LT3511 Monolithic High Voltage Isolated Flyback Converter Features Description 4.5V to 100V Input Voltage Range Internal 240mA, 150V Power Switch Boundary Mode Operation No Transformer Third Winding or Opto-Isolator Required for Regulation n Improved Primary-Side Winding Feedback Load Regulation nV OUT Set with Two External Resistors n BIAS Pin for Internal Bias Supply and Power Switch Driver n No External Start-Up Resistor n 16-Lead MSOP Package The 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 the primary-side flyback waveform. The device integrates a 240mA, 150V power switch, high voltage circuitry, and control into a high voltage 16-lead MSOP package with four leads removed. n n n n Applications Isolated Telecom Power Supplies Isolated Auxiliary/Housekeeping Power Supplies n Isolated Industrial, Automotive and Medical Power Supplies n n The LT3511 operates from an input voltage range of 4.5V to 100V and delivers up to 2.5W of isolated output power. Two external resistors and the transformer turns ratio easily set the output voltage. Off-the-shelf transformers are available for several applications. The high level of integration and the use of boundary mode operation results in a simple, clean, tightly regulated application solution to the traditionally tough problem of isolated power delivery. L, LT, LTC, LTM, Burst Mode, Linear Technology and the Linear logo are registered trademarks and No RSENSE is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. Protected by U.S. Patents, including 5438499, 7471522. Typical Application 48V to 5V Isolated Flyback Converter 1µF 4:1 1M EN/UVLO 43.2k VIN LT3511 RFB RREF 169k 10k TC SW VC 69.8k • 300µH • 5.25 VOUT+ 5V 0.3A 19µH 5.20 5.15 22µF VOUT– 5.10 VOUT (V) VIN 36V TO 72V Output Load and Line Regulation VIN = 48V 5.05 VIN = 36V 5.00 VIN = 72V 4.95 4.90 4.85 GND BIAS 4.80 16.9k 3.3nF 4.7µF 3511 TA01a 4.75 0 50 150 200 100 LOAD CURRENT (mA) 250 300 3511 TA01b 3511fa 1 LT3511 Absolute Maximum Ratings (Note 1) Pin Configuration SW (Note 4).............................................................150V VIN, EN/UVLO...........................................................100V RFB.............................................................100V, VIN ±6V BIAS....................................................................VIN, 20V RREF, TC, VC.................................................................6V Operating Junction Temperature Range (Note 2) LT3511E, LT3511I................................ –40°C to 125°C LT3511H.............................................. –40°C to 150°C LT3511MP........................................... –55°C to 150°C Storage Temperature Range................... –65°C to 150°C TOP VIEW EN/UVLO 1 16 SW VIN 3 14 RFB GND BIAS NC GND 5 6 7 8 12 11 10 9 RREF TC VC GND MS PACKAGE 16(12)-LEAD PLASTIC MSOP θJA = 90°C/W Order Information LEAD FREE FINISH TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LT3511EMS#PBF LT3511EMS#TRPBF 3511 16-Lead Plastic MSOP –40°C to 125°C LT3511IMS#PBF LT3511IMS#TRPBF 3511 16-Lead Plastic MSOP –40°C to 125°C LT3511HMS#PBF LT3511HMS#TRPBF 3511 16-Lead Plastic MSOP –40°C to 125°C LT3511MPMS#PBF LT3511MPMS#TRPBF 3511 16-Lead Plastic MSOP –55°C to 150°C Consult 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 Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 24V unless otherwise noted. PARAMETER MAX UNITS 100 15 V V 2.7 0 3.5 mA µA 1.15 1.21 1.27 V 2.0 2.6 0 3.3 µA µA Maximum Current Limit 240 330 430 mA Minimum Current Limit 35 60 90 mA 1.18 1.17 1.20 1.215 1.23 V V 0.01 0.03 %/V 80 400 nA Input Voltage Range CONDITIONS MIN l VIN = BIAS Quiescent Current Not Switching VEN/UVLO = 0.2V EN/UVLO Pin Threshold EN/UVLO Pin Voltage Rising EN/UVLO Pin Current VEN/UVLO = 1.1V VEN/UVLO = 1.4V l 6 4.5 Maximum Switching Frequency Switch VCESAT 650 ISW = 100mA l RREF Voltage Line Regulation 6V < VIN < 100V RREF Pin Bias Current (Note 3) Error Amplifier Voltage Gain ∆I = 2µA kHz 0.3 RREF Voltage Error Amplifier Transconductance TYP l V 150 V/V 140 µmhos 3511fa 2 LT3511 Electrical Characteristics The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VIN = 24V unless otherwise noted. PARAMETER CONDITIONS MIN TYP Minimum Switching Frequency TC Current into RREF RTC = 53.6k BIAS Pin Voltage Internally Regulated 3 Note 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 lifetime. Note 2: The LT3511E is guaranteed to meet performance specifications from 0°C to 125°C junction temperature. Specifications over the –40°C to 125°C operating junction temperature range are assured by design, characterization and correlation with statistical process controls. The LT3511I is guaranteed to meet performance specifications from –40°C to 125°C operating junction temperature range. The LT3511H is guaranteed Output Voltage 5.20 Quiescent Current 5.15 3 2 4.90 4.85 VIN = 24V VIN = 48V VIN = 100V 4.80 0 0 –50 –25 25 50 75 100 125 150 TEMPERATURE (°C) 0 Switch VCESAT Switch Current Limit 350 25 50 75 100 125 150 TEMPERATURE (°C) Quiescent Current vs VIN 4 MAXIMUM CURRENT LIMIT 3 300 250 IQ (mA) 800 CURRENT LIMIT (mA) SWITCH VCESAT VOLTAGE (mV) 0 3511 G03 400 200 VIN = 24V, 10mA VIN = 24V, NO LOAD 3511 G02 1000 400 V 3.0 2.0 –50 –25 25 50 75 100 125 150 TEMPERATURE (°C) 3511 G01 600 3.2 2.5 1 4.75 –50 –25 3.1 3.5 BIAS VOLTAGE (V) IQ (mA) VOUT (V) 4.95 µA BIAS Pin Voltage 4 5.00 9.5 4.0 5.10 5.05 kHz TA = 25°C, unless otherwise noted. 5 VIN = 48V UNITS to meet performance specifications from –40°C to 150°C operating junction temperature range. The LT3511MP is guaranteed over the full –55°C to 150°C operating junction range. High junction temperatures degrade operating lifetimes. Operating lifetime is derated at junction temperatures 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 waveforms of the SW pin should keep the pedestal of the flyback waveform below 100V as shown in Figure 5. Typical Performance Characteristics 5.25 MAX 40 200 2 150 100 1 MINIMUM CURRENT LIMIT 50 0 0 50 100 150 200 250 SWITCH CURRENT (mA) 300 350 3511 G04 0 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 3511 G05 0 0 20 60 40 VOLTAGE (V) 80 100 3511 G06 3511fa 3 LT3511 Typical Performance Characteristics EN/UVLO Pin (Hysteresis) Current vs Temperature EN/UVLO = 1.2V EN/UVLO PIN CURRENT (µA) EN/UVLO PIN CURRENT (µA) 4 3 2 1 0 –50 –25 0 EN/UVLO Threshold vs Temperature EN/UVLO Pin Current vs VEN/UVLO 30 3.0 25 2.5 20 15 10 2.0 1.5 1.0 0.5 5 0 25 50 75 100 125 150 TEMPERATURE (°C) EN/UVLO THRESHOLD (V) 5 TA = 25°C, unless otherwise noted. 