UC3845T

® U C3842T UC3843T UC3844T UC3845T HIGH PERFORMANCE CURRENT MODE PWM CONTROLLER . . . . . . . . . TRIMMED OSCILLATOR FOR PRECISE FREQUENCY CONTROL OSCILLATOR FREQUENCY GUARANTEED AT 250kHz CURRENT MODE OPERATION TO 500kHz 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 START-UP AND OPERATING CURRENT Minidip SO8 DESCRIPTION The UC384XT family of control ICs provides the necessary features to implement off-line or DC to DC fixed frequency current mode control schemes with a minimal external parts count. Internally implemented circuits include a trimmed oscillator for precise DUTY CYCLE CONTROL under voltage lockout featuring start-up current less than 0.5mA, a precision reference trimmed for accuracy at the error amp input, logic to insure latched operation, a PWM comparator which also provides current limit control, and a totem pole output stage designed to source or sink high peak current. The output stage, suitable for driving N-Channel MOSFETs, is low in the offstate. Differences between members of this family are the under-voltage lockout thresholds and maximum duty cycle ranges. The UC3842T and UC3844T have UVLO thresholds of 16V (on) and 10V (off), ideally suited to off-line applications The corresponding thresholds for the UC3843T and UC3845T are 8.5 V and 7.9 V. The UC3842T and UC3843T can operate to duty cycles approaching 100%. A range of zero to < 50 % is obtained by the UC3844T and UC3845T by the addition of an internal toggle flip flop which blanks the output off every other clock cycle. BLOCK DIAGRAM ( toggle flip flop used only in UC3844T and UC3845T) Vi 7 34V GROUND 5 UVLO S/R 5V REF INTERNAL BIAS VREF GOOD LOGIC RT/CT 4 OSC ERROR AMP. 2R R 1V T 8 VREF 5V 50mA 2.50V 6 OUTPUT VFB COMP CURRENT SENSE 2 1 3 + - S R CURRENT SENSE COMPARATOR D95IN331 PWM LATCH September 2001 1/15 UC3842T - UC3843T - UC3844T - UC3845T ABSOLUTE MAXIMUM RATINGS Symbol Vi Vi IO EO Parameter Supply Voltage (low impedance source) Supply Voltage (Ii < 30mA) Output Current Output Energy (capacitive load) Analog Inputs (pins 2, 3) Error Amplifier Output Sink Current Ptot Ptot Tstg TL Power Dissipation at Tamb ≤ 25 °C (Minidip) Power Dissipation at Tamb ≤ 25 °C (SO8) Storage Temperature Range Lead Temperature (soldering 10s) Value 30 Self Limiting ±1 5 – 0.3 to 5.5 10 1.25 800 – 65 to 150 300 A µJ V mA W mW °C °C Unit V * All voltages are with respect to pin 5, all currents are positive into the specified terminal. PIN CONNECTION ( top view) Minidip/SO8 COMP VFB ISENSE RT/CT 1 2 3 4 D95IN332 8 7 6 5 VREF Vi OUTPUT GROUND PIN FUNCTIONS No 1 2 3 4 5 6 7 8 Function COMP VFB ISENSE RT/CT GROUND OUTPUT VCC Vref Description This pin is the Error Amplifier output and is made available for loop compensation. This is the inverting input of the Error Amplifier. It is normally connected to the switching power supply output through a resistor divider. A voltage proportional to inductor current is connected to this input. The PWM uses this information to terminate the output switch conduction. The oscillator frequency and maximum Output duty cycle are programmed by connecting resistor RT to Vref and cpacitor CT to ground. Operation to 500kHz is possible. This pin is the combined control circuitry and power ground. This