LT3511 - Monolithic High Voltage Isolated Flyback Converter

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LT3511Monolithic High VoltageIsolated Flyback ConverterFEATURESDESCRIPTIONnThe LT 3511 is a high voltage monolithic switching regulator specifically designed for the isolated flyback topology.No third winding or opto-isolator is required for regulation as the part senses output voltage directly from theprimary-side flyback waveform. The device integrates a240mA, 150V power switch, high voltage circuitry, andcontrol into a high voltage 16-lead MSOP package withfour leads removed.nnnnnnnn4.5V to 100V Input Voltage RangeInternal 240mA, 150V Power SwitchBoundary Mode OperationNo Transformer Third Winding orOpto-Isolator Required for RegulationImproved Primary-Side Winding FeedbackLoad RegulationVOUT Set with Two External ResistorsBIAS Pin for Internal Bias Supply and PowerSwitch DriverNo External Start-Up Resistor16-Lead MSOP PackageAPPLICATIONSnnnIsolated Telecom Power SuppliesIsolated Auxiliary/Housekeeping Power SuppliesIsolated Industrial, Automotive and Medical PowerSuppliesThe LT3511 operates from an input voltage range of 4.5Vto 100V and delivers up to 2.5W of isolated output power.Two external resistors and the transformer turns ratioeasily set the output voltage. Off-the-shelf transformersare available for several applications. The high level ofintegration and the use of boundary mode operation resultsin a simple, clean, tightly regulated application solution tothe traditionally tough problem of isolated power delivery.L, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarksand No RSENSE is a trademark of Linear Technology Corporation. All other trademarks are theproperty of their respective owners. Protected by U.S. Patents, including 5438499, 7471522.TYPICAL APPLICATION48V to 5V Isolated Flyback ConverterOutput Load and Line Regulation5.251μFVOUT 10kTCSWVC69.8k19μHt5.205.1522μFVOUT–5.10VOUT (V)VIN36V TO 72VVIN 48V5.05VIN 36V5.00VIN 72V4.954.904.85GND BIAS4.8016.9k3.3nF4.7μF3511 TA01a4.75050150200100LOAD CURRENT (mA)2503003511 TA01b3511fc1

LT3511ABSOLUTE MAXIMUM RATINGSPIN CONFIGURATION(Note 1)TOP VIEWSW (Note 4) .150VVIN, EN/UVLO, RFB .100VVIN to RFB . 6VBIAS .Lesser of 20V or VINRREF, TC, VC .6VOperating Junction Temperature Range (Note 2)LT3511E, LT3511I . –40 C to 125 CLT3511H . –40 C to 150 CLT3511MP . –55 C to 150 CStorage Temperature Range . –65 C to 150 CEN/UVLO 116 SWVIN 314 RFBGNDBIASNCGND56781211109RREFTCVCGNDMS PACKAGE16(12)-LEAD PLASTIC MSOPθJA 90 C/WORDER INFORMATIONLEAD FREE FINISHTAPE AND REELPART MARKING*PACKAGE DESCRIPTIONTEMPERATURE RANGELT3511EMS#PBFLT3511EMS#TRPBF351116-Lead Plastic MSOP–40 C to 125 CLT3511IMS#PBFLT3511IMS#TRPBF351116-Lead Plastic MSOP–40 C to 125 CLT3511HMS#PBFLT3511HMS#TRPBF351116-Lead Plastic MSOP–40 C to 150 CLT3511MPMS#PBFLT3511MPMS#TRPBF351116-Lead Plastic MSOP–55 C to 150 CConsult LTC Marketing for parts specified with wider operating temperature ranges. *The temperature grade is identified by a label on the shipping container.Consult LTC Marketing for information on non-standard lead based finish parts.For more information on lead free part marking, go to: http://www.linear.com/leadfree/For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ELECTRICAL CHARACTERISTICSThe l denotes the specifications which apply over the full operatingtemperature range, otherwise specifications are at TA 25 C. VIN 24V unless otherwise mAμA1.151.211.27V2.02.603.3μAμAMaximum Current Limit240330430mAMinimum Current 00nAlInput Voltage RangeVIN BIASQuiescent CurrentNot SwitchingVEN/UVLO 0.2VEN/UVLO Pin ThresholdEN/UVLO Pin Voltage RisingEN/UVLO Pin CurrentVEN/UVLO 1.1VVEN/UVLO 1.4Vl64.5Maximum Switching FrequencySwitch VCESAT650ISW 100mARREF Voltage6V VIN 100VRREF Pin Bias Current(Note 3)Error Amplifier Voltage GainΔI 2μAkHz0.3lRREF Voltage Line RegulationError Amplifier TransconductanceTYPlV150V/V140μmhos3511fc2