1 20 40 60 80 VEN/UVLO VOLTAGE (V) 3511 G07 0 –50 –25 100 0 25 50 75 100 125 150 TEMPERATURE (°C) 3511 G09 3511 G08 Maximum Frequency vs Temperature Minimum Frequency vs Temperature 1000 100 800 80 EN/UVLO Shutdown Threshold vs Temperature 0.9 600 400 200 EN/UVLO THRESHOLD (V) MINIMUM FREQUENCY (kHz) MAXIMUM FREQUENCY (kHz) 0.8 60 40 20 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 0 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 3511 G10 0 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 3511 G11 3511 G14 Light Load Discontinuous Mode Waveform Boundary Mode Waveform 20V/DIV 20V/DIV 1µs/DIV 3511 G12 2µs/DIV 3511 G13 3511fa 4 LT3511 Pin Functions EN/UVLO (Pin 1): Enable/Undervoltage Lockout. The EN/ UVLO pin is used to start up the LT3511. Pull the pin to 0V to shut down the LT3511. This pin has an accurate 1.21V threshold and can be used to program an undervoltage lockout (UVLO) threshold using a resistor divider from supply to ground. A 2.6μA pin current hysteresis allows the programming of undervoltage lockout (UVLO) hysteresis. EN/UVLO can be directly connected to VIN. If left open circuit the part will not power up. VIN (Pin 3): Input Supply Pin. This pin supplies current to the internal start-up circuitry, and serves as a reference voltage 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 be tied directly to the local ground plane. BIAS (Pin 6): Bias Voltage. This pin supplies current to the switch driver and internal circuitry of the LT3511. This pin may also be connected to VIN if a third winding is not used and if VIN < 20V. The part can operate down to 4.5V when BIAS and VIN are connected together. If a third winding is used, the BIAS voltage should be lower than the input voltage and greater than 3.3V for proper operation. BIAS must be bypassed with a 4.7μF capacitor placed close to the pin. VC (Pin 10): Compensation Pin for Internal Error Amplifier. Connect a series RC from this pin to ground to compensate the switching regulator. An additional 100pF capacitor from this pin to ground helps eliminate noise. TC (Pin 11): Output Voltage Temperature Compensation. Connect a resistor to ground to produce a current proportional to absolute temperature to be sourced into the RREF node. ITC = 0.55V/RTC. RREF (Pin 12): Input Pin for External Ground-Referred Reference Resistor. The resistor at this pin should be 10k. For nonisolated applications, a traditional resistor voltage divider 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 the internal bandgap reference, determines the output voltage (plus the effect of any non-unity transformer turns ratio). For nonisolated applications, this pin should be connected to GND with a 1M resistor. SW (Pin 16): Switch Pin. Collector of the internal power switch. Minimize trace area at this pin to minimize EMI and voltage spikes. 3511fa 5 LT3511 Block Diagram D1 VIN VOUT + T1 C1 L1A L1B C2 R3 VOUT – N:1 TC CURRENT TC VIN Q3 SW RFB FLYBACK ERROR AMP Q2 R5 I2 1.2V –g m + – + ONE SHOT CURRENT COMPARATOR A2 – A1 + S S BIAS DRIVER BIAS R1 EN/UVLO R 1.2V R2 + A5 – 3µA Q4 INTERNAL REFERENCE AND REGULATORS Q1 Q MASTER LATCH C4 – VIN RREF R4 + V1 120mV + – A4 RSENSE 0.02Ω GND OSCILLATOR VC R6 C3 3511 BD 3511fa 6 LT3511 Operation The LT3511 is a current mode switching regulator IC designed specifically for the isolated flyback topology. The key problem in isolated topologies is how to communicate information regarding the output voltage from the isolated secondary side of the transformer to the primary side. Historically, optoisolators or extra transformer windings communicate this information across the transformer. Optoisolator circuits waste output power, and the extra components increase the cost and physical size of the power supply. Optoisolators can also exhibit trouble due to limited dynamic response, nonlinearity, unit-to-unit variation and aging over life. Circuits employing an extra transformer winding also exhibit deficiencies. Using an extra winding adds to the transformer’s physical size and cost, and dynamic response is often mediocre. In the LT3511, the primary-side flyback pulse provides information about the isolated output voltage. In this manner, neither optoisolator nor extra transformer winding is required for regulation. Two resistors program the output voltage. Since this IC operates in boundary mode, the part calculates output voltage from the switch pin when the secondary current is almost zero. The Block Diagram shows an overall view of the system. Many of the blocks are similar to those found in traditional switching regulators including internal bias regulator, oscillator, logic, current amplifier, current comparator, driver, and output switch. The novel sections include a special flyback error amplifier and a temperature compensation circuit. In addition, the logic system contains additional logic for boundary mode operation. The LT3511 features boundary mode control, where the part operates at the boundary between continuous conduction mode and discontinuous conduction mode. The VC pin controls the current level just as it does in normal current mode operation, but instead of turning the switch on at the start of the oscillator period, the part turns on the switch when the secondary-side winding current is zero. Boundary Mode Operation Boundary mode is a variable frequency, current mode switching scheme. The switch turns on and the inductor current increases until a VC pin controlled current limit. After the switch turns off, the voltage on the SW pin rises to the output voltage divided by the secondary-to-primary transformer turns ratio plus the input voltage. When the secondary current through the diode falls to zero, the SW pin voltage falls below VIN. A discontinuous conduction mode (DCM) comparator detects this event and turns the switch back on. Boundary mode returns the secondary current to zero every cycle, so parasitic resistive voltage drops do not cause load regulation errors. Boundary mode also allows the use of a smaller transformer compared to continuous conduction mode and does not exhibit subharmonic oscillation. At low output currents, the LT3511 delays turning on the switch, and thus operates in discontinuous mode. Unlike traditional flyback converters, the switch has to turn on to update the output voltage information. Below 0.6V on the VC pin, the current comparator level decreases to its minimum value, and the internal