LT3512_11

LT3512 Monolithic High Voltage Isolated Flyback Converter FEATURES n n n n n n n n n DESCRIPTION The LT3512 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 420mA, 150V power switch, high voltage circuitry, and control into a high voltage 16-lead MSOP package with four leads removed. The LT3512 operates from an input voltage range of 4.5V to 100V and delivers up to 4.5W 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. 4.5V to 100V Input Voltage Range Internal 420mA, 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 1μF 1M EN/UVLO 43.2k LT3512 RFB RREF 10k TC VC 57.6k SW GND BIAS 12.7k 4.7nF 3512 TA01a Output Load and Line Regulation VOUT+ 5V 0.5A 11μH 47μF VOUT (V) 5.25 5.20 5.15 5.10 5.05 5.00 4.95 4.90 4.85 4.80 4.75 0 100 200 300 400 LOAD CURRENT (mA) 500 3512 TA01b VIN 175μH 169k VOUT– VIN = 48V VIN = 36V VIN = 72V 4.7μF 3512f 1 LT3512 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) LT3512E, LT3512I .............................. –40°C to 125°C LT3512H ............................................ –40°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 LT3512EMS#PBF LT3512IMS#PBF LT3512HMS#PBF TAPE AND REEL LT3512EMS#TRPBF LT3512IMS#TRPBF LT3512HMS#TRPBF PART MARKING* 3512 3512 3512 PACKAGE DESCRIPTION 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 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. 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 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. CONDITIONS l MIN 6 4.5 TYP MAX 100 15 UNITS V V mA μA V μA μA kHz kHz VIN = BIAS Quiescent Current EN/UVLO Pin Threshold EN/UVLO Pin Current Maximum Switching Frequency Minimum Switching Frequency Maximum Current Limit Minimum Current Limit Switch VCESAT RREF Voltage RREF Voltage Line Regulation RREF Pin Bias Current Error Amplifier Voltage Gain Error Amplifier Transconductance ∆I = 2μA 6V < VIN < 100V (Note 3) l Not Switching VEN/UVLO = 0.2V EN/UVLO Pin Voltage Rising VEN/UVLO =1.1V VEN/UVLO =1.4V l 3.5 0 1.15 2.0 1.21 2.6 0 650 40 420 80 600 120 0.5 l 4.5 1.27 3.3 800 150 1.215 1.23 0.03 400 mA mA V V V %/V nA V/V μmhos ISW = 200mA 1.18 1.17 1.20 0.01 80 150 140 3512f 2 LT3512 ELECTRICAL CHARACTERISTICS PARAMETER TC Current into RREF BIAS Pin Voltage 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. CONDITIONS RTC = 53.6k Internally Regulated 3 MIN TYP 9.5 3.1 3.2 MAX UNITS 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 LT3512E 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 LT3512I is guaranteed to meet performance specifications from –40°C to 125°C operating junction temperature range. The LT3512H is guaranteed over the full –40°C to 150°C operating junction temperature 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) 3512 G01 TA = 25°C, unless otherwise noted. BIAS Pin Voltage 4.0 Quiescent Current 8 VIN = 48V 6 BIAS VOLTAGE (V) VIN = 24V VIN = 48V VIN = 100V 0 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 3512 G02 3.5 4 3.0 2 2.5 VIN = 24V, 10mA VIN = 24V, NO LOAD 2.0 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 3512 G03 Switch VCESAT 2000 SWITCH VCESAT VOLTAGE (mV) 800 700 1600 CURRENT LIMIT (mA) 600 500 400 300 200 100 0 0 Switch Current Limit 5 MAXIMUM CURRENT LIMIT 4 Quiescent Current vs VIN 800 IQ (mA) MINIMUM CURRENT LIMIT 0 25 50 75 100 125 150 TEMPERATURE (°C) 3512 G05 1200 3 2 400 1 300 100 200 400 SWITCH CURRENT (mA) 500 3512 G04 0 –50 –25 0 0 20 60 40 VOLTAGE (V) 80 100 3512 G06 3512f 3 LT3512 TYPICAL PERFORMANCE CHARACTERISTICS EN/UVLO Pin (Hysteresis) Current vs Temperature 5 EN/UVLO = 1.2V EN/UVLO PIN CURRENT (μA) EN/UVLO PIN CURRENT (μA) 4 25 EN/UVLO THRESHOLD (V) 20 15 10 5 0 0 25 50 75 100 125 150 TEMPERATURE (°C) 3512 G07 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 30 3 2 1 0 –50 –25 1 20 60 80 VEN/UVLO VOLTAGE (V) 40 100 3512 G08 0 25 50 75 100 125 150 TEMPERATURE (°C) 3512 G09 Maximum Frequency vs Temperature 1000 900 MAXIMUM FREQUENCY (kHz) MINIMUM FREQUENCY (kHz) 800 700 600 500 400 300 200 100 0 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 3512 G10 Minimum Frequency vs Temperature 100 90 EN/UVLO THRESHOLD (V) 80 70 60 40 40 30 20 10 0 –50 –25 0 25 50 75 100 125 150 TEMPERATURE (°C) 3512 G11 EN/UVLO Shutdown Threshold vs Temperature 0.9 0.8 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) 3512 G14 Boundary Mode Waveform Light Load Discontinuous Mode Waveform 20V/DIV 20V/DIV 2μs/DIV 3512 G12 2μs/DIV 3512 G13 3512f 4 LT3512 PIN FUNCTIONS EN/UVLO (Pin 1): Enable/Undervoltage Lockout. The EN/ UVLO pin is used to start up the LT3512. Pull the pin to 0V to shut down the LT3512. 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 LT3512. 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 VIN. SW (Pin 16): Switch Pin. Collector of the internal power switch. Minimize trace area at this pin to minimize EMI and voltage spikes. 3512f 5 LT3512 BLOCK DIAGRAM D1 VIN C1 L1A R3 N:1 VIN Q3 TC R5 I2 RREF R4 BIAS C4 R1 EN/UVLO R2 3μA Q4 R6 C3 3512 BD T1 L1B C2 VOUT + VOUT – TC CURRENT RFB Q2 FLYBACK ERROR AMP CURRENT COMPARATOR ONE SHOT A2 SW – + 1.2V – gm + + VIN – A1 + S S MASTER LATCH R Q V1 120mV DRIVER BIAS – Q1 A4 + – 1.21V + A5 – RSENSE 0.01Ω GND INTERNAL REFERENCE AND REGULATORS OSCILLATOR VC 3512f 6 LT3512 OPERATION The LT3512 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 LT3512, 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 LT3512 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 LT3512 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. 