LT3511ELECTRICAL CHARACTERISTICSThe l denotes the specifications which apply over the full operatingtemperature range, otherwise specifications are at TA 25 C. VIN 24V unless otherwise noted.PARAMETERCONDITIONSMINTYPMinimum Switching FrequencyTC Current into RREFRTC 53.6kBIAS Pin VoltageInternally Regulated3Note 1: Stresses beyond those listed under Absolute Maximum Ratingsmay cause permanent damage to the device. Exposure to any AbsoluteMaximum Rating condition for extended periods may affect devicereliability and lifetime.Note 2: The LT3511E is guaranteed to meet performance specificationsfrom 0 C to 125 C junction temperature. Specifications over the –40 Cto 125 C operating junction temperature range are assured by design,characterization and correlation with statistical process controls. TheLT3511I is guaranteed to meet performance specifications from –40 C to125 C operating junction temperature range. The LT3511H is guaranteedOutput Voltage5.20Quiescent Current5.153.13.2V3.5BIAS VOLTAGE (V)IQ (mA)VOUT (V)4.95μABIAS Pin Voltage45.009.54.05.105.05kHzTA 25 C, unless otherwise noted.5VIN 48VUNITSto meet performance specifications from –40 C to 150 C operatingjunction temperature range. The LT3511MP is guaranteed over the full–55 C to 150 C operating junction range. High junction temperaturesdegrade operating lifetimes. Operating lifetime is derated at junctiontemperatures greater than 125 C.Note 3: Current flows out of the RREF pin.Note 4: The SW pin is rated to 150V for transients. Operating waveformsof the SW pin should keep the pedestal of the flyback waveform below100V as shown in Figure 5.TYPICAL PERFORMANCE CHARACTERISTICS5.25MAX40324.903.02.54.851VIN 24VVIN 48VVIN 100V4.804.75–50 –2500–50 –2525 50 75 100 125 150TEMPERATURE ( C)0VIN 24V, 10mAVIN 24V, NO LOAD2.0–50 –2525 50 75 100 125 150TEMPERATURE ( C)3511 G01025 50 75 100 125 150TEMPERATURE ( C)3511 G033511 G02Switch VCESATSwitch Current Limit1000Quiescent Current vs VIN44008006004002003300250IQ (mA)CURRENT LIMIT (mA)SWITCH VCESAT VOLTAGE (mV)MAXIMUM CURRENT LIMIT35020021501001MINIMUM CURRENT LIMIT500050100 150 200 250SWITCH CURRENT (mA)3003503511 G040–50 –250025 50 75 100 125 150TEMPERATURE ( C)3511 G050206040VOLTAGE (V)801003511 G063511fc3

LT3511TYPICAL PERFORMANCE CHARACTERISTICSEN/UVLO Pin (Hysteresis) Currentvs TemperatureEN/UVLO PIN CURRENT (μA)EN/UVLO PIN CURRENT (μA)21303.0252.5EN/UVLO THRESHOLD (V)EN/UVLO 1.2V3EN/UVLO Thresholdvs TemperatureEN/UVLO Pin Currentvs VEN/UVLO54TA 25 C, unless otherwise noted.20151001.00–50 –25025 50 75 100 125 150TEMPERATURE ( C)1.50.550–50 –252.0120406080VEN/UVLO VOLTAGE (V)3511 G07100025 50 75 100 125 150TEMPERATURE ( C)3511 G093511 G08Maximum Frequencyvs TemperatureMinimum Frequencyvs Temperature100010080080EN/UVLO Shutdown Thresholdvs Temperature0.9600400200EN/UVLO THRESHOLD (V)MINIMUM FREQUENCY (kHz)MAXIMUM FREQUENCY (kHz)0.86040200.70.60.50.40.30.20.10–50 –25025 50 75 100 125 150TEMPERATURE ( C)0–50 –25025 50 75 100 125 150TEMPERATURE ( C)3511 G100–50 –25025 50 75 100 125 150TEMPERATURE ( C)3511 G113511 G14Light Load DiscontinuousMode WaveformBoundary Mode Waveform20V/DIV20V/DIV1μs/DIV3511 G122μs/DIV3511 G133511fc4

