UC3844, UC3845, UC2844, High Performance Current Mode ...

1 Semiconductor Components Industries, LLC, 2006 July, 2006 Rev. 71 Publication Order Number: uc3844 /DUC3844, UC3845, UC2844, UC2845 High PerformanceCurrent Mode ControllersThe uc3844 , UC3845 series are high Performance fixed frequencycurrent mode controllers. They are specifically designed for Off Lineand DC to DC converter applications offering the designer a costeffective solution with minimal external components. These integratedcircuits feature an oscillator, a temperature compensated reference, highgain error amplifier, Current sensing comparator, and a high currenttotem pole output ideally suited for driving a power included are protective features consisting of input andreference undervoltage lockouts each with hysteresis, cycle by cyclecurrent limiting, a latch for single pulse metering, and a flip flop whichblanks the output off every other oscillator cycle, allowing output deadtimes to be programmed for 50% to 70%.

2 These devices are available in an 8 pin dual in line plastic packageas well as the 14 pin plastic surface mount (SOIC 14). The SOIC 14package has separate power and ground pins for the totem pole UCX844 has UVLO thresholds of 16 V (on) and 10 V (off),ideally suited for off line converters. The UCX845 is tailored forlower voltage applications having UVLO thresholds of V (on) V (off).Features Current Mode Operation to 500 kHz Output Switching Frequency Output Deadtime Adjustable from 50% to 70% Automatic Feed Forward Compensation Latching PWM for Cycle By Cycle Current Limiting Internally Trimmed Reference with Undervoltage Lockout High Current Totem Pole Output Input Undervoltage Lockout with Hysteresis Low Startup and Operating Current Direct Interface with ON Semiconductor SENSEFETt Products Pb Free Packages are AvailableFigure 1. Simplified Block (12)VC7(11)Output6(10)PWR GND5(8)3(5)CurrentSenseVref8(14)4(7)2(3) 1(1)GND5(9)RTCTV oltageFeedbackRR+ numbers in parenthesis are for the D suffix SOIC 14 14D SUFFIXCASE 751A118 PDIP 8N SUFFIXCASE 626 PIN CONNECTIONS(Top View)Vref(Top View)CompensationVoltage FeedbackCurrent SenseRT/CTVrefVCCO utputGND12345678 CompensationNCVoltage FeedbackNCCurrent SenseNCRT/CTNCVCCVCO utputGNDP ower Ground1234567981011121314 See detailed ordering and shipping information in the packagedimensions section on page 14 of this data INFORMATIONSee general marking information in the device markingsection on page page 14 of this data MARKING INFORMATION18 SOIC 8D1 SUFFIXCASE 751 , UC3845, UC2844, UC2845 RATINGSR atingSymbolValueUnitTotal Power Supply and Zener Current (ICC + IZ)30mAOutput Current , Source or Sink (Note 1) Energy (Capacitive Load per Cycle)

3 Sense and Voltage Feedback InputsVin to + Amp Output Sink CurrentIO10mAPower Dissipation and Thermal CharacteristicsD Suffix, Plastic Package, Case 751 AMaximum Power Dissipation @ TA = 25 CThermal Resistance Junction to AirN Suffix, Plastic Package, Case 626 Maximum Power Dissipation @ TA = 25 CThermal Resistance Junction to C/WW C/WOperating Junction TemperatureTJ+ 150 COperating Ambient TemperatureUC3844, UC3845UC2844, UC2845TA0 to + 70 25 to + 85 CStorage Temperature RangeTstg 65 to + 150 CStresses exceeding Maximum Ratings may damage the device. Maximum Ratings are stress ratings only. Functional operation above theRecommended Operating Conditions is not implied. Extended exposure to stresses above the Recommended Operating Conditions may affectdevice Maximum Package power dissipation limits must be CHARACTERISTICS (VCC = 15 V, (Note 2), RT = 10 k, CT = nF, TA = Tlow to Thigh (Note 3), unless otherwise noted.)UC284 XUC384 XCharacteristicsSymbolMinTypMaxMinTypMax UnitREFERENCE SECTIONR eference Output Voltage (IO = mA, TJ = 25 C) Regulation (VCC = 12 V to 25 V)Regline Regulation (IO = mA to 20 mA)Regload StabilityTS mV/ CTotal Output Variation over Line, Load, Noise Voltage (f = 10 Hz to kHz, TJ = 25 C)Vn 50 50 mVLong Term Stability (TA = 125 C for 1000 Hours)S mVOutput Short Circuit CurrentISC 30 85 180 30 85 180mAOSCILLATOR SECTIONF requencyTJ = 25 CTA = Tlow to Thighfosc474652 5760474652 5760kHzFrequency Change with Voltage (VCC = 12 V to 25 V)Dfosc/DV Change with TemperatureTA = Tlow to ThighDfosc/DT %Oscillator Voltage Swing (Peak to Peak)Vosc VDischarge Current (Vosc = V, TJ = 25 C)Idischg mAERROR AMPLIFIER SECTIONV oltage Feedback Input (VO = V) Bias Current (VFB = V)IIB Loop Voltage Gain (VO = V to V)AVOL6590 6590 dB2.

