LT3748 – 100V Isolated Flyback Controller

1 LT374813748fbFor more information Isolated Flyback ControllerThe LT 3748 is a switching regulator Controller specifically designed for the Isolated Flyback topology and capable of high power. It drives a low side external N-channel power MOSFET from an internally regulated 7V supply. No third winding or opto-isolator is required for regulation as the part senses the Isolated output voltage directly from the primary-side Flyback waveform. The LT3748 utilizes boundary mode to provide a small magnetic solution without compromising load regulation. Operating frequency is set by load current and transformer magnetizing inductance. The gate drive of the LT3748 combined with a suitable external MOSFET allow it to deliver load power up to several tens of watts from input voltages as high as LT3748 is available in a high voltage 16-lead MSOP package with four leads , 12V Output, Isolated Telecom Supply n5V to 100V Input Voltage Range Average Gate Drive Source and Sink Current nBoundary Mode Operation nNo Transformer Third Winding or Opto-Isolator Required for Regulation nPrimary-Side Winding Feedback Load Regulation nVOUT Set with Two External Resistors nINTVCC Pin for Control of Gate Driver Voltage nProgrammable Soft Start nProgrammable Undervoltage Lockout nAvailable in MSOP Package nIsolated Telecom Converters nHigh Power Automotive Supplies nIsolated Industrial Power Supplies nMilitary and High Temperature ApplicationsOutput Load and Line Regulationtypical applicationL, LT.

2 LT C, LT M, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Protected by Patents, including 5438499 and 36V TO 72 VVOUT+12V2 AVOUT VIN4 H100 10kLOAD CURRENT (A) (V) TA01bVIN = 72 VVIN = 48 VVIN = 36 VLT374823748fbFor more information MaxiMuM ratingsVIN, RFB ..100 VVIN to RFB .. 5 VEN/UVLO .. , 100 VINTVCC ..VIN + , 20 VSS, VC, TC, .. Junction Temper atur e Range (Note 2) LT374 8E/LT374 8I .. 40 C to 125 C LT374 8H .. 40 C to 150 C LT3748 MP .. 55 C to 150 CStorage Temper atur e Range .. 65 C to 150 CorDer inForMationelectrical characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25 C. VIN = 10V, unless otherwise FREE FINISHTAPE AND REELPART MARKING*PACKAGE DESCRIPTIONTEMPERATURE RANGELT3748 EMS#PBFLT3748 EMS#TRPBF374816-Lead Plastic MSOP 40 C to 125 CLT3748 IMS#PBFLT3748 IMS#TRPBF374816-Lead Plastic MSOP 40 C to 125 CLT3748 HMS#PBFLT3748 HMS#TRPBF374816-Lead Plastic MSOP 40 C to 150 CLT3748 MPMS#PBFLT3748 MPMS#TRPBF374816-Lead Plastic MSOP 55 C to 150 CConsult LT C Marketing for parts specified with wider operating temperature ranges.

3 *The temperature grade is identified by a label on the shipping more information on lead free part marking, go to: For more information on tape and reel specifications, go to: Voltage Rangel5100 VQuiescent CurrentNot switching VEN/UVLO = 1mA AVIN Quiescent Current, INTVCC OverdrivenVINTVCC = 10V300450 AINTVCC Voltage Pin Regulation Dropout(VIN VINTVCC), IINTVCC = 10mA, VIN = VINTVCC Undervoltage LockoutFalling Pin ThresholdEN/UVLO Pin Voltage Pin Hysteresis CurrentEN/UVLO = ASoft-Start CurrentVSS = (Note 3)5 ASoft-Start Reset Current3mA(Note 1)pin conFiguration135678 VINEN/UVLOINTVCCGATESENSEGND16141211109 RFBRREFTCVCSSGNDTOP VIEWMS PACKAGE16 (12)-LEAD PLASTIC MSOP TJMAX = 150 C, JA = 90 C/WLT374833748fbFor more information 1: Stresses beyond those listed under Absolute Maximum Ratings may cause permanent damage to the device. Exposure to any Absolute Maximum Rating condition for extended periods may affect device reliability and 2: The LT3748E is guaranteed to meet performance specifications from 0 C to 125 C junction temperature.