LT3511PIN FUNCTIONSEN/UVLO (Pin 1): Enable/Undervoltage Lockout. The EN/UVLO pin is used to start up the LT3511. Pull the pin to 0Vto shut down the LT3511. This pin has an accurate 1.21Vthreshold and can be used to program an undervoltagelockout (UVLO) threshold using a resistor divider fromsupply to ground. A 2.6μA pin current hysteresis allowsthe programming of undervoltage lockout (UVLO) hysteresis. EN/UVLO can be directly connected to VIN. If leftopen circuit the part will not power up.VIN (Pin 3): Input Supply Pin. This pin supplies current tothe internal start-up circuitry, and serves as a referencevoltage for the DCM comparator and feedback circuitry.Must be locally bypassed with a capacitor.GND (Pin 5, 8, 9): Ground Pins. All three pins should betied directly to the local ground plane.BIAS (Pin 6): Bias Voltage. This pin supplies current tothe switch driver and internal circuitry of the LT3511.This pin may also be connected to VIN if a third windingis not used and if VIN 20V. The part can operate downto 4.5V when BIAS and VIN are connected together. If athird winding is used, the BIAS voltage should be lowerthan the input voltage and greater than 3.3V for properoperation. BIAS must be bypassed with a 4.7μF capacitorplaced close to the pin.VC (Pin 10): Compensation Pin for Internal Error Amplifier.Connect a series RC from this pin to ground to compensatethe switching regulator. An additional 100pF capacitor fromthis pin to ground helps eliminate noise.TC (Pin 11): Output Voltage Temperature Compensation. Connect a resistor to ground to produce a currentproportional to absolute temperature to be sourced intothe RREF node.ITC 0.55V/RTC.RREF (Pin 12): Input Pin for External Ground-ReferredReference Resistor. The resistor at this pin should be 10k.For nonisolated applications, a traditional resistor voltagedivider from VOUT may be connected to this pin.RFB (Pin 14): Input Pin for External Feedback Resistor.This pin is connected to the transformer primary (VSW).The ratio of this resistor to the RREF resistor, times theinternal bandgap reference, determines the output voltage(plus the effect of any non-unity transformer turns ratio).For nonisolated applications, this pin should be connectedto GND with a 1M resistor.SW (Pin 16): Switch Pin. Collector of the internal powerswitch. Minimize trace area at this pin to minimize EMIand voltage spikes.3511fc5

LT3511BLOCK DIAGRAMD1VINVOUT T1C1L1AL1BC2R3VOUT 2V–gm – ONESHOTCURRENTCOMPARATORA2–A1 –VINDRIVERBIASRREFSR4 V1120mVRQ1QSBIASMASTERLATCHC41.2VR1EN/UVLO A5–R23μAINTERNALREFERENCEANDREGULATORS –A4RSENSE0.02ΩGNDOSCILLATORVCQ4R6C33511 BD3511fc6