3512f 7 LT3512 APPLICATIONS INFORMATION PSUEDO DC THEORY In the Block Diagram, RREF (R4) and RFB (R3) are external resistors used to program the output voltage. The LT3512 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: ⎛ VFLBK ⎞ VBG ⎛R ⎞ o r VFLBK = VBG ⎜ FB ⎟ ⎜ R ⎟=R ⎝ FB ⎠ ⎝ RREF ⎠ REF 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 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: R 1 δ VF = − FB • • NPS δT R TC −R 1 R TC = FB • NPS δ VF / δ T (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) ⎝ R TC ⎠ 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 3512f δ VTC or, δT δV R • TC ≈ FB δT NPS (δVF / δT) = Diode’s forward voltage temperature coefficient 8 LT3512 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 off-time. 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. 5.0 N = 15 4.0 OUTPUT POWER (W) N = NPS(MAX) N = 12 N = 10 N=8 N=6 2.0 N=4 OUTPUT POWER (W) 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 3.0W at 72V but lowers to 2.5W at 36V. The equations below calculate output power: Power = η • VIN • D • IPEAK • 0.5 Efficiency = η = ~83% Duty cycle = D = ( VOUT + VF ) •NPS ( VOUT + VF ) •NPS + VIN Peak switch current = IPEAK = 0.44A 5.0 N=5 N = NPS(MAX) N=4 N=3 N=2 4.0 3.0 3.0 2.0 N=1 1.0 N=2 1.0 0 0 20 40 60 INPUT VOLTAGE (V) 80 100 3512 F01 0 0 20 40 60 INPUT VOLTAGE (V) 80 100 3512 F03 Figure 1. Output Power for 3.3V Output 5.0 N = NPS(MAX) N=8 N=7 N=6 N=5 N=4 N=3 2.0 N=2 N=1 Figure 3. Output Power for 12V Output 5.0 N = NPS(MAX) 4.0 OUTPUT POWER (W) N=2 4.0 OUTPUT POWER (W) 3.0 3.0 N=1 2.0 1.0 1.0 0 0 20 40 60 INPUT VOLTAGE (V) 80 100 3512 F02 0 0 20 40 60 INPUT VOLTAGE (V) 80 100 3512 F04 Figure 2. Output Power for 5V Output Figure 4. Output Power for 24V Output 3512f 9 LT3512 APPLICATIONS INFORMATION Table 1. Predesigned Transformers TRANSFORMER PART NUMBER 750311559 LPRI (μH) 175 LEAKAGE (μH) 1.5 NP:NS:NB 4:1:1 ISOLATION (V) 1500 SATURATION CURRENT (mA) 800 VENDOR Würth Elektronik TARGET APPLICATIONS 48V to 5V, 0.5A 24V to 5V, 0.38A 12V to 5V, 0.2A 48V to 3.3V, 0.59A 24V to 3.3V, 0.48A 12V to 3.3V, 0.29A 24V to 5V, 0.45A 12V to 5V, 0.23A 48V to 3.3V, 0.7A 24V to 3.3V, 0.59A 12V to 3.3V, 0.33A 48V to 24V, 0.11A 48V to 15V, 0.2A 48V to 12V, 0.22A 24V to 15V, 0.15A 12V to 15V, 0.075A 48V to ±15V, 0.1A 48V to ±12V, 0.11A 24V to ±15V, 0.075A 12V to ± 70V, 0.007A 12V to ± 100V, 0.005A 12V to ± 150V, 0.004A 12V to +120V& -12V, 0.005A 12V ± 70V, 0.007A 48V to 5V, 0.5A 24V to 5V, 0.38A 12V to 5V, 0.2A 48V to 3.3V, 0.59A 24V to 3.3V, 0.48A 12V to 3.3V, 0.29A 24V to 5V, 0.45A 12V to 5V, 0.23A 48V to 3.3V, 0.7A 24V to 3.3V, 0.59A 12V to 3.3V, 0.33A 48V to 24V, 0.11A 48V to 15V, 0.2A 48V to 12V, 0.22A 24V to 15V, 0.15A 12V to 15V, 0.075A 48V to ±15V, 0.1A 48V to ±12V, 0.11A 24V to ±15V, 0.075A 12V to ± 70V, 0.007A 12V to ± 100V, 0.005A 12V to ± 150V, 0.004A 750311573 200 2 6:1:2 1500 800 Würth Elektronik 750311662 750311661 151 150 2 1.85 1:1:0.2 2:1:0.66 1500 1500 800 1.1A Würth Elektronik Würth Elektronik 750311839 200 3 2:1:1 1500 800 Würth Elektronik Würth Elektronik Würth Elektronik Würth Elektronik Sumida 750311964 