LT3757

LT3511 Monolithic High Voltage Isolated Flyback Converter FeaTures n n n n DescripTion 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. 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. n n n n n 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 Improved Primary-Side Winding Feedback Load Regulation VOUT Set with Two External Resistors BIAS Pin for Internal Bias Supply and Power Switch Driver No External Start-Up Resistor 16-Lead MSOP Package applicaTions n n n Isolated Telecom Power Supplies Isolated Auxiliary/Housekeeping Power Supplies Isolated Industrial, Automotive and Medical Power Supplies Typical applicaTion 48V to 5V Isolated Flyback Converter VIN 36V TO 72V 4:1 EN/UVLO 43.2k VIN 169k 300µH VOUT+ 5V 0.3A 19µH 22µF Output Load and Line Regulation 5.25 5.20 5.15 5.10 VOUT (V) 5.05 5.00 4.95 4.90 4.85 4.80 4.75 VIN = 72V VIN = 48V VIN = 36V 1µF 1M • • LT3511 RFB RREF VOUT– 10k TC VC 69.8k SW GND BIAS 16.9k 3.3nF 3511 TA01a 4.7µF 0 50 150 200 100 LOAD CURRENT (mA) 250 300 3511 TA01b 3511fa 1 LT3511 absoluTe MaxiMuM raTings (Note 1) pin conFiguraTion TOP VIEW EN/UVLO 1 VIN 3 GND BIAS NC GND 5 6 7 8 16 SW 14 RFB 12 11 10 9 RREF TC VC GND 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 MS PACKAGE 16(12)-LEAD PLASTIC MSOP θJA = 90°C/W orDer inForMaTion LEAD FREE FINISH LT3511EMS#PBF LT3511IMS#PBF LT3511HMS#PBF LT3511MPMS#PBF TAPE AND REEL LT3511EMS#TRPBF LT3511IMS#TRPBF LT3511HMS#TRPBF LT3511MPMS#TRPBF PART MARKING* 3511 3511 3511 3511 PACKAGE DESCRIPTION 16-Lead Plastic MSOP 16-Lead Plastic MSOP 16-Lead Plastic MSOP 16-Lead Plastic MSOP TEMPERATURE RANGE –40°C to 125°C –40°C to 125°C –40°C to 125°C –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 PARAMETER Input Voltage Range Quiescent Current EN/UVLO Pin Threshold EN/UVLO Pin Current Maximum Switching Frequency Maximum Current Limit Minimum Current Limit Switch VCESAT RREF Voltage ISW = 100mA CONDITIONS VIN = BIAS 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. MIN l TYP MAX 100 15 UNITS V V mA µA V µA µA kHz mA mA V V V %/V nA V/V µmhos 3511fa 6 4.5 2.7 0 Not Switching VEN/UVLO = 0.2V EN/UVLO Pin Voltage Rising VEN/UVLO = 1.1V VEN/UVLO = 1.4V l 3.5 1.27 3.3 1.15 2.0 1.21 2.6 0 650 330 60 0.3 1.20 0.01 240 35 1.18 1.17 430 90 1.215 1.23 0.03 400 l RREF Voltage Line Regulation RREF Pin Bias Current Error Amplifier Voltage Gain Error Amplifier Transconductance 6V < VIN < 100V (Note 3) ∆I = 2µA l 80 150 140 2 LT3511 elecTrical characTerisTics PARAMETER Minimum Switching Frequency TC Current into RREF BIAS Pin Voltage RTC = 53.6k Internally Regulated 3 CONDITIONS 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. MIN TYP 40 9.5 3.1 3.2 MAX UNITS kHz µA V 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 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 Output Voltage 5.25 5.20 5.15 5.10 VOUT (V) IQ (mA) 5.05 5.00 4.95 4.90 4.85 4.80 4.75 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 3511 G01 TA = 25°C, unless otherwise noted. BIAS Pin Voltage 4.0 Quiescent Current 5 VIN = 48V 4 BIAS VOLTAGE (V) VIN = 24V VIN = 48V