LT3511OPERATIONThe LT3511 is a current mode switching regulator IC designed specifically for the isolated flyback topology. Thekey problem in isolated topologies is how to communicateinformation regarding the output voltage from the isolatedsecondary side of the transformer to the primary side.Historically, opto-isolators or extra transformer windingscommunicate this information across the transformer.Opto-isolator circuits waste output power, and the extracomponents increase the cost and physical size of thepower supply. Opto-isolators can also exhibit trouble dueto limited dynamic response, nonlinearity, unit-to-unitvariation and aging over life. Circuits employing an extratransformer winding also exhibit deficiencies. Using anextra winding adds to the transformer’s physical size andcost, and dynamic response is often mediocre.In the LT3511, the primary-side flyback pulse providesinformation about the isolated output voltage. In this manner, neither opto-isolator nor extra transformer winding isrequired for regulation. Two resistors program the outputvoltage. Since this IC operates in boundary mode, the partcalculates output voltage from the switch pin when thesecondary current is almost zero.The Block Diagram shows an overall view of the system.Many of the blocks are similar to those found in traditionalswitching regulators including internal bias regulator, oscillator, logic, current amplifier, current comparator, driver,and output switch. The novel sections include a specialflyback error amplifier and a temperature compensationcircuit. In addition, the logic system contains additionallogic for boundary mode operation.The LT3511 features boundary mode control, where thepart operates at the boundary between continuous conduction mode and discontinuous conduction mode. TheVC pin controls the current level just as it does in normalcurrent mode operation, but instead of turning the switchon at the start of the oscillator period, the part turns on theswitch when the secondary-side winding current is zero.Boundary Mode OperationBoundary mode is a variable frequency, current modeswitching scheme. The switch turns on and the inductorcurrent increases until a VC pin controlled current limit.After the switch turns off, the voltage on the SW pin risesto the output voltage divided by the secondary-to-primarytransformer turns ratio plus the input voltage. When thesecondary current through the diode falls to zero, the SWpin voltage falls below VIN. A discontinuous conductionmode (DCM) comparator detects this event and turns theswitch back on.Boundary mode returns the secondary current to zero everycycle, so parasitic resistive voltage drops do not cause loadregulation errors. Boundary mode also allows the use of asmaller transformer compared to continuous conductionmode and does not exhibit subharmonic oscillation.At low output currents, the LT3511 delays turning on theswitch, and thus operates in discontinuous mode. Unliketraditional flyback converters, the switch has to turn onto update the output voltage information. Below 0.6V onthe VC pin, the current comparator level decreases toits minimum value, and the internal oscillator frequencydecreases. With the decrease of the internal oscillator,the part starts to operate in DCM. The output current isable to decrease while still allowing a minimum switch offtime for the flyback error amplifier. The typical minimuminternal oscillator frequency with VC equal to 0V is 40kHz.3511fc7

LT3511APPLICATIONS INFORMATIONPSEUDO DC THEORYIn the Block Diagram, RREF (R4) and RFB (R3) are externalresistors used to program the output voltage. The LT3511operates similar to traditional current mode switchers,except in the use of a unique error amplifier, which derivesits feedback information from the flyback pulse.Operation is as follows: when the output switch, Q1, turnsoff, its collector voltage rises above the VIN rail. The amplitude of this flyback pulse, i.e., the difference betweenit and VIN, is given as:VFLBK (VOUT VF ISEC ESR) NPSVF D1 forward voltageISEC Transformer secondary currentESR Total impedance of secondary circuitNPS Transformer effective primary-to-secondary turnsratioRFB and Q2 convert the flyback voltage into a current. Nearlyall of this current flows through RREF to form a groundreferred voltage. The resulting voltage forms the inputto the flyback error amplifier. The flyback error amplifiersamples the voltage information when the secondary sidewinding current is zero. The bandgap voltage, 1.20V, actsas the reference for the flyback error amplifier.The relatively high gain in the overall loop will then causethe voltage at RREF to be nearly equal to the bandgapreference voltage VBG. The resulting relationship betweenVFLBK and VBG approximately equals: VFLBK VBG R or VFLBK VBG FB RFB RREF RREF VBG Internal bandgap referenceCombination of the preceding expression with earlierderivation of VFLBK results in the following equation: R 1 VOUT VBG FB – VF – ISEC (ESR) RREF NPS The expression defines VOUT in terms of the internal reference, programming resistors, transformer turns ratioand diode forward voltage drop. Additionally, it includesthe effect of nonzero secondary output impedance (ESR).Boundary control mode minimizes the effect of this impedance term.Temperature CompensationThe first term in the VOUT equation does not have temperature dependence, but the diode forward drop has asignificant negative temperature coefficient. A positivetemperature coefficient current source connects to theRREF pin to compensate. A resistor to ground from theTC pin sets the compensation current.The following equation explains the cancellation of thetemperature coefficient:R1 δVTCδVF – FB or,δTRTC NPS δTRTC δV–RFB1R TC FBNPS δVF / δT δTNPS(δVF/δT) Diode’s forward voltage temperature coefficient(δVTC/δT) 2mVVTC 0.55VExperimentally verify the resulting value of RTC and adjust asnecessary to achieve optimal regulation over temperature.The addition of a temperature coefficient current modifiesthe expression of output voltage as follows: R 1 VOUT VBG FB – VF RREF NPS V R– TC FB – ISEC (ESR) RTC NPSOutput PowerA flyback converter has a complicated relationship between the input and output current compared to a buckor a boost. A boost has a relatively constant maximuminput current regardless of input voltage and a buck has arelatively constant maximum output current regardless ofinput voltage. This is due to the continuous nonswitchingbehavior of the two currents. A flyback converter has bothdiscontinuous input and output currents which makes it3511fc8