100 0.7 1:5:5 1500 900 750311966 750311692 10396-T025 120 80 200 0.45 2 2.0 1:5:0.5 1:5:5 4:1:1.2 1500 1500 1500 900 1.0A 800 10396-T027 200 2.0 6:1:2 1500 800 Sumida 01355-T058 10396-T023 125 200 2.0 2.0 1:1:0.2 2:1:0.33 1500 1500 800 800 Sumida Sumida 10396-T029 200 2.5 2:1:1 1500 800 Sumida 01355-T061 100 2 1:5:5 1500 800 Sumida 3512f 10 LT3512 APPLICATIONS INFORMATION TRANSFORMER DESIGN CONSIDERATIONS Successful application of the LT3512 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 LT3512. Table 1 shows the details of these transformers. 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 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%. 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 LT3512. Primary Inductance Requirements The LT3512 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) = 100mA 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, 3512f 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. 11 LT3512 APPLICATIONS INFORMATION 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. 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 LT3512. 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. VSW 72V The diode needs to handle the peak switch current of the switch which was determined to be 0.45A. A 100V, 1.0A diode from Diodes Inc. (DFLS1100) 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 15k resistor and a 4.7nF 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) = Example: RFB(NEW) = 15V • 267k = 237k 16.7V VOUT VOUT(MEAS) •RFB(OLD) 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: VOUT measured at 200mA and 48VIN ΔVOUT 15.42V – 15.02V = = 2.26mV °C ΔTemp 125°C – ( −50°C) ( VOUT + VF + 0.55V ) •NPS •RREF 1.2V RFB NPS RREF = 10k R TC = Example: RFB = R TC = (15 + 0.5 + 0.55V ) • 2 • 10k = 267k 1.2V 267k = 133k 2 3512f 18 LT3512 APPLICATIONS INFORMATION Step 11: Calculate new value for RTC. R TC(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: 237k 1.85 • = 97.6k 2 2.26 The minimum load at an input voltage of 72V is: 11mA Step 16: EN/UVLO resistor values. Determine amount of hysterysis required. Voltage hysteresis = 2.6μA • R1 Example: Choose 2V of hysteresis. 15V • 237k = 243k 14.7V R1= 2V = 768k 2.6µA Example: R TC(NEW) = Step 12: Place new value for RTC, measure VOUT , and readjust RFB due to RTC change. RFB(NEW) = Example: RFB(NEW) = VOUT VOUT(MEAS) •RFB(OLD) 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 = 18.7k, CC = 4.7nF Determine UVLO Threshold. 