4 Adjust VCC above the Startup threshold before setting to 15 Low duty cycle pulse techniques are used during test to maintain junction temperature as close to ambient as C for uc3844 , UC3845 Thigh= +70 C for uc3844 , UC3845 25 C for UC2844, UC2845+85 C for UC2844, UC2845UC3844, UC3845, UC2844, UC2845 CHARACTERISTICS (VCC = 15 V, (Note 4), RT = 10 k, CT = nF, TA = Tlow to Thigh (Note 5), unless otherwise noted.)UC284 XUC384 XCharacteristicsSymbolMinTypMaxMinTypMax UnitERROR AMPLIFIER SECTION (continued)Unity Gain Bandwidth (TJ = 25 C) MHzPower Supply Rejection Ratio (VCC = 12 V to 25 V)PSRR6070 6070 dBOutput CurrentSink (VO = V, VFB = V)Source (VO = V, VFB = V) mAOutput Voltage SwingHigh State (RL = 15 k to ground, VFB = V)Low State (RL = 15 k to Vref, VFB = V) SENSE SECTIONC urrent Sense Input Voltage Gain (Notes 6 & 7) Current Sense Input Threshold (Note 6) Supply Rejection RatioVCC = 12 V to 25 V (Note 6)PSRR 70 70 dBInput Bias CurrentIIB 10 10mAPropagation Delay ( Current Sense Input to Output)tPLH(IN/OUT) 150300 150300nsOUTPUT SECTIONO utput VoltageLow State (ISink = 20 mA)(ISink = 200 mA)High State (ISink = 20 mA)(ISink = 200 mA)VOLVOH VOutput Voltage with UVLO ActivatedVCC = V, ISink = mAVOL(UVLO) Voltage Rise Time (CL = nF, TJ = 25 C)

5 Tr 50150 50150nsOutput Voltage Fall Time (CL = nF, TJ = 25 C)tf 50150 50150nsUNDERVOLTAGE LOCKOUT SECTIONS tartup Operating Voltage After Turn OnUCX844 UCX845 VCC(min) SECTIONDuty CycleMaximumMinimumDCmaxDCmin46 48 50047 48 500%TOTAL DEVICEP ower Supply Current (Note 4)Startup:(VCC = V for UCX845A, (VCC 14 V for UCX844) OperatingICC Supply Zener Voltage (ICC = 25 mA)VZ3036 3036 V4. Adjust VCC above the Startup threshold before setting to 15 Low duty cycle pulse techniques are used during test to maintain junction temperature as close to ambient as C for uc3844 , UC3845 Thigh= +70 C for uc3844 , UC3845 25 C for UC2844, UC2845+85 C for UC2844, UC28456. This parameter is measured at the latch trip point with VFB = 0 Comparator gain is defined as: AVDV Output CompensationDV Current Sense InputUC3844, UC3845, UC2844, UC2845 , ERROR AMP OUTPUT VOLTAGE (V)0, Current SENSE INPUT THRESHOLD (V) = 15 VTA = 25 CTA = 55 CTA = 125 CRT , TIMING RESISTOR (k )Figure 2.)

6 Timing Resistor versusOscillator FrequencyFigure 3. Output Deadtime versusOscillator FrequencyFigure 4. Error Amp Small SignalTransient ResponseFigure 5. Error Amp Large SignalTransient ms/DIV20 mV/DIVVCC = 15 VAV = = 25 CVCC = 15 VAV = = 25 ms/DIV200 mV/DIV10 k20 k50 k100 k200 k500 Mfosc, OSCILLATOR FREQUENCY (Hz)VCC = 15 VTA = 25 C10 k20 k50 k100 k200 k500 Mfosc, OSCILLATOR FREQUENCY (Hz)% DT, PERCENT OUTPUT DEADTIMEF igure 6. Error Amp Open Loop Gain andPhase versus FrequencyFigure 7. Current Sense Input Thresholdversus Error Amp Output VoltageNOTE: Output switches at one half the oscillator = 10 20 AVOL, OPEN LOOP VOLTAGE GAIN (dB)10 M10f, FREQUENCY (Hz)GainPhaseVCC = 15 VVO = V to VRL = 100 KTA = 25 k10 k100 M020406080100, EXCESS PHASE (DEGREES) uc3844 , UC3845, UC2844, UC2845 8. Reference Voltage Changeversus Source CurrentFigure 9. Reference Short Circuit Currentversus TemperatureFigure 10. Reference Load RegulationFigure 11.