VIN = 100V 0 25 50 75 100 125 150 TEMPERATURE (°C) 3511 G02 3.5 3 3.0 2 1 2.5 VIN = 24V, 10mA VIN = 24V, NO LOAD 0 25 50 75 100 125 150 TEMPERATURE (°C) 3511 G03 0 –50 –25 2.0 –50 –25 Switch VCESAT 1000 SWITCH VCESAT VOLTAGE (mV) 400 350 CURRENT LIMIT (mA) 300 250 Switch Current Limit 4 MAXIMUM CURRENT LIMIT 3 Quiescent Current vs VIN 800 IQ (mA) MINIMUM CURRENT LIMIT 600 200 150 100 50 2 400 200 1 0 0 50 100 150 200 250 SWITCH CURRENT (mA) 300 350 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 3511 G04 3511fa 3 LT3511 Typical perForMance characTerisTics EN/UVLO Pin (Hysteresis) Current vs Temperature 5 EN/UVLO = 1.2V EN/UVLO PIN CURRENT (µA) 30 25 EN/UVLO THRESHOLD (V) 20 15 10 5 0 TA = 25°C, unless otherwise noted. EN/UVLO Threshold vs Temperature 3.0 2.5 2.0 1.5 1.0 0.5 0 –50 –25 EN/UVLO Pin Current vs VEN/UVLO EN/UVLO PIN CURRENT (µA) 4 3 2 1 0 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 3511 G07 1 20 60 80 VEN/UVLO VOLTAGE (V) 40 100 3511 G08 0 25 50 75 100 125 150 TEMPERATURE (°C) 3511 G09 Maximum Frequency vs Temperature 1000 100 Minimum Frequency vs Temperature 0.9 0.8 EN/UVLO Shutdown Threshold vs Temperature MAXIMUM FREQUENCY (kHz) MINIMUM FREQUENCY (kHz) EN/UVLO THRESHOLD (V) 0 25 50 75 100 125 150 TEMPERATURE (°C) 3511 G11 800 80 0.7 0.6 0.5 0.4 0.3 0.2 0.1 600 60 400 40 200 20 0 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 3511 G10 0 –50 –25 0 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 3511 G14 Boundary Mode Waveform Light Load Discontinuous 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 C1 R3 N:1 TC CURRENT TC R5 VIN Q3 RFB Q2 FLYBACK ERROR AMP 1.2V CURRENT COMPARATOR ONE SHOT A2 SW L1A T1 L1B C2 VOUT – VOUT + I2 RREF – gm + – + – A1 + S S R + VIN V1 120mV DRIVER BIAS Q – R4 BIAS C4 R1 R2 1.2V Q1 MASTER LATCH A4 + – EN/UVLO + A5 – RSENSE 0.02 GND 3µA Q4 INTERNAL REFERENCE AND REGULATORS 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 ISEC = Transformer secondary current 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: 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: δVF R 1 δVTC = – FB • • or, δT RTC NPS δT 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: ⎛ VFLBK ⎞ VBG ⎛R ⎞ or VFLBK = VBG ⎜ FB ⎟ ⎜ ⎟= ⎝ RFB ⎠ RREF ⎝ 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 ⎝ 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 ⎛ 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 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. 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. 3.0 2.5 OUTPUT POWER (W) 2.0 1.5 1.0 N=2 0.5 0 N = NPS(MAX) 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 equations below calculate output power: Power = η • VIN • D • IPEAK • 0.5 Efficiency = η = ~85% Duty Cycle = D = ( VOUT + VF ) • NPS ( VOUT + VF ) • NPS + VIN Peak switch current = IPEAK = 0.26AV 3.5 3.0 OUTPUT POWER (W) N = 15 N = 12 N = 10 N=8 N=6 N=4 2.5 2.0 1.5 1.0 0.5 0 20 40 60 INPUT VOLTAGE (V) 80 100 3511 F01 N=5 N = NPS(MAX) N=4 N=3 N=2 N=1 0 0 20 40 60 INPUT VOLTAGE (V) 80 100 3511 F03 Figure 1. Output Power for 3.3V Output 3.0 2.5 OUTPUT