4 Specifications over the 40 C to 125 C operating junction temperature range are assured by design characterization and correlation with statistical process controls. The LT3748I is guaranteed over the full 40 C to 125 C operating junction temperature range. The LT3748H is guaranteed over the full 40 C to 150 C operating junction temperature range. The LT3748MP is guaranteed over the full 55 C to 150 C operating junction temperature range. High junction temperatures degrade operating lifetimes. Operating lifetime is derated at junction temperatures greater than 125 3: Current flows out of the SENSE Current Limit ThresholdVC = l95 90100 100105 110mV mVMinimum SENSE Current Limit ThresholdVC = 0V15mVMaximum to Minimum SENSE Threshold Overcurrent ThresholdVC = Input Bias CurrentVSENSE = 10mV (Note 3)101520 ARREF VoltageVC = VRREF Voltage Line Regulation5V < VIN < Pin Bias Current(Note 3)l35500nATC Current into RREFRTC = AError Amplifier Voltage Gain115V/ VError Amplifier Transconductance I = 10 A155 mhosVC Source CurrentVC = , VRREF = 45 AVC Sink CurrentVC = , VRREF = 2V48 AFlyback Comparator Trip CurrentCurrent into RFB Pin, RREF = AMinimum GATE Off-Time700nsMinimum GATE On-Time250nsMaximum Discontinuous Off-TimeVC = 0V24 sMaximum GATE Off-TimeVRREF = sMaximum GATE On-TimeVSENSE = 0V55 sGATE Output Rise TimeCL = 3300pF, 10% to 90%16nsGATE Output Fall TimeCL = 3300pF, 10% to 90%16nsGATE Output Low (VOL)

5 Output High (VOH)VINTVCC characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25 C. VIN = 10V, unless otherwise more information perForMance characteristicsINTVCC Undervoltage Lockout vs TemperatureINTVCC Voltage vs VIN VoltageINTVCC Regulator Dropout vs INTVCC CurrentSoft-Start Current vs TemperatureOutput Regulation vs TemperatureQuiescent Current vs TemperatureINTVCC Voltage vs TemperatureTA = 25 C, unless otherwise Current vs VIN VoltageINTVCC Dropout vs TemperatureTEMPERATURE ( C) (V) 252575125 FIGURE 16 CIRCUITIOUT = 150mA ON EACH OUTPUTVIN = 12 VTEMPERATURE ( C) 55 QUIESCENT CURRENT (mA) = 0 VINTVCC = OPENVIN = 72 VVIN = 36 VVIN = 12 VVIN = 6 VVIN (V)0 QUIESCENT CURRENT (mA) = 0 VINTVCC = OPENTEMPERATURE ( C) VOLTAGE (V) 2525100125150 IINTVCC = 0mAIINTVCC = 10mAVIN VOLTAGE (V)46 VINTVCC (V) = 0mAIINTVCC = 10mATEMPERATURE ( C) 55 INTVCC UVLO (V) 1253748 THRESHOLDFALLING THRESHOLDINTVCC CURRENT (mA)

6 00 INTVCC REGULATOR DROPOUT (V) = 5V102030403748 G07150 C100 C25 C 50 CTEMPERATURE ( C) 550 INTVCC DROPOUT (V) 252575125 VIN = 5 VIINTVCC = 20mAIINTVCC = 10mAIINTVCC = 5mATEMPERATURE ( C) 550 SOFT-START CURRENT ( A)12340501001503748 G0956 252575125LT374853748fbFor more information perForMance characteristicsError Amplifier Transconductance vs TemperatureError Amplifier Output Current vs RREF Pin VoltageSENSE Pin Threshold vs TemperatureEN/UVLO Current vs TemperatureEN/UVLO Threshold vs TemperatureTC Pin Voltage vs TemperatureTA = 25 C, unless otherwise Discontinuous Off-Time vs TemperatureGATE Rise and Fall Time vs ChargeGATE Rise and Fall Time vs INTVCC VoltageTEMPERATURE ( C) 550EN/UVLO CURRENT ( A) 252575125 VEN/UVLO = = = ( C) 55EN/UVLO THRESHOLD (V) ( C) 55TC VOLTAGE (V) ( C) 55100 TRANSCONDUCTANCE ( mhos)110130140150200170050753748 G13120180190160 2525100125150 VIN = 100 VVIN = 6 VVREF (V)0 IVC ( A) G14 20 40 10103050 30 50 C100 C25 C 50 CTEMPERATURE ( C) 55 SENSE THRESHOLD (mV)801201503748 G15400050100 2525751251606010020140 OVERCURRENTVC = = ( C) 5520 MAXIMUM DISCONTINUOUS OFF-TIME ( s)212324253027050753748 G1622282926 2525100125150 TOTAL GATE CHARGE (nC)00 GATE RISE AND FALL TIME (ns)AVERAGE GATE SOURCE, SINK CURRENT (A)10203040204060803748 TIMEFALL TIMEQ = C VVINTVCC = 7 Vtr, tf 10% TO 90%AVERAGECURRENTVINTVCC (V)0 GATE RISE AND FALL TIME (ns)1015203748 G1850510152520 FALLINGRISINGCGATE = , tf 10% TO 90%LT374863748fbFor more information FunctionsVIN (Pin 1) Input Voltage.