1.2V • (R1+ R2) R2 1.2V •R1 R2 = VIN(UVLO,FALLING) – 1.2V VIN(UVLO,FALLING) = 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 3512f 19 LT3512 TYPICAL APPLICATIONS 48V to 5V Isolated Flyback Converter VIN 36V TO 72V 4:1:1 C1 1μF R1 1M EN/UVLO R2 43.2k LT3512 RFB RREF R3 169k R4 10k D3 VIN Z1 D1 VOUT+ 5V 0.5A C4 47μF VOUT– C1: TAIYO YUDEN HMK316B7105KL-T C3: TAIYO YUDEN EMK212B7475KG C4: TAIYO YUDEN LMK325B7476MM-TR D1: DIODES INC. SBR2A40P1 D2: CENTRAL SEMI CMDSH-3 D3: DIODES INC. DFLS1100 T1: WÜRTH 750311559 Z1: ON SEMI MMSZ5266BT1G T1 175μH 11μH TC VC R5 57.6k GND R6 12.7k C2 4.7nF SW BIAS D2 L1C 11μH 3512 TA02 C3 4.7μF OPTIONAL THIRD WINDING FOR HV OPERATION 48V to 15V Isolated Flyback Converter VIN 36V TO 72V C1 1μF 2:1 R1 1M EN/UVLO R2 43.2k LT3512 RFB RREF R3 243k R4 10k D2 VIN Z1 T1 200μH 50μH D1 VOUT+ 15V 0.2A C4 22μF VOUT– C1: TAIYO YUDEN HMK316B7105KL-T C3: TAIYO YUDEN EMK212B7475KG C4: MURATA GRM32ER71E226KE15B D1: DIODES INC. DFLS160 D2: DIODES INC. DFLS1100 T1: SUMIDA 10396-T023 Z1: ON SEMI MMSZ5266BT1G TC VC R5 97.6k GND R6 18.7k C2 4.7nF 3512 TA03 SW BIAS C3 4.7μF 48V to 24V Isolated Flyback Converter VIN 36V TO 72V C1 1μF 1:1 R1 1M EN/UVLO R2 43.2k LT3512 RFB RREF R3 187k R4 10k D2 VIN Z1 T1 151μH D1 VOUT+ 24V 110mA C4 10μF VOUT– C1: TAIYO YUDEN HMK316B7105KL-T C3: TAIYO YUDEN EMK212B7475KG C4: TAIYO YUDEN UMK316AB7475KL-T D1: DIODES INC. SBR1U150SA D2: DIODES INC. DFLS1100 T1: WÜRTH 750311662 Z1: ON SEMI MMSZ5266BT1G 151μH TC VC R5 162k GND R6 24.3k C2 2.2nF 3512 TA04 SW BIAS C3 4.7μF 3512f 20 LT3512 TYPICAL APPLICATIONS 24V to 5V Isolated Flyback Converter VIN 20V TO 30V 6:1 C1 4.7μF R1 1M EN/UVLO R2 80.6k LT3512 RFB RREF R3 249k R4 10k D2 VIN Z1 T1 200μH 5.5μH D1 VOUT+ 5V 0.45A C4 47μF VOUT– C1: TAIYO YUDEN UMK316AB7475KL-T C3: TAIYO YUDEN EMK212B7475KG C4: TAIYO YUDEN LMK32587476MM-TR D1: DIODES INC. SBR2A30P1 D2: DIODES INC. DFLS1100 T1: SUMIDA 10396-T027 Z1: ON SEMI MMSZ5270BT1G TC VC R5 69.8k GND R6 6.49k C2 4.7nF 3512 TA05 SW BIAS C3 4.7μF 24V to 15V Isolated Flyback Converter VIN 20V TO 30V 2:1 C1 4.7μF R1 1M EN/UVLO R2 80.6k LT3512 RFB RREF R3 237k R4 10k D2 VIN Z1 T1 200μH D1 VOUT+ 15V 0.15A C4 22μF VOUT– C1: TAIYO YUDEN UMK316AB7475KL-T C3: TAIYO YUDEN EMK212B7475KG C4: MURATA GRM32ER71E226KE158 D1: DIODES INC. SBR140S3 D2: DIODES INC. DFLS1100 T1: SUMIDA 10396-T023 Z1: ON SEMI MMSZ5270BT1G 50μH TC VC R5 150k GND R6 20k C2 4.7nF 3512 TA06 SW BIAS C3 4.7μF 12V to 15V Isolated Flyback Converter VIN 8V TO 20V 2:1 C1 4.7μF R1 1M EN/UVLO R2 562k LT3512 RFB RREF R3 237k R4 10k D2 VIN Z1 T1 150μH D1 VOUT+ 15V 70mA C4 10μF Z2 VOUT– OPTIONAL MINIMUM LOAD C1: TAIYO YUDEN UMK316AB7475KL-T C3: TAIYO YUDEN EMK212B7475KG C4: MURATA GRM32ER7IE226K D1: DIODES INC. SBR2A40P1 D2: DIODES INC. DFLS1100 T1: WÜRTH 750311661 Z1: ON SEMI MMSZ5270BT1G 38μH TC VC R5 107k GND R6 21.5k C2 6.8nF 3512 TA08 SW BIAS C3 4.7μF 3512f 21 LT3512 TYPICAL APPLICATIONS 12V to ±70V Isolated Flyback Converter C6 R7 10pF 3k VIN 10V TO 20V 1:5:5 C1 2.2μF R1 1M EN/UVLO R2 562k R3 100k LT3512 RFB RREF D2 R4 10k VOUT2+ 7mA D3 C7 R8 10pF 3k VIN Z1 T1 100μH D1 VOUT1+ 70V 7mA C4 0.47μF VOUT1– TC VC R5 1M GND R6 24.9k C2 6.8nF 3512 TA07 SW BIAS C3 4.7μF C5 0.47μF VOUT2– –70V C1: TAIYO YUDEN UMK316AB7475KL-T C3: TAIYO YUDEN