oscillator frequency decreases. With the decrease of the internal oscillator, the part starts to operate in DCM. The output current is able to decrease while still allowing a minimum switch off time for the flyback error amplifier. The typical minimum internal oscillator frequency with VC equal to 0V is 40kHz. 3511fa 7 LT3511 Applications Information PSUEDO DC THEORY In the Block Diagram, RREF (R4) and RFB (R3) are external resistors used to program the output voltage. The LT3511 operates similar to traditional current mode switchers, except in the use of a unique error amplifier, which derives its feedback information from the flyback pulse. Operation is as follows: when the output switch, Q1, turns off, its collector voltage rises above the VIN rail. The amplitude of this flyback pulse, i.e., the difference between it and VIN, is given as: VFLBK = (VOUT + VF + ISEC • ESR) • NPS VF = D1 forward voltage the effect of nonzero secondary output impedance (ESR). Boundary control mode minimizes the effect of this impedance term. Temperature Compensation The first term in the VOUT equation does not have temperature dependence, but the diode forward drop has a significant negative temperature coefficient. A positive temperature coefficient current source connects to the RREF pin to compensate. A resistor to ground from the TC pin sets the compensation current. The following equation explains the cancellation of the temperature coefficient: ISEC = Transformer secondary current δVF R 1 δVTC = – FB • • or, δT RTC NPS δT ESR = Total impedance of secondary circuit NPS = Transformer effective primary-to-secondary turns ratio RFB and Q2 convert the flyback voltage into a current. Nearly all of this current flows through RREF to form a groundreferred voltage. The resulting voltage forms the input to the flyback error amplifier. The flyback error amplifier samples the voltage information when the secondary side winding current is zero. The bandgap voltage, 1.20V, acts as the reference for the flyback error amplifier. The relatively high gain in the overall loop will then cause the voltage at RREF to be nearly equal to the bandgap reference voltage VBG. The resulting relationship between VFLBK and VBG approximately equals: ⎛ VFLBK ⎞ VBG ⎛R ⎞ or VFLBK = VBG ⎜ FB ⎟ ⎜ ⎟= ⎝ RREF ⎠ ⎝ RFB ⎠ RREF VBG = Internal bandgap reference Combination of the preceding expression with earlier derivation 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 ratio and diode forward voltage drop. Additionally, it includes 8 RTC = δV –RFB 1 R • • TC ≈ FB NPS δVF / δT δT NPS (δVF/δT) = Diode’s forward voltage temperature coefficient (δVTC/δT) = 2mV VTC = 0.55V Experimentally verify the resulting value of RTC and adjust as necessary to achieve optimal regulation over temperature. The addition of a temperature coefficient current modifies the expression of output voltage as follows: ⎛ R ⎞⎛ 1 ⎞ VOUT = VBG ⎜ FB ⎟ ⎜ ⎟ – VF ⎝ RREF ⎠ ⎝ NPS ⎠ ⎛V ⎞ R – ⎜ TC ⎟ • FB – ISEC (ESR) ⎝ RTC ⎠ NPS Output Power A flyback converter has a complicated relationship between the input and output current compared to a buck or a boost. A boost has a relatively constant maximum input current regardless of input voltage and a buck has a relatively constant maximum output current regardless of input voltage. This is due to the continuous nonswitching behavior of the two currents. A flyback converter has both discontinuous input and output currents which makes it 3511fa LT3511 Applications Information similar to a nonisolated buck-boost. The duty cycle will affect the input and output currents, making it hard to predict output power. In addition, the winding ratio can be changed to multiply the output current at the expense of a higher switch voltage. One design example would be a 5V output converter with a minimum input voltage of 36V and a maximum input voltage of 72V. A four-to-one winding ratio fits this design example perfectly and outputs close to 1.6W at 72V but lowers to 1W at 36V. The graphs in Figures 1-4 show the typical maximum output power possible for the output voltages 3.3V, 5V, 12V and 24V. The maximum power output curve is the calculated output power if the switch voltage is 100V during the offtime. 50V of margin is left for leakage voltage spike. To achieve this power level at a given input, a winding ratio value must be calculated to stress the switch to 100V, resulting in some odd ratio values. The following curves are examples of common winding ratio values and the amount of output power at given input voltages. The equations below calculate output power: Power = η • VIN • D • IPEAK • 0.5 Efficiency = η = ~85% Duty Cycle = D = Peak switch current = IPEAK = 0.26AV 3.0 3.5 3.0 N = NPS(MAX) 2.0 N = 15 N = 12 N = 10 N=8 OUTPUT POWER (W) OUTPUT POWER (W) 2.5 N=6 1.5 N=4 1.0 N=2 0.5 0 ( VOUT + VF ) • NPS ( VOUT + VF ) • NPS + VIN N=5 2.5 N=4 N = NPS(MAX) N=3 2.0 N=2 1.5 N=1 1.0 0.5 0 20 40 60 INPUT VOLTAGE (V) 0 100 80 0 20 40 60 INPUT VOLTAGE (V) 80 3511 F01 3511 F03 Figure 1. Output Power for 3.3V Output Figure 3. Output Power for 12V Output 3.0 3.0 OUTPUT POWER (W) N = NPS(MAX) 2.0 1.5 N=3 N=2 1.0 N=1 0.5 0 20 40 60 INPUT VOLTAGE (V) 80 N = NPS(MAX) 2.5 N=7 N=6 N=5 N=4 OUTPUT POWER (W) N=8 2.5 0 100 N=2 2.0 N=1 1.5 1.0 0.5 100 3511 F02 Figure 2. Output Power for 5V Output 0 0 20 40 60 INPUT VOLTAGE (V) 80 100 3511 F04 Figure 4. Output Power for 24V Output 3511fa 9 LT3511 Applications Information TRANSFORMER DESIGN CONSIDERATIONS Successful application of the LT3511 relies on proper transformer specification and design. Carefully consider the following information in addition to the traditional guidelines associated with high frequency isolated power supply transformer design. Linear Technology has worked with several leading magnetic component manufacturers to produce pre-designed flyback transformers for use with the LT3511. Table 1 shows the details of these transformers. Table 1. Predesigned Transformers TRANSFORMER PART NUMBER LPRI (μH) LEAKAGE (μH) NP:NS:NB ISOLATION (V) SATURATION CURRENT (mA) VENDOR 750311558 300 1.5 4:1:1 1500 500 Würth Elektronik 48V to 5V, 0.3A 24V to 5V, 0.2A 12V to 5V, 0.13A 48V to 3.3V, 0.33A 24V to 3.3V, 0.28A 12V to 3.3V, 0.18A 750311019 400 5 6:1:2 1500 750 Würth Elektronik 24V to 5V, 0.26A 12V to 5V, 0.17A 48V to 3.3V, 0.43A 24V to 3.3V, 0.35A 12V to 3.3V, 0.2A 750311659 300 2 1:1:0.2 1500 560 Würth Elektronik 48V to 24V, 0.07A 750311660 350 3 2:1:0.33 1500 520 Würth Elektronik 48V to 15V, 