output directly drives the gate of a power MOSFET. Peak currents up to 1A are sourced and sunk by this pin. This pin is the positive supply of the control IC. This is the reference output. It provides charging current for capacitor C T through resistor RT. ORDERING NUMBERS SO8 UC3842TD UC3843TD UC3844TD UC3845TD Minidip UC3842TN UC3843TN UC3844TN UC3845TN 2/15 U3842T - UC3843T - UC3844T - UC3845T THERMAL DATA Symbol Rth j-amb Description Thermal Resistance Junction-ambient. max. Minidip 100 SO8 150 Unit °C/W ELECTRICAL CHARACTERISTICS ( [note 1] Unless otherwise stated, these specifications apply for 0 < Tamb < 105°C; Vi = 15V (note 5); RT = 10K; CT = 3.3nF) Symbol Parameter Test Conditions Min. Value Typ. 2 3 0.2 4.85 50 5 -30 Tj = 25°C TA = Tlow to Thigh TJ = 25°C (RT = 6.2k, CT = 1nF) VCC = 12V to 25V TA = Tlow to Thigh (peak to peak) TA = Tlow to Thigh VPIN1 = 2.5V VFB = 5V 2V ≤ Vo ≤ 4V TJ = 25°C 12V ≤ Vi ≤ 25V VPIN2 = 2.7V VPIN1 = 1.1V VPIN2 = 2.3V VPIN1 = 5V VPIN2 = 2.3V; RL = 15KΩ to Ground VPIN2 = 2.7V; RL = 15KΩ to Pin 8 (note 3 & 4) VPIN1 = 5V (note 3) 12 ≤ Vi ≤ 25V (note 3) 2.85 0.9 65 0.7 60 2 -0.5 5 49 48 225 – – – 7.3 2.42 -100 52 – 250 0.2 1 1.6 – 2.50 -0.1 90 1 70 12 -1 6.2 0.8 1.1 25 -180 55 56 275 1 – – 8.8 2.58 -2 5.15 Max. 20 25 Unit REFERENCE SECTION ∆VREF ∆VREF Line Regulation Load Regulation 12V ≤ Vi ≤ 25V 1 ≤ Io ≤ 20mA (Note 2) Line, Load, Temperature 10Hz ≤ f ≤ 10KHz Tj = 25°C (note 2) Tamb = 125°C, 1000Hrs (note 2) mV mV mV/°C V µV mV mA KHz KHz KHz % % V mA V µA dB MHz dB mA mA V V ∆VREF/∆T Temperature Stability Total Output Variation eN Output Noise Voltage Long Term Stability ISC fOSC Output Short Circuit Frequency OSCILLATOR SECTION ∆fOSC/∆V ∆fOSC/∆T VOSC Idischg V2 Ib Frequency Change with Volt. Frequency Change with Temp. Oscillator Voltage Swing Discharge Current (VOSC =2V) Input Voltage Input Bias Current AVOL ERROR AMP SECTION BW PSRR Io Io Unity Gain Bandwidth Power Supply Rejec. Ratio Output Sink Current Output Source Current VOUT High VOUT Low CURRENT SENSE SECTION GV V3 SVR Ib Gain Maximum Input Signal Supply Voltage Rejection Input Bias Current Delay to Output 3 1 70 -2 100 -10 300 3.15 1.1 V/V V dB µA ns 3/15 UC3842T - UC3843T - UC3844T - UC3845T ELECTRICAL CHARACTERISTICS (continued) Symbol OUTPUT SECTION VOL VOH VOLS tr tf Output Low Level ISINK = 20mA ISINK = 200mA Output High Level ISOURCE = 20mA ISOURCE = 200mA UVLO Saturation Rise Time Fall Time Start Threshold VCC = 6V; ISINK = 1mA Tj = 25°C CL = 1nF (2) Tj = 25°C CL = 1nF (2) UC3842T/4T UC3843T/5T Min Operating Voltage After Turn-on PWM SECTION Maximum Duty Cycle UC3842T/3T UC3844T/5T Minimum Duty Cycle TOTAL STANDBY CURRENT Ist Ii Viz Start-up Current Vi = 6.5V for UC3843T/45T Vi = 14V for UC3842T/44T Operating Supply Current Zener Voltage VPIN2 = VPIN3 = 0V Ii = 25mA 30 0.3 0.3 12 36 0.5 0.5 17 mA mA mA V 94 47 96 48 100 50 0 % % % UC3842T/4T UC3843T/5T 15 7.8 9 7.0 13 12 0.1 1.6 13.5 13.5 0.1 50 50 16 8.4 10 7.6 1.1 150 150 17 9.0 11 8.2 0.4 2.2 V V V V V ns ns V V V V Parameter Test Conditions Min. Value Typ. Max. Unit UNDER-VOLTAGE LOCKOUT SECTION N otes : 1. Max package power dissipation limits must be respected; low duty cycle pulse techniques are used during test maintain Tj as close to Tamb as possible. 