Simple Step-Up Voltage Regulator - TI.com

1 Simple Step-Up Voltage Regulator Requires Few External Components NPN Output Switches , 65V(max) Extended Input Voltage Range: to 40V Current Mode Operation for ImprovedTransient Response, Line Regulation, andCurrent Limiting Soft Start Function Provides ControlledStartup 52kHz Internal Oscillator Output Switch Protected by Current Limit,Undervoltage Lockout and ThermalShutdown Improved Replacement for LM2577-ADJS eriesThe UC2577-ADJ device provides all the active functions neces-sary to implement Step-Up (boost), flyback, and forward converterswitching regulators . Requiring only a few components, these sim-ple regulators efficiently provide up to 60V as a Step-Up Regulator ,and even higher voltages as a flyback or forward converter UC2577-ADJ features a wide input Voltage range of to40V and an adjustable output Voltage . An on-chip NPN switchis included with undervoltage lockout, thermal protection circuitry,and current limiting, as well as soft start mode operation to reducecurrent during startup.

2 Other features include a 52kHz fixed fre-quency on-chip oscillator with no external components and currentmode control for better line and load standard series of inductors and capacitors are available fromseveral manufacturers optimized for use with these regulators andare listed in this data DIAGRAMBLOCK DIAGRAMUDG-94034 Simple Boost and Flyback Converters SEPIC Topology Permits Input Voltage tobe Higher or Lower than Output Voltage Transformer Coupled Forward regulators Multiple Output DesignsTYPICAL APPLICATIONS5-Pin TO-220 (Top View)Also available in TO-263 Pac CONDITIONSMINTYPMAX UNITSS ystem Parameters Circuit Figure 1 (Note 3)Output VoltageVIN = 5V to 10V, ILOAD = 100mA to = 25 RegulationVIN = to 10V, ILOAD = 300mA20100mVTJ = 25 C50mVLoad RegulationVIN = 5V, ILOAD = 100mA to 800mA20100mVTJ = 25 C50mVEfficiencyVIN = 5V, ILOAD = 800mA80%Device ParametersInput Supply CurrentVFB = (Switch Off) = 25 C10mAISWITCH = , VCOMP = (Max Duty Cycle)4585mATJ = 25 C70mAInput Supply UVLOISWITCH = = 25 FrequencyMeasured at SWITCH Pin, ISWITCH = 100mA425262kHzTJ = 25 C4856kHzReference VoltageMeasured at FB Pin, VIN = to 40V, VCOMP = = 25 Voltage Line RegulationVIN = to Amp Input Bias CurrentVCOMP = = 25 C300nAError Amp TransconductanceICOMP = 30 A to +30 A, VCOMP = mhoTJ = 25 C24004800 mhoError Amp Voltage GainVCOMP = to , RCOMP = (Note 4)250800V/VTJ = 25 C500V/VError Amplifier Output SwingUpper Limit VFB = = 25 Limit VFB = = 25 Amp Output CurrentVFB = to , VCOMP = 90 200 400 ATJ = 25 C 130 300 ASoft Start CurrentVFB = , VCOMP = ATJ = 25 AMaximum Duty CycleVCOMP = , ISWITCH = 100mA9095%TJ = 25 C93%Unless otherwise stated, these specifications apply for TA = 40 C to +125 C, VIN =5V, VFB = VREF, ISWITCH = 0, and TA =TJ.