LT3511APPLICATIONS INFORMATIONsimilar to a nonisolated buck-boost. The duty cycle willaffect the input and output currents, making it hard topredict output power. In addition, the winding ratio canbe changed to multiply the output current at the expenseof a higher switch voltage.One design example would be a 5V output converter witha minimum input voltage of 36V and a maximum inputvoltage of 72V. A four-to-one winding ratio fits this designexample perfectly and outputs close to 1.6W at 72V butlowers to 1W at 36V.The graphs in Figures 1-4 show the typical maximum outputpower possible for the output voltages 3.3V, 5V, 12V and24V. The maximum power output curve is the calculatedoutput power if the switch voltage is 100V during the offtime. 50V of margin is left for leakage voltage spike. Toachieve this power level at a given input, a winding ratiovalue must be calculated to stress the switch to 100V,resulting in some odd ratio values. The following curvesare examples of common winding ratio values and theamount of output power at given input voltages.The equations below calculate output power:Power η VIN D IPEAK 0.5Efficiency η 85%Duty Cycle D Peak switch current IPEAK 0.26A3.03.53.0N NPS(MAX)2.0N 15 N 12N 10N 8OUTPUT POWER (W)2.5OUTPUT POWER (W)( VOUT VF ) NPS( VOUT VF ) NPS VINN 61.5N 41.0N 20.5N 52.5N 4N NPS(MAX)N 32.0N 21.5N 11.00.500204060INPUT VOLTAGE (V)0100800204060INPUT VOLTAGE (V)803511 F013511 F03Figure 1. Output Power for 3.3V OutputFigure 3. Output Power for 12V Output3.03.0N 8N NPS(MAX)2.01.5N 3N 21.0N 10.5N NPS(MAX)2.5N 7N 6N 5N 4OUTPUT POWER (W)2.5OUTPUT POWER (W)100N 22.0N 11.51.00.5000204060INPUT VOLTAGE (V)801003511 F02Figure 2. Output Power for 5V Output0204060INPUT VOLTAGE (V)801003511 F04Figure 4. Output Power for 24V Output3511fc9