0.1A 48V to 12V, 0.12A 24V to 15V, 0.09A 12V to 15V, 0.045A 750311838 350 3 2:1:1 1500 520 Würth Elektronik 48V to ±15V, 0.05A 48V to ±12V, 0.06A 24V to ±15V, 0.045A 750311963 200 0.4 1:5:5 1500 650 Würth Elektronik 12V to ±70V, 0.004A 12V to ±100V, 0.003A 12V to ±150V, 0.002A 750311966 120 0.45 1:5:0.5 1500 900 Würth Elektronik 12V to +120V and –12V, 0.002A 10396-T024 300 2.0 4:1:1 1500 500 Sumida 48V to 5V, 0.3A 24V to 5V, 0.2A 12V to 5V, 0.13A 48V to 3.3V, 0.33A 24V to 3.3V, 0.28A 12V to 3.3V, 0.18A 10396-T026 300 2.5 6:1:2 1500 500 Sumida 24V to 5V, 0.26A 12V to 5V, 0.17A 48V to 3.3V, 0.43A 24V to 3.3V, 0.35A 12V to 3.3V, 0.2A 01355-T057 250 2.0 1:1:0.2 1500 500 Sumida 48V to 24V, 0.07A 10396-T022 300 2.0 2:1:0.33 1500 500 Sumida 48V to 15V, 0.1A 48V to 12V, 0.12A 24V to 15V, 0.09A 12V to 15V, 0.045A 10396-T028 300 2.5 2:1:1 1500 500 Sumida 48V to ±15V, 0.05A 48V to ±12V, 0.06A 24V to ±15V, 0.045A TARGET APPLICATIONS 3511fa 10 LT3511 Applications Information Turns Ratio Saturation Current Note that when using an RFB/RREF resistor ratio to set output voltage, the user has relative freedom in selecting a transformer turns ratio to suit a given application. In contrast, the use of simple ratios of small integers, e.g., 1:1, 2:1, 3:2, provides more freedom in setting total turns and mutual inductance. The current in the transformer windings should not exceed its rated saturation current. Energy injected once the core is saturated will not be transferred to the secondary and will instead be dissipated in the core. Information on saturation current should be provided by the transformer manufacturers. Table 1 lists the saturation current of the transformers designed for use with the LT3511. Typically, choose the transformer turns to maximize available output power. For low output voltages (3.3V or 5V), a N:1 turns ratio can be used with multiple primary windings relative to the secondary to maximize the transformer’s current gain (and output power). However, remember that the SW pin sees a voltage that is equal to the maximum input supply voltage plus the output voltage multiplied by the turns ratio. In addition, leakage inductance will cause a voltage spike (VLEAKAGE) on top of this reflected voltage. This total quantity needs to remain below the absolute maximum rating of the SW pin to prevent breakdown of the internal power switch. Together these conditions place an upper limit on the turns ratio, N, for a given application. Choose a turns ratio low enough to ensure: N< 150V – VIN(MAX) – VLEAKAGE VOUT + VF For larger N:1 values, a transformer with a larger physical size is needed to deliver additional current and provide a large enough inductance value to ensure that the off-time is long enough to accurately measure the output voltage. For larger N:1 values, choose a transformer with a larger physical size to deliver additional current. In addition, choose a large enough inductance value to ensure that the 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:N transformer will minimize the magnetizing inductance (and minimize size), but will also limit the available output power. A higher 1:N turns ratio makes it possible to have very high output voltages without exceeding the breakdown voltage of the internal power switch. The turns ratio is an important element in the isolated feedback scheme. Make sure the transformer manufacturer guarantees turns ratio accuracy within ±1%. Primary Inductance Requirements The LT3511 obtains output voltage information from the reflected output voltage on the switch pin. The conduction of secondary winding current reflects the output voltage on the primary. The sampling circuitry needs a minimum of 400ns to settle and sample the reflected output voltage. In order to ensure proper sampling, the secondary winding needs to conduct current for a minimum of 400ns. The following equation gives the minimum value for primaryside magnetizing inductance: LPRI ≥ tOFF(MIN) • NPS • ( VOUT + VF ) IPEAK(MIN) tOFF(MIN) = 400ns IPEAK(MIN) = 55mA Leakage Inductance and Clamp Circuits Transformer leakage inductance (on either the primary or secondary) causes a voltage spike to appear at the primary after the output switch turns off. This spike is increasingly prominent at higher load currents where more stored energy must be dissipated. When designing an application, adequate margin should be kept for the effect of leakage voltage spikes. In most cases the reflected output voltage on the primary plus VIN should be kept below 100V. This leaves at least 50V of margin for the leakage spike across line and load conditions. A larger voltage margin will be needed for poorly wound transformers or for excessive leakage inductance. Figure 5 illustrates this point. Minimize transformer leakage inductance. A clamp circuit is recommended for most applications. 3511fa 11 LT3511 Applications Information VSW VSW <150V <150V <140V VLEAKAGE <100V <100V t OFF > 400ns t OFF > 400ns TIME tSP < 150ns tSP < 150ns without Clamp 3511 F05 TIME with Clamp Figure 5. Maximum Voltages for SW Pin Flyback Waveform Two circuits that can protect the internal power switch include the RCD (resistor-capacitor-diode) clamp and the DZ (diode-Zener) clamp. The clamp circuits dissipate the stored energy in the leakage inductance. The DZ clamp is the recommended clamp for the LT3511. Simplicity of design, high clamp voltages, and low power levels make the DZ clamp the preferred solution. Additionally, a DZ clamp ensures well defined and consistent clamping voltages. Figure 5 shows the clamp effect on the switch waveform and Figure 6 shows the connection of the DZ clamp. Proper care must be taken when choosing both the diode and the Zener diode. Schottky diodes are typically the best choice, but some PN diodes can be used if they turn on fast enough to limit the leakage inductance spike. Choose a diode that has a reverse-voltage rating higher than the maximum input voltage. The Zener diode breakdown voltage should be chosen to balance power loss and switch voltage protection. The best compromise is to choose the largest voltage breakdown. Use the following equation to make 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, which will 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 highest at maximum load and minimum input voltage. The switch LS Z D 3511 F06 Figure 6. DZ Clamp current is highest at this point along with the energy stored in the leakage inductance. A 0.5W Zener will satisfy