7 Reference Line Regulation , OUTPUT VOLTAGE CHANGE ( mV/DIV) ms/DIVV , OUTPUT VOLTAGE CHANGE ( mV/DIV) ms/DIVVVCC = 12 V to 25 VTA = 25 C , REFERENCE VOLTAGE CHANGE (mV)ref020406080100120 Iref, REFERENCE SOURCE Current (mA)VVCC = 15 VTA = 55 CTA = 25 CTA = 125 C, REFERENCE SHORT CIRCUIT Current (mA)SC 55 250255075100125TA, AMBIENT TEMPERATURE ( C)VCC = 15 VRL WIFigure 12. Output Saturation Voltageversus Load CurrentFigure 13. Output Waveform50 ns/DIVVCC = 15 VCL = nFTA = 25 C8006004002000IO, OUTPUT LOAD Current (mA), OUTPUT SATURATION VOLTAGE (V)satVVCCTA = 25 CTA = 55 CSink Saturation(Load to VCC)GNDS ource Saturation(Load to Ground)TA = 55 CVCC = 15 V80 ms Pulsed Load120 Hz RateTA = 25 CVCC = 15 VIO = mA to 20 mATA = 25 C0 12 16 20 2411090705090%10% , UC3845, UC2844, UC2845 , SUPPLY CURRENTF igure 14. Output Cross ConductionFigure 15. Supply Current versusSupply Voltage100 ns/DIVVCC = 30 VCL = 15 pFTA = 255C100 mA/DIV20 V/DIV25201510500 10203040 VCC, SUPPLY VOLTAGE (V)RT = 10 kCT = nFVFB = 0 VISense = 0 VTA = 255 CUCX845 UCX844 VCC, OUTPUT VOLTAGEICC, SUPPLY Current (mA)PIN FUNCTION DESCRIPTION PinFunctionDescription8 Pin14 Pin11 CompensationThis pin is Error Amplifier output and is made available for loop is the inverting input of the Error Amplifier.

8 It is normally connected to the switchingpower supply output through a resistor SenseA voltage proportional to inductor Current is connected to this input. The PWM uses thisinformation to terminate the output switch Oscillator frequency and maximum Output duty cycle are programmed by connectingresistor RT to Vref and capacitor CT to ground. Operation to MHz is GNDThis pin is combined control circuitry and power ground (8 pin package only).610 OutputThis output directly drives the gate of a power MOSFET. Peak currents up to A aresourced and sunk by this pin. The output switches at one half the oscillator pin is the positive supply of the control is the reference output. It provides charging Current for capacitor CT through resistor RT. 8 Power GroundThis pin is a separate power ground return (14 pin package only) that is connected back tothe power source. It is used to reduce the effects of switching transient noise on the controlcircuitry. 11 VCThe Output high state (VOH) is set by the voltage applied to this pin (14 pin package only).

9 With a separate power source connection, it can reduce the effects of switching transientnoise on the control circuitry. 9 GNDThis pin is the control circuitry ground return (14 pin package only) and is connected to backto the power source ground. 2,4,6,13 NCNo connection (14 pin package only). These pins are not internally , UC3845, UC2844, UC2845 DESCRIPTIONThe uc3844 , UC3845 series are high Performance , fixedfrequency, Current mode controllers. They are specificallydesigned for Off Line and DC to DC converterapplications offering the designer a cost effective solutionwith minimal external components. A representative blockdiagram is shown in Figure oscillator frequency is programmed by the valuesselected for the timing components RT and CT. Capacitor CTis charged from the V reference through resistor RT toapproximately V and discharged to V by an internalcurrent sink. During the discharge of CT, the oscillatorgenerates an internal blanking pulse that holds the centerinput of the NOR gate high.

10 This causes the Output to be ina low state, thus producing a controlled amount of outputdeadtime. An internal flip flop has been incorporated in theUCX844/5 which blanks the output off every other clockcycle by holding one of the inputs of the NOR gate high. Thisin combination with the CT discharge period yields outputdeadtimes programmable from 50% to 70%. Figure 2 showsRT versus Oscillator Frequency and Figure 3, OutputDeadtime versus Frequency, both for given values of that many values of RT and CT will give the sameoscillator frequency but only one combination will yield aspecific output deadtime at a given many noise sensitive applications it may be desirable tofrequency lock the converter to an external system can be accomplished by applying a clock signal to thecircuit shown in Figure 18. For reliable locking, thefree running oscillator frequency should be set about 10%less than the clock frequency.