LT3511APPLICATIONS INFORMATIONTRANSFORMER DESIGN CONSIDERATIONSSuccessful application of the LT3511 relies on propertransformer specification and design. Carefully considerthe following information in addition to the traditionalguidelines associated with high frequency isolated powersupply transformer design.Linear Technology has worked with several leading magnetic component manufacturers to produce pre-designedflyback transformers for use with the LT3511. Table 1shows the details of these transformers.Table 1. Predesigned TransformersTRANSFORMERPART NUMBERLPRI (μH)LEAKAGE (μH)NP:NS:NBISOLATION (V)SATURATIONCURRENT (mA)VENDOR7503115583001.54:1:11500500Würth Elektronik48V to 5V, 0.3A24V to 5V, 0.2A12V to 5V, 0.13A48V to 3.3V, 0.33A24V to 3.3V, 0.28A12V to 3.3V, 0.18A75031101940056:1:21500750Würth Elektronik24V to 5V, 0.26A12V to 5V, 0.17A48V to 3.3V, 0.43A24V to 3.3V, 0.35A12V to 3.3V, 0.2A75031165930021:1:0.21500560Würth Elektronik48V to 24V, 0.07A75031166035032:1:0.331500520Würth Elektronik48V to 15V, 0.1A48V to 12V, 0.12A24V to 15V, 0.09A12V to 15V, 0.045A75031183835032:1:11500520Würth Elektronik48V to 15V, 0.05A48V to 12V, 0.06A24V to 15V, 0.045A7503119632000.41:5:51500650Würth Elektronik12V to 70V, 0.004A12V to 100V, 0.003A12V to 150V, 0.002A7503119661200.451:5:0.51500900Würth Elektronik12V to 120V and–12V, 0.002A10396-T0243002.04:1:11500500Sumida48V to 5V, 0.3A24V to 5V, 0.2A12V to 5V, 0.13A48V to 3.3V, 0.33A24V to 3.3V, 0.28A12V to 3.3V, 0.18A10396-T0263002.56:1:21500500Sumida24V to 5V, 0.26A12V to 5V, 0.17A48V to 3.3V, 0.43A24V to 3.3V, 0.35A12V to 3.3V, 0.2A01355-T0572502.01:1:0.21500500Sumida48V to 24V, 0.07A10396-T0223002.02:1:0.331500500Sumida48V to 15V, 0.1A48V to 12V, 0.12A24V to 15V, 0.09A12V to 15V, 0.045A10396-T0283002.52:1:11500500Sumida48V to 15V, 0.05A48V to 12V, 0.06A24V to 15V, 0.045ATARGETAPPLICATIONS3511fc10

LT3511APPLICATIONS INFORMATIONTurns RatioSaturation CurrentNote that when using an RFB/RREF resistor ratio to setoutput voltage, the user has relative freedom in selectinga transformer turns ratio to suit a given application. Incontrast, the use of simple ratios of small integers, e.g.,1:1, 2:1, 3:2, provides more freedom in setting total turnsand mutual inductance.The current in the transformer windings should not exceed its rated saturation current. Energy injected once thecore is saturated will not be transferred to the secondaryand will instead be dissipated in the core. Information onsaturation current should be provided by the transformermanufacturers. Table 1 lists the saturation current of thetransformers designed for use with the LT3511.Typically, choose the transformer turns to maximize available output power. For low output voltages (3.3V or 5V), aN:1 turns ratio can be used with multiple primary windingsrelative to the secondary to maximize the transformer’scurrent gain (and output power). However, remember thatthe SW pin sees a voltage that is equal to the maximuminput supply voltage plus the output voltage multiplied bythe turns ratio. In addition, leakage inductance will causea voltage spike (VLEAKAGE) on top of this reflected voltage.This total quantity needs to remain below the absolutemaximum rating of the SW pin to prevent breakdown ofthe internal power switch. Together these conditions placean upper limit on the turns ratio, N, for a given application.Choose a turns ratio low enough to ensure:N Primary Inductance RequirementsThe LT3511 obtains output voltage information from thereflected output voltage on the switch pin. The conductionof secondary winding current reflects the output voltageon the primary. The sampling circuitry needs a minimumof 400ns to settle and sample the reflected output voltage.In order to ensure proper sampling, the secondary windingneeds to conduct current for a minimum of 400ns. Thefollowing equation gives the minimum value for primaryside magnetizing inductance:LPRI 150V – VIN(MAX) – VLEAKAGEVOUT VFFor larger N:1 values, choose a transformer with a largerphysical size to deliver additional current. In addition,choose a large enough inductance value to ensure thatthe off-time is long enough to measure the output voltage.For lower output power levels, choose a 1:1 or 1:N transformer for the absolute smallest transformer size. A 1:Ntransformer will minimize the magnetizing inductance(and minimize size), but will also limit the available outputpower. A higher 1:N turns ratio makes it possible to havevery high output voltages without exceeding the breakdownvoltage of the internal power switch.The turns ratio is an important element in the isolatedfeedback scheme. Make sure the transformer manufacturerguarantees turns ratio accuracy within 1%.tOFF(MIN) NPS ( VOUT VF )IPEAK(MIN)tOFF(MIN) 400nsIPEAK(MIN) 55mAIn addition to the primary inductance requirement forsampling time, the LT3511 has internal circuit constraintsthat prevent the switch from staying on for less than 100ns.If the inductor current exceeds the desired current limitduring that time, oscillation may occur at the output asthe current control loop will lose its ability to regulate.The following equation, based on maximum input voltage,must also be followed in selecting primary-side magnetizing inductance:LPRI tON(MIN) VIN(MAX)IPEAK(MIN)tON(MIN) 100nsIPEAK(MIN) 55mA3511fc11