most applications when the highest VZENER is chosen. Choosing a low value for VZENER will cause excessive power loss as shown in the following equations: 1 • L • IPK(VIN(MIN))2 • fSW • 2 ⎛ ⎞ NPS • ( VOUT + VF ) ⎜⎜1+ ⎟⎟ ⎝ VZENER – NPS • ( VOUT + VF ) ⎠ L = Leakage Inductance VOUT • IOUT • 2 IPK(VIN(MIN)) = η • VIN(MIN) • DVIN(MIN) DZ Power Loss = fSW = 1 1 = tON + tOFF LPRI • IPK(VIN(MIN)) LPRI • IPK(VIN(MIN)) + VIN(MIN) NPS • ( VOUT + VF ) 3511fa 12 LT3511 Applications Information Table 2 and 3 show some recommended diodes and Zener diodes. Table 2. Recommended Zener Diodes VZENER (V) POWER (W) CASE VENDOR MMSZ5266BT1G 68 0.5 SOD-123 On Semi MMSZ5270BT1G 91 0.5 SOD-123 CMHZ5266B 68 0.5 SOD-123 CMHZ5267B 75 0.5 SOD-123 BZX84J-68 68 0.5 SOD323F NXP BZX100A 100 0.5 SOD323F PART Central Semiconductor Table 3. Recommended Diodes pulse. The smaller flyback pulse results in a higher regulated output voltage. The inductive divider effect of secondary leakage inductance is load independent. RFB/RREF ratio adjustments can accommodate this effect to the extent secondary leakage inductance is a constant percentage of mutual inductance (over manufacturing variations). Winding Resistance Effects Resistance in either the primary or secondary will reduce overall efficiency (POUT/PIN). Good output voltage regulation will be maintained independent of winding resistance due to the boundary mode operation of the LT3511. Bifilar Winding PART I (A) VREVERSE (V) BAV19W 0.625 100 SOD-123 Diodes Inc. BAV20W 0.625 150 SOD-123 CASE VENDOR Leakage Inductance Blanking When the power switch turns off, the flyback pulse appears. However, a finite time passes before the transformer primary-side voltage waveform approximately represents the output voltage. Rise time on the SW node and transformer leakage inductance cause the delay. The leakage inductance also causes a very fast voltage spike on the primary side of the transformer. The amplitude of the leakage spike is largest when power switch current is highest. Introduction of an internal fixed delay between switch turn-off and the start of sampling provides immunity to the phenomena discussed above. The LT3511 sets internal blanking to 150ns. In certain cases leakage inductance spikes last longer than the internal blanking, but will not significantly affect output regulation. Secondary Leakage Inductance In addition to primary leakage inductance, secondary leakage inductance exhibits an important effect on application design. Secondary leakage inductance forms an inductive divider on the transformer secondary. The inductive divider effectively reduces the size of the primary-referred flyback A bifilar, or similar winding technique, is a good way to minimize troublesome leakage inductances. However, remember that this will also increase primary-to-secondary capacitance and limit the primary-to-secondary breakdown voltage, so bifilar winding is not always practical. The Linear Technology applications group is available and extremely qualified to assist in the selection and/or design of the transformer. APPLICATION DESIGN CONSIDERATIONS Iterative Design Process The LT3511 uses a unique sampling scheme to regulate the isolated output voltage. The use of this isolated scheme requires a simple iterative process to choose feedback resistors and temperature compensation. Feedback resistor values and temperature compensation resistance is heavily dependent on the application, transformer and output diode chosen. Once resistor values are fixed after iteration, the values will produce consistent output voltages with the chosen transformer and output diode. Remember, the turns ratio of the transformer must be guaranteed within ±1%. The transformer vendors mentioned in this data sheet can build transformers to this specification. 3511fa 13 LT3511 Applications Information Selecting RFB and RREF Resistor Values VIN The following section provides an equation for setting RFB and RREF values. The equation should only serve as a guide. Follow the procedure outlined in the Design Procedure to set accurate values for RFB, RREF and RTC using the iterative design procedure. R1 EN/UVLO Rearrangement of the expression for VOUT in the Temperature Compensation section, developed in the Operations section, yields the following expression for RFB: RFB = RREF • NPS ⎡⎣( VOUT + VF ) + VTC ⎤⎦ VBG where: VOUT = Output voltage GND In addition, the EN/UVLO pin draws 2.6μA when the voltage at the pin is below 1.21V. This current provides user programmable hysteresis based on the value of R1. The effective UVLO thresholds are: VIN(UVLO,RISING) = NPS = Effective primary-to-secondary turns ratio This equation assumes: RTC R = FB NPS The equation assumes the temperature coefficients of the diode and VTC are equal, which is a good first order approximation. Strictly speaking, the above equation defines RFB not as an absolute value, but as a ratio of RREF. So the next question is, what is the proper value for RREF? The answer is that RREF should be approximately 10k. The LT3511 is trimmed and specified using this value of RREF. If the impedance of RREF varies considerably from 10k, additional errors will result. However, a variation in RREF of several percent is acceptable. This yields a bit of freedom in selecting standard 1% resistor values to yield nominal RFB/RREF ratios. Undervoltage Lockout (UVLO) A resistive divider from VIN to the EN/UVLO pin implements undervoltage lockout (UVLO). Figure 7 shows this configuration. The EN/UVLO pin threshold is set at 1.21V. 3511 F07 Figure 7. Undervoltage Lockout (UVLO) VF = Switching diode forward voltage VTC = 0.55V RUN/STOP CONTROL (OPTIONAL) R2 LT3511 1.2V • (R1+ R2) VIN(UVLO,FALLING) = + 2.6µA • R1 R2 1.2V • (R1+ R2) R2 Figure 7 also shows the implementation of external shutdown control while still using the UVLO function. The NMOS grounds the EN/UVLO pin when turned on, and puts the LT3511 in shutdown with quiescent current draw of less than 1μA. Minimum Load Requirement The LT3511 recovers output voltage information using the flyback pulse. The flyback pulse occurs once the switch turns off and the secondary winding conducts current. In order to regulate the output voltage, the LT3511 needs to sample the flyback pulse. The LT3511 delivers a minimum amount of energy even during light load conditions to ensure accurate output voltage information. The minimum delivery of energy creates a minimum load requirement of 10mA to 15mA depending on the specific application. Verify minimum load requirements for each application. A Zener diode with a Zener breakdown of 20% higher than the output voltage can serve as a minimum load if pre-loading is not acceptable. For a 5V output, use a 6V Zener with cathode connected to the output. 