3 ELECTRICAL CHARACTERISTICSRECOMMENDED OPERATING RANGES upply Voltage .. 45 VOutput Switch Voltage .. 65 VOutput Switch Current (Note 2) .. Dissipation .. Internally LimitedStorage Temperature Range .. 65 C to +150 CLead Temperature (Soldering, 10 sec.) .. 260 CMaximum Junction Temperature .. 150 CMinimum ESD Rating (C = 100pF, R = 15k ) .. 2kVABSOLUTE MAXIMUM RATINGS (Note 1)UC2577-ADJS upply Voltage .. VIN 40 VOutput Switch Voltage .. 0V VSWITCH 60 VOutput Switch Current .. ISWITCH Temperature Range .. 40 C TJ +125 C2UC2577-ADJPARAMETERTEST CONDITIONSMINTYPMAX UNITSD evice Parameters (cont.)Switch Leakage CurrentVSWITCH = 65V, VFB = (Switch Off)10600 ATJ = 25 C300 ASwitch Saturation VoltageISWITCH = , VCOMP = (Max Duty Cycle) = 25 Switch Current LimitVCOMP = ResistanceJunction to Ambient65 C/WJunction to Case2 C/WCOMP Pin CurrentVCOMP = 02550 ATJ = 25 C40 AUnless otherwise stated, these specifications apply for TA = 40 C to +125 C, VIN =5V, VFB = VREF, ISWITCH = 0, and TA =TJ.

4 ELECTRICAL CHARACTERISTICSNote 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating ratingsindicate conditions during which the device is intended to be functional, but device parameter specifications may not beguaranteed under these conditions. For guaranteed specifications and test conditions, see the Electrical 2: Output current cannot be internally limited when the UC2577 is used as a Step-Up Regulator . To prevent damage tothe switch, its current must be externally limited to However, output current is internally limited when the UC2577 isused as a flyback or forward converter 3. External components such as the diode, inductor, input and output capacitors can affect switching regulatorperformance. When the UC2577 is used as shown in the Test Circuit, system performance will be as specified by thesystem 4: A resistor is connected to the compensation pin (which is the error amplifier s output) to ensure accuracy inmeasuring AVOL.

5 In actual applications, this pin s load resistance should be 10M , resulting in AVOL that is typically twicethe guaranteed minimum 1. Circuit Used to Specify System ParametersUDG-94035L = 415-0930 (AIE)D = any manufacturerCOUT = Sprague Type 673 DElectrolytic 680 F, 2 0 VR1 = in series with 511 (1%)R2 = (1%)3 The Block Diagram shows a Step-Up switching regulatorutilizing the UC2577. The Regulator produces an outputvoltage higher than the input Voltage . The UC2577 turnsits switch on and off at a fixed frequency of 52kHz, thusstoring energy in the inductor (L). When the NPN switchis on, the inductor current is charged at a rate of the switch is off, the Voltage at the SWITCH termi-nal of the inductor rises above VIN, discharging thestored current through the output diode (D) into the out-put capacitor (COUT) at a rate of (VOUT - VIN)/L. The en-ergy stored in the inductor is thus transferred to output Voltage is controlled by the amount of energytransferred, which is controlled by modulating the peakinductor current.

6 This modulation is accomplished byfeeding a portion of the output Voltage to an error ampli-fier which amplifies the difference between the feedbackvoltage and an internal precision reference volt-age. The output of the error amplifier is then compared toa Voltage proportional to the switch current, or the induc-tor current, during the switch on time. A comparator ter-minates the switch on time when the two voltages areequal and thus controls the peak switch current to main-tain a constant output Voltage . Figure 2 shows voltageand current waveforms for the circuit. Formulas for calcu-lation are shown in Figure Regulator DESIGN PROCEDURER efer to the Block DiagramGiven:VINmin = Minimum input supply voltageVOUT = Regulated output voltageUC2577-ADJStep-up (Boost) RegulatorDuty CycleDVOUT + VF VINVOUT + VF VSAT VOUT VINVOUTAvg. InductorCurrentIIND(AVG)ILOAD1 DInductorCurrent Ripple IINDVIN VSATL D52,000 Peak InductorCurrentIIND(PK)ILOAD1 D + IIND2 Peak SwitchCurrentISW(PK)ILOAD1 D + IIND2 Switch Voltagewhen OffVSW(OFF)VOUT + VFDiode ReverseVoltageVRVOUT - VSATAvg.