LT3511APPLICATIONS INFORMATIONVSWVSW 150V 150V 140VVLEAKAGE 100V 100Vt OFF 400nst OFF 400nsTIMEtSP 150nswithout ClamptSP 150ns3511 F05TIMEwith ClampFigure 5. Maximum Voltages for SW Pin Flyback WaveformLSLeakage Inductance and Clamp CircuitsZTransformer leakage inductance (on either the primary orsecondary) causes a voltage spike to appear at the primaryafter the output switch turns off. This spike is increasinglyprominent at higher load currents where more stored energy must be dissipated. When designing an application,adequate margin should be kept for the effect of leakagevoltage spikes. In most cases the reflected output voltageon the primary plus VIN should be kept below 100V. Thisleaves at least 50V of margin for the leakage spike acrossline and load conditions. A larger voltage margin will beneeded for poorly wound transformers or for excessiveleakage inductance. Figure 5 illustrates this point. Minimizetransformer leakage inductance.A clamp circuit is recommended for most applications.Two circuits that can protect the internal power switchinclude the RCD (resistor-capacitor-diode) clamp and theDZ (diode-Zener) clamp. The clamp circuits dissipate thestored energy in the leakage inductance. The DZ clampis the recommended clamp for the LT3511. Simplicity ofdesign, high clamp voltages, and low power levels make theDZ clamp the preferred solution. Additionally, a DZ clampensures well defined and consistent clamping voltages.Figure 5 shows the clamp effect on the switch waveformand Figure 6 shows the connection of the DZ clamp.D3511 F06Figure 6. DZ ClampProper care must be taken when choosing both the diodeand the Zener diode. Schottky diodes are typically the bestchoice, but some PN diodes can be used if they turn onfast enough to limit the leakage inductance spike. Choosea diode that has a reverse-voltage rating higher than themaximum switch voltage. The Zener diode breakdownvoltage should be chosen to balance power loss and switchvoltage protection. The best compromise is to choose thelargest voltage breakdown. Use the following equation tomake the proper choice:VZENER(MAX) 150V – VIN(MAX)For an application with a maximum input voltage of 72V,choose a 68V VZENER which has VZENER(MAX) at 72V, whichwill be below the 78V maximum.The power loss in the clamp will determine the power rating of the Zener diode. Power loss in the clamp is highest3511fc12