POWER (W) N = NPS(MAX) 2.0 1.5 1.0 0.5 0 N=8 N=7 N=6 N=5 N=4 N=3 N=2 N=1 Figure 3. Output Power for 12V Output 3.0 2.5 OUTPUT POWER (W) 2.0 1.5 1.0 0.5 0 N=1 N = NPS(MAX) N=2 0 20 40 60 INPUT VOLTAGE (V) 80 100 3511 F02 0 20 40 60 INPUT VOLTAGE (V) 80 100 3511 F04 Figure 2. Output Power for 5V Output 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. Table 1. Predesigned Transformers TRANSFORMER PART NUMBER 750311558 LPRI (μH) 300 LEAKAGE (μH) 1.5 NP:NS:NB 4:1:1 ISOLATION (V) 1500 SATURATION CURRENT (mA) 500 VENDOR Würth Elektronik TARGET APPLICATIONS 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 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 48V to 24V, 0.07A 48V to 15V, 0.1A 48V to 12V, 0.12A 24V to 15V, 0.09A 12V to 15V, 0.045A 48V to ±15V, 0.05A 48V to ±12V, 0.06A 24V to ±15V, 0.045A 12V to ±70V, 0.004A 12V to ±100V, 0.003A 12V to ±150V, 0.002A 12V to +120V and –12V, 0.002A 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 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 48V to 24V, 0.07A 48V to 15V, 0.1A 48V to 12V, 0.12A 24V to 15V, 0.09A 12V to 15V, 0.045A 48V to ±15V, 0.05A 48V to ±12V, 0.06A 24V to ±15V, 0.045A 3511fa 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. 750311019 400 5 6:1:2 1500 750 Würth Elektronik 750311659 750311660 300 350 2 3 1:1:0.2 2:1:0.33 1500 1500 560 520 Würth Elektronik Würth Elektronik 750311838 350 3 2:1:1 1500 520 Würth Elektronik 750311963 200 0.4 1:5:5 1500 650 Würth Elektronik 750311966 10396-T024 120 300 0.45 2.0 1:5:0.5 4:1:1 1500 1500 900 500 Würth Elektronik Sumida 10396-T026 300 2.5 6:1:2 1500 500 Sumida 01355-T057 10396-T022 250 300 2.0 2.0 1:1:0.2 2:1:0.33 1500 1500 500 500 Sumida Sumida 10396-T028 300 2.5 2:1:1 1500 500 Sumida 10 LT3511 applicaTions inForMaTion Turns Ratio 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. 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 Saturation Current 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. 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 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%. 11 LT3511 applicaTions inForMaTion VSW 72V 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 = 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. RFB(NEW) = 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) ( VOUT + VF + 0.55V ) • NPS • RREF 1.2V RREF = 10k RTC R = FB NPS Example: Example: RFB = RTC 1.2V 267k = = 133k 2 VOUT measured at 100mA and 48VIN (15 + 0.5 + 0.55V ) • 2 • 10k = 267k 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 Step 15: Ensure minimum load. 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: 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. 2V = 768k 2.6µA Example: RTC(NEW) = 237k 1.85 • = 118k 2 1.9 Step 12: Place new value for RTC, measure VOUT, and readjust RFB due to RTC change. RFB(NEW) = VOUT VOUT(MEAS) • RFB(OLD) Example: RFB(NEW) = 15V • 237k = 237k 15V R1 = Step 13: Verify new values of RFB and RTC over temperature. Measure output voltage over temperature with RTC connected. Step 14: Optimize compensation. 