LT3825 Isolated No-Opto Synchronous Flyback Controller with Wide Input Supply Range DESCRIPTION FEATURES n n n n n n n n n The LT®3825 is an isolated switching regulator controller designed for medium power flyback topologies. A typical application is 10W to 60W with input voltage limited only by external power path components. A third transformer winding provides output voltage feedback. Senses Output Voltage Directly from Primary Side Winding—No Optoisolator Required Synchronous Driver for High Efficiency Input Voltage Limited Only by External Power Components Accurate Output Regulation Without User Trims Switching Frequency from 50kHz to 250kHz Synchronizable Load Compensation Programmable Undervoltage Lockout Available in a Thermally Enhanced 16-Lead TSSOP Package The LT3825 is a current mode controller that regulates output voltage based on sensing secondary voltage via a transformer winding during flyback. This allows for tight output regulation without the use of an optoisolator, improving dynamic response and reliability. Synchronous rectification increases converter efficiency and improves output cross regulation in multiple output converters. APPLICATIONS n n n n The LT3825 operates in forced continuous conduction mode which improves cross regulation in multiple winding applications. Switching frequency is user programmable and can be externally synchronized. The part also has load compensation, undervoltage lockout and soft-start circuity. Isolated Medium Power (10W to 60W) Supplies Isolated Telecom, Medical Converters Instrumentation Power Supplies Isolated Power over Ethernet Supplies L, LTC and LT are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. Protected by U.S. Patents, including 6948466, 5841643. Efficiency TYPICAL APPLICATION 92 90 36VIN 48VIN 48V to 3.3V at 12V Isolated Supply VIN+ 36V TO 72V 100pF T1 28.7k 20Ω • 3.01k FB VCC SG PG SENSE+ UVLO LT3825 15k PGDLY tON SYNC RCMP ENDLY 100k 2.1k 150k 47pF 2 3 4 VC OSC GND SFST 3.38 15Ω 2.2nF • 36VIN 48VIN 72VIN 3.33 CCMP 0.1μF 3825 TA01b 1μF 0.1μF 0.22μF 10 11 12 3.43 330Ω SENSE– 5 6 7 8 9 LOAD CURRENT (A) Regulation ×2 SG 0.02Ω 10nF 12k 78 SG 402k 2.2μF 80 47Ω + 47μF 84 • 20Ω 47k 72VIN 86 82 OUTPUT (V) T1 470μF ×4 • VOUT+ 3.3V 12A EFFICIENCY (%) 88 • 10k 100k 3825 TA01a 3.28 3.23 3.18 3.13 2 3 4 5 6 7 8 9 LOAD CURRENT (A) 10 11 12 3825 TA01c 3825fa 1 LT3825 ABSOLUTE MAXIMUM RATINGS PIN CONFIGURATION (Note 1) VCC to GND Low Impedance Source ........................ –0.3V to 18V Current Fed (VCC has Internal 19.5V Clamp) ...........30mA into VCC UVLO, SYNC Pin Voltage ........................... –0.3V to VCC SENSE–, SENSE+ Pin Voltage ...................... –0.5V, +0.5V FB Pin Current........................................................±2mA VC Pin Current....................................................... ±1mA Operating Junction Temperature Range (Notes 2, 3) .......................................... –40°C to 125°C Storage Temperature Range.................. –65°C to 150°C Lead Temperature (Soldering, 10 sec) .................. 