EMK212B7475KG C4, C5: NIPPON CHEMI-CON KTS251B474M43N0T00 D1, D2: DIODES INC. ES1G D3: DIODES INC. DFLS1100 T1: WÜRTH 750311692 Z1: ON SEMI MMS2527OBT1G 48V to 3.3V Non-Isolated Flyback Converter VIN 36V TO 72V 6:1 C1 1μF R1 1M EN/UVLO R2 43.2k LT3512 RREF VIN RFB R3 1M 8.66k VOUT R4 5k C1: TAIYO YUDEN HMK316B7105KL-T C3: TAIYO YUDEN EMK212B7475KG C4: TAIYO YUDEN LMK325B7476MM-TR ×2 D1: DIODES INC. SBR3U30P1 D2: DIODES INC. DFLS1100 T1: WÜRTH 750311573 Z1: ON SEMI MMSZ5266BT1G Z1 D2 T1 200μH D1 VOUT+ 3.3V 0.7A 5.5μH C4 47μF ×2 VOUT– TC VC R5 1M GND R6 9.53k C2 4.7nF SW BIAS C3 4.7μF 3512 TA09 3512f 22 LT3512 TYPICAL APPLICATIONS 48V to 12V Isolated Flyback Converter VIN 36V TO 72V C1 1μF 2:1 R1 1M EN/UVLO R2 43.2k LT3512 RFB RREF R3 191k R4 10k D2 VIN Z1 T1 200μH 50μH D1 VOUT+ 12V 0.2A C4 10μF VOUT– C1: TAIYO YUDEN HMK316B7105KL-T C3: TAIYO YUDEN EMK212B7475KG C4: TAIYO YUDEN TMK316AB7106KL-T D1: DIODES INC. DFLS160 D2: DIODES INC. DFLS1100 T1: SUMIDA 10396-T023 Z1: ON SEMI MMSZ5266BT1G TC VC R5 75k GND R6 5.23k C2 4.7nF SW BIAS C3 4.7μF 3512 TA10 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 (.035 0.127 .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 DETAIL “A” 0 – 6 TYP 3.00 0.102 (.118 .004) (NOTE 4) 0.254 (.010) GAUGE PLANE 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 (.004 0.0508 .002) 3512f 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. 23 LT3512 TYPICAL APPLICATION 48V to ±15V Isolated Flyback Converter VIN 36V TO 72V 2:1:1 C1 1μF R1 1M EN/UVLO R2 43.2k LT3512 RFB RREF R4 10k R3 237k D3 D2 VIN Z1 T1 200μH D1 VOUT1+ 15V 100mA C4 10μF VOUT1– VOUT2+ 100mA C5 10μF VOUT2– –15V 50μH 50μH TC VC R5 287k GND R6 8.66k C2 6.8nF SW BIAS C3 4.7μF 3512 TA11 C1: TAIYO YUDEN HMK316B7105KL-T C3: TAIYO YUDEN EMK212B7475KG C4, C5: TAIYO YUDEN TMK316AB7106KL-T D1, D2: DIODES INC. SBR0560S1 D3: DIODES INC. DFLS1100 T1: WÜRTH 750311839 Z1: ON SEMI MMSZ5266BT16 RELATED PARTS PART NUMBER LT3511 LT3748 LT3958 LT3957 LT3956 LT3575 LT3573 LT3574 LT3757 LT3758 DESCRIPTION Monolithic High Voltage Isolated Flyback Converter 100V Isolated Flyback Controller High Input Voltage Boost, Flyback, SEPIC and Inverting Converter 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 4.5V ≤ VIN ≤ 100V, 240mA/150V Onboard Power Switch, MSOP-16 with High Voltage Spacing 5V ≤ VIN ≤ 100V, No Opto-Isolator or “Third Winding” Required, Onboard Gate Driver, MSOP-16 with High Voltage Pin Spacing 5V ≤ VIN ≤ 80V, 3.3A/84V Onboard Power Switch, 5mm × 6mm QFN-36 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 at Light Loads, MSOP-10 LTC1871/LTC1871-1/ No RSENSE™ Low Quiescent Current Flyback, Boost LTC1871-7 and SEPIC Controller 3512f 24 Linear Technology Corporation (408) 432-1900 ● FAX: (408) 434-0507 ● LT 0211 • PRINTED IN USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 www.linear.com © LINEAR TECHNOLOGY CORPORATION 2011