LT3511APPLICATIONS INFORMATIONat maximum load and minimum input voltage. The switchcurrent is highest at this point along with the energy storedin the leakage inductance. A 0.5W Zener will satisfy mostapplications when the highest VZENER is chosen. Choosinga low value for VZENER will cause excessive power loss asshown in the following equations:1 L IPK(VIN(MIN))2 fSW 2 C NPS ( VOUT VF ) 1 VZENER – NPS ( VOUT VF ) L C Leakage InductanceVOUT IOUT 2IPK(VIN(MIN)) η VIN(MIN) DVIN(MIN)DZ Power Loss fSW 11 tON tOFF LPRI IPK(VIN(MIN)) LPRI IPK(VIN(MIN)) VIN(MIN)NPS ( VOUT VF )Tables 2 and 3 show some recommended diodes andZener diodes.Table 2. Recommended Zener SOD-123On MHZ5267B750.5SOD-123BZX84J-68680.5SOD323F NXPBZX100A1000.5SOD323FPARTSecondary Leakage InductanceIn addition to primary leakage inductance, secondaryleakage inductance exhibits an important effect on application design. Secondary leakage inductance formsan inductive divider on the transformer secondary. Theinductive divider effectively reduces the size of theprimary-referred flyback pulse. The smaller flybackpulse results in a higher regulated output voltage. Theinductive divider effect of secondary leakage inductanceis load independent. RFB/RREF ratio adjustments can accommodate this effect to the extent secondary leakageinductance is a constant percentage of mutual inductance(over manufacturing variations).Winding Resistance EffectsCentralSemiconductorTable 3. Recommended DiodesPARTI (A)VREVERSE(V)BAV21W0.625200SOD-123 Diodes Inc.BAV20W0.625150SOD-123CASEnode and transformer leakage inductance cause thedelay. The leakage inductance also causes a very fastvoltage spike on the primary side of the transformer.The amplitude of the leakage spike is largest whenpower switch current is highest. Introduction of aninternal fixed delay between switch turn-off and thestart of sampling provides immunity to the phenomenadiscussed above. The LT3511 sets internal blanking to150ns. In certain cases leakage inductance spikes lastlonger than the internal blanking, but will not significantly affect output regulation.VENDORLeakage Inductance BlankingWhen the power switch turns off, the flyback pulseappears. However, a finite time passes before the transformer primary-side voltage waveform approximatelyrepresents the output voltage. Rise time on the SWResistance in either the primary or secondary will reduce overall efficiency (POUT/PIN). Good output voltageregulation will be maintained independent of windingresistance due to the boundary mode operation of theLT3511.Bifilar WindingA bifilar, or similar winding technique, is a good way tominimize troublesome leakage inductances. However, remember that this will also increase primary-to-secondarycapacitance and limit the primary-to-secondary breakdownvoltage, so bifilar winding is not always practical. TheLinear Technology applications group is available andextremely qualified to assist in the selection and/or designof the transformer.3511fc13

LT3511APPLICATIONS INFORMATIONAPPLICATION DESIGN CONSIDERATIONSIterative Design ProcessThe LT3511 uses a unique sampling scheme to regulatethe isolated output voltage. The use of this

Isolated Flyback Converter The LT 3511 is a high voltage monolithic switching regula-tor specifically designed for the isolated flyback topology. No third winding or opto-isolator is required for regula-tion as the part senses output voltage directly from

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The MAX253 monolithic oscillator/power-driver is specifically designed to provide isolated power for an isolated RS-485 or RS-232 data interface. The device drives a center-tapped transformer primary from a 5V or 3.3V DC power supply. The secondary can be wound to provide any isolated voltage needed at power levels up to 1W.

The SN6501 is a monolithic oscillator/power-driver,specifically designed for small form factor, isolated power supplies in isolated interface applications. It drives a low-profile,center-tappedtransformer primary from a 3.3 V or 5 V DC power supply. The secondary can be wound to provide any isolated voltage based on transformer turns ratio.

B ) Use of different industrial waste as refractory raw materials as below Fused Alumina Chrome grains for bricks and monolithic. Forsterite bonded Fused Alumina chrome grains for bricks and monolithic. MgO enriched Fused Alumina chrome grains for bricks and monolithic. Fused corundum spinel grains for bricks and monolithic.

IDS 700 1.0 V A Pin 3 Gnd Pin 3 1000 µF to ground Vin 400 500 Power Supply Voltage (Pin 3) VCC 40 V T Input Voltage Range Voltage Feedback Input (Pin 10) Compensation (Pin 9) Overvoltage Protection Input (Pin 11) R (Pin 6) CT (Pin 7) VIR –1.0 to Vreg V θ Thermal Characteristics P Suff

APPLICATIONS OF MONOLITHIC BRIDGE DRIVERS High power monolithic bridge drivers are an attractive replacement for discrete transistors and half bridges in applications such as DC motor and stepper motor driving. This application guide describes three such de-vices - the L293, L293E and L29

Amplifier Configurations . Voltage Amplifier: Voltage input and Voltage output The controlled source is a Voltage -controlled-Voltage Source A. v Open Circuit Voltage Gain can be found by applying a voltage source with R. s 0, and measuring the open circuit output voltage(no load or R. L infinity) Source Amplifier Load . Amplifier Input .

need a basic knowledge of school guidance and counselling techniques to address the personal and social problems of students that they may encounter in the classroom. This course will assist Student Teachers in understanding the role of various members of a guidance and counselling system in supporting students in addressing their future and social challenges. They will master the basic skills .