300°C TOP VIEW SG 1 16 PG VCC 2 15 PGDLY tON 3 14 RCMP ENDLY 4 SYNC 5 SFST 6 11 SENSE– OSC 7 10 UVLO FB 8 9 13 CCMP 17 12 SENSE+ VC FE PACKAGE 16-LEAD PLASTIC TSSOP TJMAX = 125°C, θJA = 40°C/W, θJC = 10°C/W EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB ORDER INFORMATION LEAD FREE FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE LT3825EFE#PBF LT3825EFE#TRPBF 3825EFE 16-Lead Plastic 4.4mm TSSOP –40°C to 125°C LEAD BASED FINISH TAPE AND REEL PART MARKING PACKAGE DESCRIPTION TEMPERATURE RANGE LT3825EFE LT3825EFE#TR 3825EFE 16-Lead Plastic 4.4mm TSSOP –40°C to 125°C Consult LTC Marketing for parts specified with wider operating temperature ranges. 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. VCC = 14V; PG, SG Open; VC = 1.5V, VSENSE– = 0V; RCMP = 1k, RtON = 90k, RPGDLY = 27.4k, RENDLY = 90k, unless otherwise specified. PARAMETER CONDITIONS MIN TYP MAX UNITS 16.0 V Power Supply VCC Turn-On Voltage l 14.0 15.3 VCC Turn-Off Voltage ● 8 9.7 11 V VCC(ON) – VCC(OFF) ● 4.0 5.6 6.5 V VCC Hysteresis VCC Shunt Clamp VUVLO = 0V, IVCC = 15mA ● 19.5 20.5 VCC Supply Current (Note 5) (ICC) VC = Open ● 4 6.4 10 mA VCC Start-Up Current VCC = 10V ● 180 400 μA 1.251 V μmho V Feedback Amplifier Feedback Regulation Voltage (VFB) Feedback Pin Input Bias Current RCMP Open Feedback Amplifier Transconductance ΔIC = ±10μA Feedback Amplifier Source or Sink Current Feedback Amplifier Clamp Voltage VFB = 0.9V VFB = 1.4V ● 1.220 1.237 ● 700 1000 1400 ● 25 55 90 200 2.56 nA μA V 3525fa 2 LT3825 ELECTRICAL CHARACTERISTICS The l denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C. VCC = 14V; PG, SG Open; VC = 1.5V, VSENSE– = 0V; RCMP = 1k, RtON = 90k, RPGDLY = 27.4k, RENDLY = 90k, unless otherwise specified. PARAMETER CONDITIONS MIN ● TYP MAX UNITS 0.005 0.02 %/V Reference Voltage Line Regulation 12V ≤ VCC ≤ 18V Feedback Amplifier Voltage Gain VC = 1.2V to 1.7V Soft-Start Charging Current VSFST = 1.5V 16 20 Soft-Start Discharge Current VSFST = 1.5V, VUVLO = 0V 0.8 1.3 mA Control Pin Threshold (VC) Duty Cycle = Min 1.0 V 1400 V/V 25 μA Gate Outputs ● PG, SG Output High Level PG, SG Output Low Level PG, SG Output Shutdown Strength VUVLO = 0V; IPG, ISG = 20mA PG Rise Time CPG = 1nF 6.6 7.4 8.0 ● 0.01 0.05 V ● 1.6 2.3 V 11 V ns SG Rise Time CSG = 1nF 15 ns PG, SG Fall Time CPG, CSG = 1nF 10 ns Current Amplifier Switch Current Limit at Maximum VC VSENSE+ ● 88 ΔVSENSE/ΔVC VSENSE+, VSFST < 1V ● Switching Frequency (fOSC) COSC = 100pF ● Oscillator Capacitor Value (COSC) (Note 6) Sense Voltage Overcurrent Fault Voltage 98 110 0.07 mV V/V 206 230 mV 100 110 kHz Timing 84 33 Minimum Switch On Time (tON(MIN)) 200 200 pF ns Flyback Enable Delay Time (tED) 265 ns PG Turn-On Delay Time (tPGDLY) 200 ns 88 % Maximum Switch Duty Cycle ● SYNC Pin Threshold ● 85 1.53 SYNC Pin Input Resistance 2.1 V 40 kΩ Load Compensation Load Comp to VSENSE Offset Voltage VRCMP with VSENSE+ = 0V 1 mV Feedback Pin Load Compensation Current VSENSE+ = 20mV, VFB = 1.230V 20 μA UVLO Function ● UVLO Pin Threshold (VUVLO) UVLO Pin Bias Current VUVLO = 1.2V VUVLO = 1.3V 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: This IC includes overtemperature protection that is intended to protect the device during momentary overload conditions. Junction temperature will exceed 125°C when overtemperature protection is active. Continuous operation above the specified maximum operating junction temperature may impair device reliability. Note 3: The LT3825E is guaranteed to meet performance specifications from 0°C to 125°C. Specifications over the –40°C to 125°C operating 1.215 1.240 1.265 V –0.25 –4.50 0 –3.4 ±0.25 –2.50 μA μA temperature range are assured by design, characterization and correlation with statistical process controls. Note 4: TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formula: TJ = TA + (PD • 40°C/W) Note 5: Supply current does not include gate charge current to the MOSFETs. See Applications Information. Note 6: Component value range guaranteed by design. 