3511fa 14 LT3511 Applications Information BIAS Pin Considerations The BIAS pin powers the internal circuitry of the LT3511. Three unique configurations exist for regulation of the BIAS pin. In the first configuration, the internal LDO drives the BIAS pin internally from the VIN supply. In the second setup, the VIN supply directly drives the BIAS pin through a direct connection bypassing the internal LDO. This configuration will allow the part to operate down to 4.5V and up to 15V. In the third configuration, an external supply or third winding drives the BIAS pin. Use this option when a voltage supply exists lower than the input supply. Drive the BIAS pin with a voltage supply higher than 3.3V to disable the internal LDO. The lower voltage supply provides a more efficient source of power for internal circuitry. LT3511 VIN 6V TO 100V LDO 3V BIAS LT3511 VIN 4.5V TO 15V BIAS OPTIONAL VIN Use the following design procedure as a guide to designing applications for the LT3511. Remember, the unique sampling architecture requires an iterative process for choosing correct resistor values. VIN(MIN) = 36V, VIN(NOM) = 48V, VIN(MAX) = 72V, VOUT = 15V and IOUT = 100mA LDO BIAS An external resistor-capacitor network compensates the LT3511 on the VC pin. Typical compensation values are in the range of RC = 20k and CC = 2.2nF (see the numerous schematics in the Typical Applications section for other possible values). Proper choice of both RC and CC is important to achieve stability and acceptable transient response. For example, vulnerability to high frequency noise and jitter result when RC is too large. On the other hand, if RC is too small, transient performance suffers. The inverse is true with respect to the value of CC. Transient response suffers with too large of a CC, and instability results from too small a CC. The specific value for RC and CC will vary based on the application and transformer choice. Verify specific choices with board level evaluation and transient response performance. The design example involves designing a 15V output with a 100mA load current and an input range from 36V to 72V. 6V TO 100V 3.3V < BIAS < 20V Loop Compensation DESIGN PROCEDURE/DESIGN EXAMPLE LDO LT3511 an additional winding (often called a third winding) improves overall system efficiency. Design the third winding to output a voltage between 3.3V and 12V. For a typical 48VIN application, overdriving the BIAS pin improves efficiency 4% to 5%. EXTERNAL SUPPLY Step 1: Select the transformer turns ratio. 3511 F08 Figure 8. BIAS Pin Configurations Overdriving the BIAS Pin with a Third Winding The LT3511 provides excellent output voltage regulation without the need for an opto-coupler, or third winding, but for some applications with higher input voltages (>20V), NPS < VSW(MAX) – VIN(MAX) – VLEAKAGE VOUT + VF VSW(MAX) = Max rating of internal switch = 150V VLEAKAGE = Margin for transformer leakage spike = 40V VF = Forward voltage of output diode = assume approximately ~ 0.5V 3511fa 15 LT3511 Applications Information Example: 150V – 72V – 40V 15V + 0.5V < 2.45 NPS < NPS NPS = 2 The choice of turns ratio is critical in determining output power as shown earlier in the Output Power section. At this point, a third winding can be added to the transformer to drive the BIAS pin of the LT3511 for higher efficiencies. Choose a turns ratio that sets the third winding voltage to regulate between 3.3V and 6V for maximum efficiency. Step 3: Determine primary inductance, switching frequency and saturation current. Primary inductance for the transformer must be set above a minimum value to satisfy the minimum off time requirement. tOFF(MIN) • NPS • ( VOUT + VF ) LPRI ≥ IPEAK(MIN) tOFF(MIN) = 400ns IPEAK(MIN) = 55mA Example: LPRI ≥ Choose a third winding ratio to drive BIAS winding with 5V. (Optional) 400ns • 2 • (15 + 0.5) 0.055 LPRI ≥ 225µH Example: NTHIRD VTHIRD 5V = = = 0.33 NS VOUT 15V In addition, primary inductance will determine switching frequency. The turns ratio of the transformer chosen is as follows NPRIMARY: NSECONDARY: NTHIRD = 2:1:0.33. fSW = Step 2: Calculate maximum power output at minimum VIN. POUT(VIN(MIN)) = η • VIN(MIN) • IIN = η • VIN(MIN) • D • IPEAK • 0.5 D= ( VOUT + VF ) • NPS ( VOUT + VF ) • NPS + VIN(MIN) η = Efficiency = ~75% IPEAK = Let’s calculate switching frequency at our nominal VIN of 48V. D= Example: The chosen turns ratio satisfies the output current requirement of 100mA. If the output current was too low, the minimum input voltage could be adjusted higher. The turns ratio in this example is set to its highest ratio given switch voltage requirements and margin for leakage inductance voltage spike. 16 VOUT • IOUT • 2 η • VIN • D Example: IPEAK = Peak switch current = 0.26A D = 0.46 POUT(VIN(MIN)) = 1.62 IOUT(VIN(MIN)) = POUT(VIN(MIN))/VOUT = 0.11A 1 1 = tON + tOFF LPRI • IPEAK + LPRI • IPEAK VIN NPS • ( VOUT + VF ) (15 + 0.5) • 2 = 0.39 (15 + 0.5) • 2 + 48 IPEAK = 15V • 0.1A • 2 = 0.21A 0.75 • 48V • 0.39 Let’s choose LPRI = 350μH. Remember, most transformers specify primary inductance with a tolerance of ±20%. fSW = 256kHz Finally, the transformer needs to be rated for the correct saturation current level across line and load conditions. In the given example, the worst-case condition for switch current is at minimum VIN and maximum load. 3511fa LT3511 Applications Information Step 5: Choose an output capacitor. VOUT • IOUT • 2 η • VIN • D 15V • 0.1A • 2 = = 0.24A 0.75 • 36V • 0.46 IPEAK = IPEAK The output capacitor choice should minimize output voltage ripple and balance the trade-off between size and cost for a larger capacitor. Use the equation below at nominal VIN: Ensure that the saturation current covers steady-state operation, start-up and transient conditions. To satisfy these conditions, choose a saturation current 50% or more higher than the steady-state calculation. In this example, a saturation current between 400mA and 500mA is chosen. Table 1 presents a list of pre-designed flyback transformers. For this application, the Würth 750311660 transformer will be used. Step 4: Choose the correct output diode. The two main criteria for choosing the output diode include forward current rating