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. Example: The optimal compensation for the application is: RC = 22.1k, CC = 4.7nF Determine UVLO Threshold. VIN(UVLO,FALLING) = R2 = 1.2V • (R1+ R2) R2 1.2V • R1 VIN(UVLO,FALLING) – 1.2V Set UVLO falling threshold to 30V. R2 = 1.2V • 768k = 32.4k 30V – 1.2V 1.2V • (R1+ R2) VIN(UVLO,FALLING) = R2 1.2V • (768k + 32.4k ) = = 30V 32.4k 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 R2 43.2k EN/UVLO R3 169k R4 10k D3 VIN Z1 T1 300µH D1 VOUT+ 5V 0.3A 19µH C4 22µF 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 LT3511 RFB RREF TC VC R5 69.8k GND R6 16.9k C2 3.3nF SW BIAS D2 L1C 19µH 3511 TA02 C3 4.7µF OPTIONAL THIRD WINDING FOR HV OPERATION 48V to 15V Isolated Flyback Converter VIN 36V TO 72V C1 1µF 2:1 VIN EN/UVLO R3 237k R4 10k D2 Z1 T1 350µH 88µH D1 VOUT+ 15V 0.1A R1 1M R2 43.2k 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 LT3511 RFB RREF TC VC R5 97.6k GND R6 13k C2 6.8nF 3511 TA03 SW BIAS C3 4.7µF 3511fa 20 LT3511 Typical applicaTions 48V to 24V Isolated Flyback Converter VIN 36V TO 72V C1 1µF 1:1 VIN EN/UVLO R3 187k R4 10k D2 Z1 T1 300µH D1 VOUT+ 24V 65mA R1 1M R2 43.2k 300µH 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 LT3511 RFB RREF TC VC R5 200k GND R6 33.2k C2 3.3nF 3511 TA04 SW BIAS C3 4.7µF 24V to 5V Isolated Flyback Converter VIN 20V TO 30V C1 4.7µF 6:1 R1 1M R2 80.6k VIN EN/UVLO R3 249k R4 10k D2 Z1 T1 300µH 8µH D1 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 LT3511 RFB RREF TC VC R5 73.2k GND R6 9.31k C2 15nF 3511 TA05 SW BIAS C3 4.7µF 3511fa 21 LT3511 Typical applicaTions 24V to 15V Isolated Flyback Converter VIN 20V TO 30V 2:1 R1 1M R2 80.6k VIN EN/UVLO R3 237k R4 10k D2 Z1 T1 350µH 88µH D1 VOUT+ 15V 0.09A C1 4.7µF C4 10µF LT3511 RFB RREF 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 TC VC R5 133k GND R6 20k C2 4.7nF 3511 TA06 SW BIAS C3 4.7µF 12V to 15V Isolated Flyback Converter VIN 8V TO 20V C1 4.7µF 2:1 R1 1M R2 562k VIN EN/UVLO RFB RREF R3 237k R4 10k D2 Z1 T1 350µH D1 C4 4.7µF VOUT+ 15V 40mA 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 88µH LT3511 TC VC R5 133k GND R6 26.1k C2 4.7nF 3511 TA08 SW BIAS C3 4.7µF 3511fa 22 LT3511 Typical applicaTions 12V to ±70V Isolated Flyback Converter VIN 8V TO 20V 1:5:5 C1 2.2µF R1 1M R2 562k EN/UVLO R3 105k R4 10k D3 VIN Z1 T1 80µH D2 D1 VOUT1+ 70V 4mA C4 0.47µF VOUT1– VOUT2+ 4mA C5 0.47µF VOUT2– –70V LT3511 RFB RREF TC VC R5 191k GND R6 90k C2 6.8nF 3511 TA07 SW BIAS C3 4.7µF 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 48V to 3.3V Non-Isolated Flyback Converter VIN 36V TO 72V C1 1µF 6:1 R1 1M R2 43.2k VIN EN/UVLO RFB R3 1M 8.66k R4 5k Z1 D2 T1 400µH 11µH D1 VOUT 3.3V 0.4A C4 47µF VOUT– VOUT 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 LT3511 RREF TC VC R5 1M GND R6 8k C2 4.7nF SW BIAS C3 4.7µF 3511 TA09 48V to 12V Isolated Flyback Converter VIN 36V TO 72V C1 1µF 2:1 R1 1M R2 43.2k VIN EN/UVLO R3 191k R4 10k D2 Z1 T1 300µH 75µH D1 VOUT+ 12V 0.1A C4 4.7µF LT3511 RFB RREF 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 TC VC R5 143k GND R6 15k C2 6.8nF SW BIAS 