7 DiodeCurrentID(AVG)ILOADPeak DiodeCurrentID(PK)ILOAD1 D + ILOAD1 D 2 D + ILOAD D VIN50 (1 D)VF = Forward Biased Diode Voltage , ILOAD = Output LoadFigure 2. Step-Up Regulator WaveformsFirst, determine if the UC2577 can provide these valuesof VOUT and ILOADmax when operating with the minimumvalue of VIN. The upper limits for VOUT and ILOADmax aregiven by the following 60V andVOUT 10 VINminILOADmax VINminVOUTT hese limits must be greater than or equal to the valuesspecified in this Output Voltage SectionResistors R1 and R2 are used to select the desired out-put Voltage . These resistors form a Voltage divider andpresent a portion of the output Voltage to the error ampli-fier which compares it to an internal reference. Se-lect R1 and R2 such that:R1R2 = 1 Figure 3. Step-Up Regulator FormulasAPPLICATIONS INFORMATION42. Inductor Selection (L)A. Preliminary CalculationsTo select the inductor, the calculation of the followingthree parameters is necessary:Dmax, the maximum switch duty cycle (0 D ):Dmax = VOUT + VF VINminVOUT + VF typically VF = for Schottky diodes and VF = for fast recovery T, the product of volts time that charges the induc-tor:E T = Dmax (VINmin )10652,000Hz (V s)IIND, DC, the average inductor current under full load:IIND, DC = ILOADmax1 DmaxB.

8 Identify Inductor Value:1. From Figure 4, identify the inductor code for the regionindicated by the intersection of E T and IIND, DC. Thiscode gives the inductor value in microhenries. The L or Hprefix signifies whether the inductor is rated for a maxi-mum E T of 90V s (L) or 250V s (H).2. If D < , go to step C. If D , calculate theminimum inductance needed to ensure the switchingregulator s T (V s)IIND, DC(A)Figure 4. Inductor Selection GraphIf Lmin is smaller than the inductor values found in stepB1, go on to step C. Otherwise, the inductor value foundin step B1 is too low; an appropriate inductor codeshould be obtained from the graph as follows:1. Find the lowest value inductor that is greater than Lmin .2. Find where E T intersects this inductor value todetermine if it has an L or H prefix. If E T intersectsboth the L and H regions, select the inductor with anH Inductor SelectionSelect an inductor from the table of Figure 5 which crossreferences the inductor codes to the part numbers of thethree different manufacturers.

9 The inductors listed in thistable have the following characteristics:AIE (ferrite, pot-core inductors): Benefits of this typeare low etectromagnetic interference (EMI), smallphysical size, and very low power dissipation (coreloss).Pulse (powdered iron, toroid core inductors): Bene-fits are low EMI and ability to withstand E T andpeak current above rated value better than (ferrite, bobbin-core inductors): Benefits arelow cost and best ability to withstand E T and peakcurrent above rated value. Be aware that these in-ductors generate more EMI than the other types, andthis may interfere with signals sensitive to : This chart assumes that the inductor ripple current inductor is approximately 20% to 30% of the average inductor current(when the Regulator is under full load). Greater ripple current causes higher peak switch currents and greater output ripple volt-age. Lower ripple current is achieved with larger value inductors. The factor of 20% to 30% is chosen as a convenient balancebetween the two INFORMATION (cont.)

10 5UC2577-ADJ3. Compensation Network (RC, CC) and OutputCapacitor (COUT) SelectionThe compensation network consists of resistor RC andcapacitor CC which form a Simple pole-zero network andstabilize the Regulator . The values of RC and CC dependupon the Voltage gain of the Regulator , ILOADmax, the in-ductor L, and output capacitance COUT. A procedure tocalculate and select the values for RC, CC, and COUT which ensures stability is described below. It should benoted, however, that this may not result in optimum com-pensation. To guarantee optimum compensation a stand-ard procedure for testing loop stability is recommended,such as measuring VOUT transient responses to Calculate the maximum value for 750 ILOADmax VOUT2 VINmin2 Select a resistor less than or equal to this value, not toexceed 3k .B. Calculate the minimum value for COUT using the fol-lowing two L RC ILOADmaxVINmin VOUT andCOUT VINmin RC (VINmin + ( 105 L))487,800 VOUT3 The larger of these two values is the minimum value thatensures Calculate the minimum value of VOUT2 COUTRC2 VINminThe compensation capacitor is also used in the soft startfunction of the Regulator .