3825fa 3 LT3825 TYPICAL PERFORMANCE CHARACTERISTICS VCC(0N) and VCC(OFF) vs Temperature VCC Start-Up Current vs Temperature 16 VCC Current vs Temperature 10 300 VCC(ON) 15 9 250 14 12 11 IVCC (mA) IVCC (μA) VCC (V) 8 200 13 150 7 6 STATIC PART CURRENT 100 VCC(OFF) 5 10 50 9 8 –50 –25 0 50 75 25 TEMPERATURE (°C) 100 4 0 –50 –25 125 50 25 75 0 TEMPERATURE (°C) 3825 G01 110 100 98 96 94 108 104 205 200 195 90 –50 –25 50 25 0 75 TEMPERATURE (°C) 100 98 92 0 50 75 25 TEMPERATURE (°C) 100 90 –50 125 –25 50 25 0 75 TEMPERATURE (°C) 3825 G05 3825 G04 1.240 100 125 3825 G06 Feedback Pin Input Bias vs Temperature VFB vs Temperature VFB Reset vs Temperature 1.04 300 RCMP OPEN 1.238 1.237 VFB (V) 1.236 1.235 1.234 1.233 1.232 1.03 250 1.02 200 VFB RESET (V) FEEDBACK PIN INPUT BIAS (nA) 1.239 150 100 –25 50 25 0 75 TEMPERATURE (°C) 100 125 3825 G07 1.01 1.00 0.99 0.98 50 0.97 1.231 1.230 –50 100 94 180 –50 –25 125 102 96 190 185 92 COSC = 100pF 106 210 fOSC (kHz) 102 125 Oscillator Frequency vs Temperature SENSE = VSENSE+ – 215 WITH VSENSE = 0V 104 100 3825 G03 220 FB = 1.1V SENSE = VSENSE+ WITH VSENSE– = 0V SENSE VOLTAGE (mV) SENSE VOLTAGE (mV) 125 SENSE Fault Voltage vs Temperature 110 106 100 VCC = 14V 3 50 –50 –25 25 75 0 TEMPERATURE (°C) 3825 G02 SENSE Voltage vs Temperature 108 DYNAMIC CURRENT CPG = 1nF, CSG = 1nF, fOSC = 100kHz 0 –50 –25 50 25 75 0 TEMPERATURE (°C) 100 125 3825 G08 0.96 –50 –25 0 50 75 25 TEMPERATURE (°C) 100 125 3825 G09 3525fa 4 LT3825 TYPICAL PERFORMANCE CHARACTERISTICS Feedback Amplifier Source and Sink Current vs Temperature 70 70 25°C 10 –10 1050 SINK CURRENT VFB = 1.4V 60 IVC (μA) IVC (μA) 65 –40°C 30 1100 SOURCE CURRENT VFB = 1.1V 125°C 50 Feedback Amplifier gm vs Temperature gm (μmho) Feedback Amplifier Output Current vs VFB 1000 55 50 –30 950 45 –50 40 –50 –70 0.9 1 1.1 1.2 VFB (V) 1.3 1.5 1.4 50 25 75 0 TEMPERATURE (°C) –25 100 –25 75 0 25 50 TEMPERATURE (°C) 3825 G11 3825 G10 Feedback Amplifier Voltage Gain vs Temperature 1700 900 –50 125 100 125 3825 G12 IUVLO Hysteresis vs Temperature UVLO vs Temperature 3.7 1.250 1650 1600 3.6 1.245 1550 1500 3.5 AV (V/V) UVLO (V) 1450 1400 1350 1300 IUVLO (μA) 1.240 1.235 3.4 3.3 1.230 3.2 1250 1200 1.225 3.1 1150 1100 –50 –25 0 50 75 25 TEMPERATURE (°C) 100 125 1.220 –50 –25 50 25 75 0 TEMPERATURE (°C) 100 3825 G13 80 22 70 21 60 20 50 VCC Clamp Voltage vs Temperature 21.5 TA = 25°C ICC = 10mA 21.0 20.5 FALL TIME 40 20.0 30 18 125 VCC (V) TIME (ns) 23 100 3825 G15 PG, SG Rise and Fall Times vs Load Capacitance 19 50 25 75 0 TEMPERATURE (°C) 3825 G14 Soft-Start Charge Current vs Temperature SFST CHARGE CURRENT (μA) 3.0 –50 –25 125 RISE TIME 17 20 16 10 15 –50 –25 19.5 0 75 50 25 TEMPERATURE (°C) 0 100 125 3825 G16 0 1 2 3 4 5 6 7 CAPACITANCE (nF) 8 9 10 3825 G17 19.0 –50 –25 50 25 0 75 TEMPERATURE (°C) 100 125 3825 G18 3825fa 5 LT3825 TYPICAL PERFORMANCE CHARACTERISTICS Minimum PG On Time vs Temperature 340 RtON(MIN) = 158k 330 260 250 RENDLY = 90k 240 RPGDLY = 27.4k 320 200 300 290 220 tED (ns) 310 tPGDLY (ns) tON(MIN) (ns) Enable