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 0.889 ± 0.127 (.035 ± .005) 5.23 (.206) MIN 3.20 – 3.45 (.126 – .136) 4.039 ± 0.102 (.159 ± .004) (NOTE 3) 16 14 121110 9 0.305 ± 0.038 (.0120 ± .0015) TYP 0.50 (.0197) BSC 4.90 ± 0.152 (.193 ± .006) 0.280 ± 0.076 (.011 ± .003) REF RECOMMENDED SOLDER PAD LAYOUT 0.254 (.010) GAUGE PLANE DETAIL “A” 0° – 6° TYP 3.00 ± 0.102 (.118 ± .004) (NOTE 4) 1 0.53 ± 0.152 (.021 ± .006) DETAIL “A” 0.18 (.007) SEATING PLANE 1.10 (.043) MAX 3 5678 1.0 (.0394) BSC 0.86 (.034) REF 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 0.17 – 0.27 (.007 – .011) TYP 0.50 (.0197) BSC 0.1016 ± 0.0508 (.004 ± .002) 3511fa 24 LT3511 revision hisTory REV A DATE 4/11 DESCRIPTION Added MP-grade. Revised RFB pin description in the Pin Functions section. Updated efficiency equation and Table 1 in the Applications Information section. Revised the Typical Applications drawings. PAGE NUMBER 2, 3 5 9, 10 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 R2 43.2k EN/UVLO R3 237k R4 10k D3 VIN Z1 T1 350µH D1 C4 4.7µF VOUT1+ 15V 50mA 88µH LT3511 RFB RREF D2 C5 4.7µF VOUT1– VOUT2+ 50mA 88µH TC VC R5 154k GND R6 20k C2 6.8nF SW BIAS 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 C3 4.7µF 3511 TA11 relaTeD parTs PART NUMBER LT3958 LT3748 LT3957 LT3956 LT3575 LT3573 LT3574 LT3757 LT3758 DESCRIPTION High Input Voltage Boost, Flyback, SEPIC and Inverting Converter 100V Isolated Flyback Controller Boost, Flyback, SEPIC and Inverting Converter Constant-Current, Constant-Voltage Boost, Buck, Buck-Boost, SEPIC or Flyback Converter Isolated Flyback Switching Regulator with 60V/2.5A Integrated Switch Isolated Flyback Switching Regulator with 60V/1.25A Integrated Switch Isolated Flyback Switching Regulator with 60V/0.65A Integrated Switch Boost, Flyback, SEPIC and Inverting Controller Boost, Flyback, SEPIC and Inverting Controller COMMENTS 5V ≤ VIN ≤ 80V, 3.3A/84V Onboard Power Switch, 5mm × 6mm QFN-36 with High Voltage Pin Spacing 5V ≤ VIN ≤ 100V, No Opto-Isolator or “Third Winding” Required, Onboard Gate Driver, MSOP-16 with High Voltage Pin Spacing 3V ≤ VIN ≤ 40V, 5A/40V Onboard Power Switch, 5mm × 6mm QFN-36 with High Voltage Pin Spacing 4.5V ≤ VIN ≤ 80V, 3.3A/84V Onboard Power Switch, True PWM Dimming, 5mm × 6mm QFN-36 with High Voltage Pin Spacing 3V ≤ VIN ≤ 40V, No Opto-Isolator or “Third Winding” Required, Up to 14W, TSSOP-16E 3V ≤ VIN ≤ 40V, No Opto-Isolator or “Third Winding” Required, Up to 7W, MSOP-16E 3V ≤ VIN ≤ 40V, No Opto-Isolator or “Third Winding” Required, Up to 3W, MSOP-16 2.9V ≤ VIN ≤ 40V, 100kHz to 1MHz Programmable Operating Frequency, 3mm × 3mm DFN-10 and MSOP-10E Package 5.5V ≤ VIN ≤ 100V, 100kHz to 1MHz Programmable Operating Frequency, 3mm × 3mm DFN-10 and MSOP-10E Package 2.5V ≤ VIN ≤ 36V, Burst Mode® Operation LTC1871/LTC1871-1/ No RSENSE™ Low Quiescent Current Flyback, Boost and SEPIC Controller LTC1871-7 3511fa 26 Linear Technology Corporation 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● LT 0411 REV A • PRINTED IN USA www.linear.com  LINEAR TECHNOLOGY CORPORATION 2010