and reverse voltage rating. The maximum load requirement is a good first-order guess at the average current requirement for the output diode. A better metric is RMS current. IRMS = IPEAK(VIN(MIN)) • NPS • 1– DVIN(MIN) 3 Example: IRMS = 0.24 • 2 • 1– 0.46 = 0.2A 3 Next calculate reverse voltage requirement using maximum VIN: VREVERSE = VOUT + VIN(MAX) NPS Example: VREVERSE = 15V + 72V = 51V 2 A 0.5A, 60V diode from Diodes Inc. (SBR0560S1) will be used. C= IOUT • D ΔVOUT • fSW Example: Design for ripple levels below 50mV. C= 0.1A • 0.39 = 3.1µF 0.05V • 256kHz A 10μF, 25V output capacitor is chosen. Remember ceramic capacitors lose capacitance with applied voltage. The capacitance can drop to 40% of quoted capacitance at the max voltage rating. Step 6: Design clamp circuit. The clamp circuit protects the switch from leakage inductance spike. A DZ clamp is the preferred clamp circuit. The Zener and the diode need to be chosen. The maximum Zener value is set according to the maximum VIN: VZENER(MAX) ≤ 150V – VIN(MAX) Example: VZENER(MAX) ≤ 150V – 72V VZENER(MAX) ≤ 78V In addition, power loss in the clamp circuit is inversely related to the clamp voltage as shown previously. Higher clamp voltages lead to lower power loss. A 68V Zener with a maximum of 72V will provide optimal protection and minimize power loss. Half-watt Zeners will satisfy most clamp applications involving the LT3511. Power loss can be calculated using the equations presented in the Leakage Inductance and Clamp Circuit section. The Zener chosen is a 68V 0.5W Zener from On Semiconductor (MMSZ5266BT1G). 3511fa 17 LT3511 Applications Information Choose a diode that is fast and has sufficient reverse voltage breakdown: VREVERSE > VIN(MAX) Example: Step 9: Adjust RFB based on output voltage. Power up the application with application components connected and measure the regulated output voltage. Readjust RFB based on the measured output voltage. VREVERSE > 72V RFB(NEW) = The diode needs to handle the peak switch current of the switch which was determined to be 0.24A. A 100V, 0.6A diode from Diodes Inc. (BAV19W) is chosen. Step 7: Compensation. Compensation will be optimized towards the end of the design procedure. Connect a resistor and capacitor from the VC node to ground. Use a 20k resistor and a 2.2nF capacitor. Step 8: Select RFB and RTC Resistors. Use the following equations to choose starting values for RFB and RTC. Set RREF to 10k. RFB = ( VOUT + VF + 0.55V ) • NPS • RREF 1.2V RREF = 10k R = FB NPS RTC VOUT VOUT(MEAS) • RFB(OLD) Example: RFB(NEW) = 15V • 267k = 237k 16.8V Step 10: Remove RTC and measure output voltage over temperature. Measure output voltage in a controlled temperature environment like an oven to determine the output temperature coefficient. Measure output voltage at a consistent load current and input voltage, across the temperature range of operation. This procedure will optimize line and load regulation over temperature. Calculate the temperature coefficient of VOUT: ΔVOUT VOUT(HOT) – VOUT(COLD) = ΔTemp THOT(°C) – TCOLD(°C) Example: Example: VOUT measured at 100mA and 48VIN RFB = RTC (15 + 0.5 + 0.55V ) • 2 • 10k = 267k 1.2V 267k = = 133k 2 ΔVOUT 15.70V – 15.37V = = 1.9mV/°C ΔTemp 125°C – ( –50°C) 3511fa 18 LT3511 Applications Information Step 11: Calculate new value for RTC. RTC(NEW) = RFB 1.85mV / °C • ΔVOUT NPS ΔTemp Example: RTC(NEW) = 237k 1.85 • = 118k 2 1.9 VOUT VOUT(MEAS) • RFB(OLD) Example: RFB(NEW) = Check minimum load requirement at maximum input voltage. The minimum load occurs at the point where the output voltage begins to climb up as the converter delivers more energy than what is consumed at the output. Example: Step 12: Place new value for RTC, measure VOUT, and readjust RFB due to RTC change. RFB(NEW) = Step 15: Ensure minimum load. The minimum load at an input voltage of 72V is: 7mA Step 16: EN/UVLO resistor values. Determine amount of hysterysis required. Voltage hysteresis = 2.6μA • R1 Example: Choose 2V of hysteresis. 15V • 237k = 237k 15V Step 13: Verify new values of RFB and RTC over temperature. R1= 2V = 768k 2.6µA Determine UVLO Threshold. Measure output voltage over temperature with RTC connected. VIN(UVLO,FALLING) = Step 14: Optimize compensation. R2 = Now that values for RFB and RTC are fixed, optimize the compensation. Compensation should be optimized for transient response to load steps on the output. Check transient response across the load range. 1.2V • (R1+ R2) R2 1.2V • R1 VIN(UVLO,FALLING) – 1.2V Set UVLO falling threshold to 30V. 1.2V • 768k = 32.4k 30V – 1.2V 1.2V • (R1+ R2) VIN(UVLO,FALLING) = R2 1.2V • (768k + 32.4k ) = = 30V 32.4k R2 = Example: The optimal compensation for the application is: RC = 22.1k, CC = 4.7nF VIN(UVLO,RISING) = VIN(UVLO,FALLING) + 2.6μA • R1 = 30V + 2.6μA • 768k = 32V 3511fa 19 LT3511 Typical Applications 48V to 5V Isolated Flyback Converter VIN 36V TO 72V 4:1:1 C1 1µF R1 1M Z1 VIN R2 43.2k EN/UVLO RFB RREF LT3511 TC D3 R3 169k T1 300µH R5 69.8k GND VOUT– C1: TAIYO YUDEN HMK316B7105KL-T C3: TAIYO YUDEN EMK212B7475KG C4: MURATA GRM32ER71C226KE18B D1, D2: DIODES INC. SBR140S3 D3: DIODES INC. BAV19W T1: WÜRTH 750311558 Z1: ON SEMI MMSZ5266BT1G R4 10k BIAS D2 R6 16.9k C2 3.3nF VOUT+ 5V 0.3A C4 22µF 19µH SW VC D1 L1C 19µH C3 4.7µF 3511 TA02 OPTIONAL THIRD WINDING FOR HV OPERATION 48V to 15V Isolated Flyback Converter VIN 36V TO 72V C1 1µF D1 2:1 R1 1M Z1 VIN R2 43.2k T1 350µH EN/UVLO RFB RREF LT3511 TC R4 10k SW VC R5 97.6k R3 237k GND BIAS R6 13k C2 6.8nF D2 88µH VOUT+ 15V 0.1A C4 10µF VOUT– C1: TAIYO YUDEN HMK316B7105KL-T C3: TAIYO YUDEN EMK212B7475KG C4: MURATA GRM31CR71E106KA12 D1: DIODES INC. SBR0560S1 D2: DIODES INC. BAV19W T1: WÜRTH 750311660 Z1: ON SEMI MMSZ5266BT1G C3 4.7µF 3511 TA03 3511fa 20 LT3511 Typical Applications 48V to 24V Isolated Flyback Converter VIN 36V TO 72V C1 1µF Z1 VIN R2 43.2k T1 300µH EN/UVLO R3 187k RFB RREF LT3511 TC R5 200k GND C4 4.7µF VOUT– C1: TAIYO YUDEN HMK316B7105KL-T C3: TAIYO YUDEN EMK212B7475KG C4: MURATA GRM32ER71H475KA88B D1: DIODES INC. SBR1U150SA D2: DIODES INC. BAV19W T1: WÜRTH 750311659 Z1: ON SEMI MMSZ5266BT1G R4 10k BIAS R6 33.2k C2 3.3nF 300µH D2 SW VC VOUT+ 24V 65mA D1 1:1 R1 1M C3 4.7µF 3511 TA04 24V to 5V Isolated Flyback Converter VIN 20V TO 30V C1 4.7µF D1 6:1 R1 1M Z1 VIN R2 80.6k EN/UVLO LT3511 RFB RREF SW TC VC R5 73.2k GND BIAS R6 9.31k C2 15nF R3 249k R4 10k D2 T1 300µH 8µH VOUT+ 5V 0.25A C4 22µF VOUT– C1: MURATA GRM31CR71H475KA12B C3: TAIYO YUDEN EMK212B7475KG C4: MURATA