Delay Time vs Temperature PG Delay Time vs Temperature 300 150 RPGDLY = 16.9k 100 200 180 280 260 –50 –25 160 50 270 75 50 25 TEMPERATURE (°C) 0 100 125 0 –50 –25 25 0 75 50 TEMPERATURE (°C) 3825 G19 100 125 3825 G20 140 –50 –25 50 25 75 0 TEMPERATURE (°C) 100 125 3825 G21 PIN FUNCTIONS SG (Pin 1): Synchronous Gate Drive Output. This pin provides an output signal for a secondary-side synchronous switch. Large dynamic currents may flow during voltage transitions. See the Applications Information for details. VCC (Pin 2): Supply Voltage Pin. Bypass this pin to ground with a 4.7μF capacitor or more. This pin has a 19.5V clamp to ground. VCC has an undervoltage lockout function that turns the part on when VCC is approximately 15.3V and off at 9.7V. In a conventional “trickle-charge” bootstrapped configuration, the VCC supply current increases significantly during turn-on causing a benign relaxation oscillation action on the VCC pin if the part does not start normally. tON (Pin 3): Pin for external programming resistor to set the minimum time that the primary switch is on for each cycle. Minimum turn-on facilitates the isolated feedback method. See Applications Information for details. ENDLY (Pin 4): Pin for external programming resistor to set enable delay time. The enable delay time disables the feedback amplifier for a fixed time after the turn-off of the primary-side MOSFET. This allows the leakage inductance voltage spike to be ignored for flyback voltage sensing. See Applications Information for details. SYNC (Pin 5): Pin for synchronizing the internal oscillator with an external clock. The positive edge on a pulse causes the oscillator to discharge causing PG to go low (off) and SG high (on). The sync threshold is typically 1.6V. See Applications Information for details. Tie to ground if unused. SFST (Pin 6): This pin, in conjunction with a capacitor to ground, controls the ramp-up of peak primary current as sensed through the sense resistor. This is used to control converter inrush current at start-up. The VC pin voltage cannot exceed the SFST pin voltage, so as SFST increases, the maximum voltage on VC increases commensurately, allowing higher peak currents. Total VC ramp time is approximately 70ms per μF of capacitance. Leave pin open if not using the soft-start function. OSC (Pin 7): This pin in conjunction with an external capacitor defines the controller oscillator frequency. The frequency is approximately 100kHz • 100/COSC(pF). FB (Pin 8): Pin for the feedback node for the power supply feedback amplifier. Feedback is usually sensed via a third winding and enabled during the flyback period. This pin also sinks additional current to compensate for load current variation as set by the RCMP pin. Keep the Thevenin equivalent resistance of the feedback divider at roughly 3k. 3525fa 6 LT3825 PIN FUNCTIONS VC (Pin 9): Pin used for frequency compensation for the switcher control loop. It is the output of the feedback amplifier and the input to the current comparator. Switcher frequency compensation components are normally placed on this pin to ground. The voltage on this pin is proportional to the peak primary switch current. The feedback amplifier output is enabled during the synchronous switch on time. UVLO (Pin 10): A resistive divider from VIN to this pin sets an undervoltage lockout based upon VIN level (not VCC). When the UVLO pin is below its threshold, the gate drives are disabled, but the part draws its normal quiescent current from VCC. The VCC undervoltage lockout supersedes this function so VCC must be great enough to start the part. The bias current on this pin has hysteresis such that the bias current is sourced when the UVLO threshold is exceeded. This introduces a hysteresis at the pin equivalent to the bias current change times the impedance of the upper divider resistor. The user can control the amount of hysteresis by adjusting the impedance of the divider. See the Applications Information for details. Tie the UVLO pin to VCC if you are not using this function. SENSE– (Pin 11), SENSE+ (Pin 12): These pins are used to measure primary side switch current through an external sense resistor. Peak primary side current is used in the converter control loop. Make Kelvin connections to the sense resistor to reduce noise problems. SENSE– connects to the ground side. At maximum current (VC at its maximum voltage) it has a 98mV threshold. The signal is blanked (ignored) during the minimum turn-on time. CCMP (Pin 13): Pin for external filter capacitor for the optional load compensation function. Load compensation reduces the effects of parasitic resistances in the feedback sensing path. A 0.1μF ceramic capacitor suffices for most applications. Short this pin to GND in less demanding applications that don’t require load compensation. RCMP (Pin 14): Pin for optional external load compensation resistor. Use of this pin allows for nominal compensation of parasitic resistances in the feedback sensing path. In less demanding applications, this resistor is not needed and this pin can be left open. See Applications Information for details. PGDLY (Pin 15): Pin for external programming resistor to set delay from synchronous gate turn-off to primary gate turn-on. See Applications Information for details. PG (Pin 16): Gate Drive Pin for the Primary Side MOSFET Switch. Large dynamic currents flow during voltage transitions. See the Applications Information for details. GND (Exposed Pad, Pin 17): This is the ground connection for both signal ground and gate driver grounds. This GND should be connected to the PCB ground plane. Careful attention must be paid to ground layout. See Applications Information for details. 3825fa 7 LT3825 BLOCK DIAGRAM CLAMPS VCC 19.5V 15.3V FB 1.3 – 1.235V REFERENCE (VFB) – INTERNAL REGULATOR VC + 3V S Q R Q – UVLO + 10 – 9 COLLAPSE DETECT + UVLO 8 ERROR AMP CURRENT COMPARATOR IUVLO TSD SFST 1V 6 OVERCURRENT FAULT – – 0.7 + – VCC UVLO + + 2 SENSE– 11 – CURRENT SENSE AMP + + CURRENT TRIP SENSE+ SLOPE COMPENSATION 7 5 3 15 4 OSC OSCILLATOR RCMPF 50k CCMP ENABLE SET + SYNC ENDLY 13 – LOAD COMPENSATION tON PGDLY 12 LOGIC BLOCK RCMP TO FB 14 VCC PGATE GATE DRIVE PG 16 SGATE + – 3V VCC GATE DRIVE SG GND 1 17 3525fa 8 LT3825 FLYBACK FEEDBACK AMPLIFIER T1 VFLBK FLYBACK LT3825 FEEDBACK AMP R1 8 FB • – 1V R2 VFB 1.25V VC 9 + CVC VIN • PRIMARY SECONDARY • COUT ISOLATED OUTPUT MP + – COLLAPSE DETECT MS R ENABLE S Q 3825 FFA TIMING DIAGRAM PRIMARY SIDE MOSFET DRAIN VOLTAGE VFLBK VIN 0.8 • VFLBK PG VOLTAGE SG VOLTAGE 3825 TD tON(MIN) MIN ENABLE ENABLE DELAY PG DELAY FEEDBACK AMPLIFIER ENABLED 3825fa 9 LT3825 OPERATION The LT3825 is a current mode switcher controller IC designed specifically for use in an