2. These parameters, although guaranteed, are not 100% tested in production. 3. Parameter measured at trip point of latch with VPIN2 = 0. 4. Gain defined as : ∆ VPIN1 A= ; 0 ≤ VPIN3 ≤ 0.8 V ∆ VPIN3 5. Adjust Vi above the start threshold before setting at 15 V. 4/15 U3842T - UC3843T - UC3844T - UC3845T Figure 1: Open Loop Test Circuit. VREF 4.7KΩ 2N2222 100KΩ ERROR AMP. ADJUST 4.7KΩ 1KΩ ISENSE ADJUST 5KΩ COMP VFB ISENSE RT/CT RT VREF 1 2 3 4 8 7 Vi 0.1µF 6 5 OUTPUT GROUND 1W 1KΩ OUTPUT A 0.1µF Vi D.U.T. CT D95IN343 GROUND High peak currents associated with capacitive loads necessitate careful grounding techniques. Timing and bypass capacitors should be connected close to pin 5 in a single point ground. The transistor and 5 KΩ potentiometer are used to sample the oscillator waveform and apply an adjustable ramp to pin 3. Figure 2: Timing Resistor vs. Oscillator Frequency RT (KΩ) 50 C T= Figure 3: Output Dead-Time vs. Oscillator Frequency D95IN334 D95IN333 % 20 0p F C 50 10 0p F T= C 20 T= 50 CT=5nF C 0p 30 20 CT=1nF CT=500pF CT=200pF CT=2nF CT=5nF CT=10nF F T= 1n F 10 10 5 CT=2nF CT=10nF 5 3 2 CT=100pF 2 Vi=15V TA=25˚C 1 0.8 10K Vi=15V TA=25˚C 20K 30K 50K 100K 200K 300K 500K fOSC(KHz) 1 10K 20K 30K 50K 100K 200K 300K 500K fOSC(KHz) 5/15 UC3842T - UC3843T - UC3844T - UC3845T Figure 4: Oscillator Discharge Current vs. Temperature. Idischg (mA) D95IN335 Figure 5: Maximum Output Duty Cycle vs. Timing Resistor. D95IN336 Dmax (%) 90 Idischg=7.5mA 80 Idischg=8.8mA Vi=15V VOSC=2V 8.5 8.0 70 60 7.5 50 Vi=15V CT=3.3nF TA=25˚C 7.0 -55 -25 0 25 50 75 100 TA(˚C) 40 0.8 1 2 3 5 RT(KΩ) Figure 6: Error Amp Open-Loop Gain and Phase vs. Frequency. (dB) 80 D95IN337 Figure 7: Current Sense Input Threshold vs. Error Amp Output Voltage. φ 30 60 90 120 150 Vth (V) 1.0 D95IN338 Gain 60 40 20 0 -20 10 Vi=15V VO=2V to 4V RL=100K TA=25˚C Vi=15V TA=25˚C 0.8 TA=125˚C Phase 0.6 0.4 TA=-40˚C 0.2 0.0 100 1K 10K 100K 1M 180 f(Hz) 0 2 4 6 VO(V) Figure 8: Reference Voltage Change vs. Source Current. 60 D95IN339 Figure 9: Reference Short Circuit Current vs. Temperature. ISC (mA) 100 Vi=15V RL≤0.1Ω D95IN340 Vi=15V 50 40 TA=-40˚C TA=125˚C 90 80 30 20 10 0 0 20 40 60 80 100 Iref(mA) TA=25˚C 70 60 50 -55 -25 0 25 50 75 100 TA(˚C) 6/15 U3842T - UC3843T - UC3844T - UC3845T Figure 10: Output Saturation Voltagevs. Load Current. Vsat (V) D95IN341 Figure 11: Supply Current vs. Supply Voltage. Ii (mA) 20 D95IN342 Vi -1 -2 Source Saturation (Load to Ground) TA=25˚C TA=-40˚C Vi=15V 80µs Pulsed Load 120Hz Rate 15 RT=10K CT=3.3nF VFB=0V ISense=0V TA=25˚C UCX843/45 10 2 1 0 0 TA=-40˚C TA=25˚C 5 Sink Saturation (Load to Vi) 200 400 600 GND 0 IO(mA) 0 10 UCX842/44 3 20 30 Vi(V) Figure 12: Output Waveform. Figure 13: Output Cross Conduction Vi =30V CL = 15pF TA = 25°C VO 20V/DIV 90% Vi =15V CL = 1.0nF TA = 25°C ICC 10% 50ns/DIV 100ns/DIV 100mA/DIV Figure 14: Oscillator and Output Waveforms. Vi 7 8 5V REG PWM RT CLOCK 4 OSCILLATOR ID CT 5 GND D95IN344 CT OUTPUT 6 OUTPUT LARGE RT/SMALL CT CT OUTPUT SMALL RT/LARGE CT 7/15 UC3842T - UC3843T - UC3844T - UC3845T Figure 15 : Error Amp Configuration. 2.5V 1mA + VFB COMP Zf D95IN345 Zi 2 1 - Figure 16 : Under Voltage Lockout. Vi 7 ON/OFF COMMAND TO REST OF IC ICC UC3842T UC3844T VON VOFF 16V 10V UC3843T UC3845T 8.4V 7.6V