82 ----50 --5.0 -30 -85 Max 5.1 20 25 --5.18 -----180 Unit V m V m V mV/oC V µV m V mA V 47 46 --------52 --0.2 5.0 1.6 10.8 57 60 1.0 ------- % % V mA 4-31 UC3845A High Performance Current Mode Controller ELECTRICAL CHARACTERISTICS ERROR AMPLIFIER SECTION Item Symbol Voltage Feedback Input (VO = 2.5V) VFB IIB Input Bias Current (VFB = 2.7V) Open Loop Voltage Gain (VO = 2.0V to 4.0V) AVOL Unity Gain Bandwidth (TJ = 25oC) BW Power Supply Rejection Ratio (VCC = 12V to 25V) PSRR Output Current Sink (VO = 1.1V, VFB = 2.7V) ISink Source (VO = 5.0V, VFB = 2.3V) ISource Output Voltage Swing High State (RL = 15k to GND, VFB = 2.3V) VOH Low State (RL = 15k to VREF, VFB = 2.3V) VOL CURRENT SENSE SECTION Current Sense Input Voltage Gain (Notes 4 & 5) AV Maximum Current Sense Input Threshold (Note 4) VTH Power Supply Rejection Ratio (VCC = 12V to 25V) PSRR Input Bias Current IIB Propagation Delay (Current Sense Input to Output) tPLH(in/out) OUTPUT SECTION Output Voltage Low State (ISink = 20mA) VOL (ISink = 200mA) High State (ISink = 20mA) VOH (ISink = 200mA) Output Voltage with UVLO Activated VOL(UVLO) (VCC = 6.0V, ISink = 1.0mA) Output Voltage Rise Time (CL = 1.0nF, TJ = 25oC) tr Output Voltage Fall Time (CL = 1.0nF, TJ = 25oC) tf UNDERVOLTAGE LOCKOUT SECTION Startup Threshold Vth Minimum Operating Voltage After Turn-On VCC(min) PWM SECTION Duty Cycle Max. DCmax Min. DCmin TOTAL DEVICE Power Supply Current (VCC = 6.5V) (Note 2) ICC Startup Operating Power Supply Zener Voltage VZ Min Typ 2.42 2.5 ---0.1 65 90 0.7 1.0 60 70 2.0 -0.5 5.0 --2.85 0.9 ------12 -1.0 6.2 0.8 3.0 1.0 70 -2.0 150 Max 2.58 -2.0 ----------V --1.1 3.15 1.1 ---10 300 V/V V dB µA ns V ----13 12 ------7.8 7.0 47 --0.1 1.6 13.5 13.4 0.1 50 50 8.4 7.6 48 --0.4 2.2 ----V 1.1 150 150 9.0 8.2 50 0 ns ns V V % Unit V µA dB MHz dB mA mA ----30 0.17 12 36 0.3 17 --- V 4-32 UC3845A High Performance Current Mode Controller FIGURE 1 - TIMING RESISTOR versus OSCILLATOR FREQUENCY PERCENT OUTPUT DEADTIME OSCILLATOR FREQUENCY (Hz) FIGURE 3 - ERROR AMP SMALL SIGNAL TRANSIENT RESPONSE FIGURE 2 - OUTPUT DEADTIME versus OSCILLATOR FREQUENCY TIMING RESISTOR (kΩ) OSCILLATOR FREQUENCY (Hz) FIGURE 4 - ERROR AMP LARGE SIGNAL TRANSIENT RESPONSE FIGURE 5 - ERROR AMP OPEN-LOOP GAIN AND PHASE versus FREQUENCY OPEN-LOOP VOLTAGE GAIN (dB) EXCESS PHASE (DEGREES) FREQUENCY (Hz) FIGURE 6 - CURRENT SENSE INPUT THRESHOLD versus ERROR AMP OUTPUT VOLTAGE CURRENT SENSE INPUT THRESHOLD (V) ERROR AMP OUTPUT VOLTAGE (V) 4-33 UC3845A High Performance Current Mode Controller REFERENCE VOLTAGE CHANGE (mV) FIGURE 7 - REFERENCE VOLTAGE CHANGE versus SOURCE CURRENT FIGURE 8 - REFERENCE SHORT CIRCUIT CURRENT versus TEMPERATURE REFERENCE SHORT CIRCUIT CURRENT (mA) REFERENCE SOURCE CURRENT (mA) OUTPUT VOLTAGE CHANGE (2.0mV/DIV) OUTPUT VOLTAGE CHANGE (2.0mV/DIV) FIGURE 9 - REFERENCE LOAD REGULATION AMBIENT TEMPERATURE (oC) FIGURE 10 - REFERENCE LINE REGULATION 2.0 mS/DIV FIGURE 11 - OUTPUT SATURATION VOLTAGE versus LOAD CURRENT 2.0 mS/DIV FIGURE 12 - OUTPUT WAVEFORM OUTPUT SATURATION VOLTAGE (V) OUTPUT LOAD CURRENT (mA) 4-34 UC3845A High Performance Current Mode Controller OUTPUT VOLTAGE FIGURE 13 - OUTPUT CROSS CONDUCTION FIGURE 14 - SUPPLY CURRENT versus SUPPLY VOLTAGE SUPPLY CURRENT SUPPLY CURRENT (mA) SUPPLY VOLTAGE (V) 4-35