GRM32ER71C226KE18B D1: DIODES INC. SBR2A30P1 D2: DIODES INC. BAV19W T1: SUMIDA 10396-T026 Z1: ON SEMI MMSZ5270BT1G C3 4.7µF 3511 TA05 3511fa 21 LT3511 Typical Applications 24V to 15V Isolated Flyback Converter VIN 20V TO 30V C1 4.7µF R1 1M Z1 VIN EN/UVLO R2 80.6k R3 237k RFB RREF LT3511 D2 T1 350µH VC R5 133k GND VOUT– C1: MURATA GRM31CR71H475KA12B C3: TAIYO YUDEN EMK212B7475KG C4: MURATA GRM31CR71E106KA12B D1: DIODES INC. SBR140S3 D2: DIODES INC. BAV19W T1: WÜRTH 750311660 Z1: ON SEMI MMSZ5270BT1G R4 10k BIAS R6 20k C2 4.7nF C4 10µF 88µH SW TC VOUT+ 15V 0.09A D1 2:1 C3 4.7µF 3511 TA06 12V to 15V Isolated Flyback Converter VIN 8V TO 20V C1 4.7µF 2:1 R1 1M R2 562k Z1 VIN EN/UVLO LT3511 RFB RREF SW TC VC R5 133k GND BIAS R6 26.1k C2 4.7nF C3 4.7µF R3 237k R4 10k D2 T1 350µH VOUT+ 15V 40mA D1 88µH C4 4.7µF Z2 VOUT– OPTIONAL MINIMUM LOAD C1: MURATA GRM31CR71H475KA12B C3: TAIYO YUDEN EMK212B7475KG C4: MURATA GRM31CR71H475KA12 D1: DIODES INC. SBR130S3 D2: DIODES INC. BAV19W T1: WÜRTH 750311660 Z1: ON SEMI MMSZ5270BT1G 3511 TA08 3511fa 22 LT3511 Typical Applications 12V to ±70V Isolated Flyback Converter VIN 8V TO 20V 1:5:5 C1 2.2µF R1 1M VIN Z1 EN/UVLO R2 562k LT3511 D3 R3 105k RFB RREF D1 T1 80µH C4 0.47µF D2 R4 10k VC R5 191k GND BIAS R6 90k C2 6.8nF VOUT1– VOUT2+ 4mA C5 0.47µF VOUT2– –70V SW TC VOUT1+ 70V 4mA C1: MURATA GRM21BR71E225KA73B C3: TAIYO YUDEN EMK212B7475KG C4, C5: NIPPON CHEMI-CON KTS251B474M43N0T00 D1, D2: CENTRAL SEMI CRM1U-06M D3: DIODES INC. BAV19W T1: WÜRTH 750311692 Z1: NXP BZX100A C3 4.7µF 3511 TA07 48V to 3.3V Non-Isolated Flyback Converter VIN 36V TO 72V C1 1µF D1 6:1 R1 1M Z1 VIN R2 43.2k RFB EN/UVLO LT3511 D2 8.66k RREF 11µH VOUT R4 5k VC GND C1: TAIYO YUDEN HMK316B7105KL-T C3: TAIYO YUDEN EMK212B7475KG C4: TAIYO YUDEN LMK325B7476MM-TR D1: DIODES INC. SBR2A30P1 D2: DIODES INC. BAV19W T1: WÜRTH 750311019 Z1: ON SEMI MMSZ5266BT1G BIAS R6 8k C2 4.7nF C4 47µF VOUT– SW TC R5 1M R3 1M T1 400µH VOUT 3.3V 0.4A C3 4.7µF 3511 TA09 48V to 12V Isolated Flyback Converter VIN 36V TO 72V C1 1µF D1 2:1 R1 1M Z1 VIN R2 43.2k EN/UVLO LT3511 RFB RREF GND BIAS TC R4 10k SW VC R5 143k R3 191k R6 15k C2 6.8nF D2 T1 300µH 75µH VOUT+ 12V 0.1A C4 4.7µF VOUT– C1: TAIYO YUDEN HMK316B7105KL-T C3: TAIYO YUDEN EMK212B7475KG C4: MURATA GRM31CR71H475KA12 D1: DIODES INC. SBR0560S1 D2: DIODES INC. BAV19W T1: SUMIDA 10396-T022 Z1: ON SEMI MMSZ5266BT1G C3 4.7µF 3511 TA10 3511fa 23 LT3511 Package Description MS Package Varitation: MS16 (12) 16-Lead Plastic MSOP with 4 Pins Removed (Reference LTC DWG # 05-08-1847 Rev A) 1.0 (.0394) BSC 5.23 (.206) MIN 0.889 ± 0.127 (.035 ± .005) 3.20 – 3.45 (.126 – .136) 4.039 ± 0.102 (.159 ± .004) (NOTE 3) 16 14 121110 9 0.50 (.0197) BSC 0.305 ± 0.038 (.0120 ± .0015) TYP RECOMMENDED SOLDER PAD LAYOUT 0.254 (.010) 0.280 ± 0.076 (.011 ± .003) REF 3.00 ± 0.102 (.118 ± .004) (NOTE 4) 4.90 ± 0.152 (.193 ± .006) DETAIL “A” 0° – 6° TYP 1 GAUGE PLANE 0.53 ± 0.152 (.021 ± .006) DETAIL “A” 0.18 (.007) SEATING PLANE 1.10 (.043) MAX 0.17 – 0.27 (.007 – .011) TYP 0.50 (.0197) BSC 3 567 8 1.0 (.0394) BSC 0.86 (.034) REF 0.1016 ± 0.0508 (.004 ± .002) MSOP (MS12) 0510 REV A NOTE: 1. DIMENSIONS IN MILLIMETER/(INCH) 2. DRAWING NOT TO SCALE 3. DIMENSION DOES NOT INCLUDE MOLD FLASH, PROTRUSIONS OR GATE BURRS. MOLD FLASH, PROTRUSIONS OR GATE BURRS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 4. DIMENSION DOES NOT INCLUDE INTERLEAD FLASH OR PROTRUSIONS. INTERLEAD FLASH OR PROTRUSIONS SHALL NOT EXCEED 0.152mm (.006") PER SIDE 5. LEAD COPLANARITY (BOTTOM OF LEADS AFTER FORMING) SHALL BE 0.102mm (.004") MAX 3511fa 24 LT3511 Revision History REV DATE DESCRIPTION A 4/11 Added MP-grade. PAGE NUMBER 2, 3 Revised RFB pin description in the Pin Functions section. 5 Updated efficiency equation and Table 1 in the Applications Information section. 9, 10 Revised the Typical Applications drawings. 20, 21 3511fa Information furnished by Linear Technology Corporation is believed to be accurate and reliable. However, no responsibility is assumed for its use. Linear Technology Corporation makes no representation that the interconnection of its circuits as described herein will not infringe on existing patent rights. 25 LT3511 Typical Application 48V to ±15V Isolated Flyback Converter VIN 36V TO 72V 2:1:1 C1 1µF R1 1M Z1 VIN R2 43.2k EN/UVLO RFB LT3511 VC D3 R3 237k RREF TC R5 154k D1 T1 350µH 88µH C4 4.7µF VOUT1– D2 R4 10k 88µH VOUT2+ 50mA C5 4.7µF SW GND BIAS R6 20k C2 6.8nF C3 4.7µF 3511 TA11 VOUT1+ 15V 50mA C1: TAIYO YUDEN HMK316B7105KL-T C3: TAIYO YUDEN EMK212B7475KG C4, C5: MURATA GRM31CR71H475KA12 D1, D2: DIODES INC. SBR0560S1 D3: DIODES INC. BAV19W T1: WÜRTH 750311838 Z1: CENTRAL SEMI CMHZ5266B VOUT2– –15V Related Parts PART NUMBER DESCRIPTION COMMENTS LT3958 High Input Voltage Boost, Flyback, SEPIC and Inverting Converter 5V ≤ VIN ≤ 80V, 3.3A/84V Onboard Power Switch, 5mm × 6mm QFN-36 with High Voltage Pin Spacing LT3748 100V Isolated Flyback Controller 5V ≤ VIN ≤ 100V, No Opto-Isolator or “Third Winding” Required, Onboard Gate Driver, MSOP-16 with High Voltage Pin Spacing LT3957 Boost, Flyback, SEPIC and Inverting Converter 3V ≤ VIN ≤ 40V, 5A/40V Onboard Power Switch, 5mm × 6mm QFN-36 with High Voltage Pin Spacing LT3956 Constant-Current, Constant-Voltage Boost, Buck, Buck-Boost, SEPIC or Flyback Converter 4.5V ≤ VIN ≤ 80V, 3.3A/84V Onboard Power Switch, True PWM Dimming, 5mm × 6mm QFN-36 with High Voltage Pin Spacing LT3575 Isolated Flyback Switching Regulator with 60V/2.5A Integrated Switch 3V ≤ VIN ≤ 40V, No Opto-Isolator or “Third Winding” Required, Up to 14W, TSSOP-16E LT3573 Isolated Flyback Switching Regulator with 60V/1.25A Integrated Switch 3V ≤ VIN ≤ 40V, No Opto-Isolator or “Third Winding” Required, Up to 7W, MSOP-16E LT3574 Isolated Flyback Switching Regulator with 60V/0.65A Integrated Switch 3V ≤ VIN ≤ 40V, No Opto-Isolator or “Third Winding” Required, Up to 3W, MSOP-16 LT3757 Boost, Flyback, SEPIC and Inverting Controller 2.9V ≤ VIN ≤ 40V, 100kHz to 1MHz Programmable Operating Frequency, 3mm × 3mm DFN-10 and MSOP-10E Package LT3758 Boost, Flyback, SEPIC and Inverting Controller 5.5V ≤ VIN ≤ 100V, 100kHz to 1MHz Programmable Operating Frequency, 3mm × 3mm DFN-10 and MSOP-10E Package LTC1871/LTC1871-1/ No RSENSE™ Low Quiescent Current Flyback, Boost and SEPIC Controller LTC1871-7 2.5V ≤ VIN ≤ 36V, Burst Mode® Operation 3511fa 26 Linear Technology Corporation LT 0411 REV A • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com LINEAR TECHNOLOGY CORPORATION 2010