7 This pin supplies current to the internal start-up circuitry and is the reference voltage for the feedback circuitry connected to the RFB pin. This pin must be locally bypassed with a (Pin 3): Enable/ Undervoltage Lockout. A resistor divider connected to VIN is tied to this pin to program the minimum input voltage at which the LT3748 will operate. At a voltage below ~ , the part draws less than 1 A quiescent current. When below but above ~ , the part will draw quiescent current but will not regulate the INTVCC supply or power the gate drive circuitry. Above , all internal circuitry will start and the SS pin will source 5 A. When EN/UVLO falls below , A is sunk from the pin to provide programmable hysteresis for undervoltage (Pin 5): Gate Driver Bias Voltage. This pin supplies current to the internal gate driver circuitry of the LT3748 . The INTVCC pin must be locally bypassed with a capacitor.

8 This pin may also be connected to VIN if a third winding is not used and if VIN 20V. If a third winding is used, the INTVCC voltage should be lower than the input voltage for proper (Pin 6): N-Channel MOSFET Gate Driver Output. Switches between INTVCC and (Pin 7): The Current Sense Input for the Control Loop. Kelvin connect this pin to the positive terminal of the switch current sense resistor, RSENSE, in the source of the N-channel MOSFET. The negative terminal of the current sense resistor should be connected to the GND plane close to the (Pins 8, 9): (Pin 10): Soft-Start Pin. This pin delays start-up and clamps VC pin voltage. Soft-start timing is set by the size of the external capacitor at the pin. switching starts when VSS reaches ~ (Pin 11): Compensation Pin for the Internal Error Amplifier. Connect a series RC from this pin to ground to compensate the switching regulator.

9 A 100pF capacitor in parallel helps eliminate (Pin 12): Output Voltage Temperature Compensation. Connect a resistor to ground to produce a current pro-portional to absolute temperature to be sourced into the RREF node. ITC = RREF (Pin 14): Input Pin for the External Ground-Referred Reference Resistor. The resistor at this pin should be , but for convenience in selecting a resistor divider ratio, the value may range from to The resistor should be as close to the LT3748 as (Pin 16): Input Pin for the External Feedback Resistor. This pin is connected to the transformer primary at the external MOSFET power switch. The ratio of this resistor to the RREF resistor, times the internal bandgap reference, determines the output voltage (plus the effect of any non-unity transformer turns ratio). The average current through this resistor during the Flyback period should be approximately 200 A.

10 The resistor should be as close to the LT3748 as more information DiagraMMASTERLATCHBOUNDARYMODE DETECTVARIABLEDELAY TIMERINTVCCCBIASR1R2 CSSRTCVOUT+VOUT VINTCSSVINSENSEVCT1 NPS:15 A20 ACOUTDOUTCINLPRILSECRFBRREFRSENSERCCC+ INTERNALREFERENCEANDREGULATORSCURRENTLIM IT50 s MAXON TIMER50 s MAXOFF TIMERTCCURRENTRQSRSA1 ERROR AMP100mV+ + A3+ + AGND8, 9 3748 more information LT3748 is a current mode switching regulator con-troller designed specifically for the Isolated Flyback topol-ogy. The special problem normally encountered in such circuits is that information relating to the output voltage on the Isolated secondary side of the transformer must be communicated to the primary side in order to maintain regulation. Historically, this has been done with opto- isolators or extra transformer windings. Opto-isolator circuits waste output power and the extra components increase the cost and physical size of the power supply.