isolated flyback topology employing synchronous rectification. The LT3825 operation is similar to traditional current mode switchers. The major difference is that output voltage feedback is derived via sensing the output voltage through the transformer. This precludes the need of an optoisolator in isolated designs greatly improving dynamic response and reliability. The LT3825 has a unique feedback amplifier that samples a transformer winding voltage during the flyback period and uses that voltage to control output voltage. The internal blocks are similar to many current mode controllers. The differences lie in the flyback feedback amplifier and load compensation circuitry. The logic block also contains circuitry to control the special dynamic requirements of flyback control. For more information on the basics of current mode switcher/controllers and isolated flyback converters see Application Note 19. Feedback Amplifier—Pseudo DC Theory For the following discussion refer to the simplified Flyback Feedback Amplifier diagram. When the primary side MOSFET switch MP turns off, its drain voltage rises above the VIN rail. Flyback occurs when the primary MOSFET is off and the synchronous secondary MOSFET is on. During flyback the voltage on nondriven transformer pins is determined by the secondary voltage. The amplitude of this flyback pulse as seen on the third winding is given as: compares the voltage to the internal bandgap reference. The feedback amp is actually a transconductance amplifier whose output is connected to VC only during a period in the flyback time. An external capacitor on the VC pin integrates the net feedback amp current to provide the control voltage to set the current mode trip point. The regulation voltage at the FB pin is nearly equal to the bandgap reference VFB because of the high gain in the overall loop. The relationship between VFLBK and VFB is expressed as: VFLBK = R1+ R2 • VFB R2 Combining this with the previous VFLBK expression yields an expression for VOUT in terms of the internal reference, programming resistors and secondary resistances: R1+ R2 VOUT = • VFB • NSF – ISEC • ESR + RDS(ON) R2 ( ) The effect of nonzero secondary output impedance is discussed in further detail; see Load Compensation Theory. The practical aspects of applying this equation for VOUT are found in the Applications Information. Feedback Amplifier Dynamic Theory RDS(ON) = on resistance of the synchronous MOSFET MS So far, this has been a pseudo-DC treatment of flyback feedback amplifier operation. But the flyback signal is a pulse, not a DC level. Provision must be made to enable the flyback amplifier only when the flyback pulse is present. This is accomplished by the “Enable” line in the diagram. Timing signals are then required to enable and disable the flyback amplifier. There are several timing signals which are required for proper LT3825 operation. Please refer to the Timing Diagram. ISEC = transformer secondary current Minimum Output Switch On Time (tON(MIN)) ESR = impedance of secondary circuit capacitor, winding and traces The LT3825 affects output voltage regulation via flyback pulse action. If the output switch is not turned on, there is no flyback pulse and output voltage information is not available. This causes irregular loop response and start-up/latch-up problems. The solution is to require the primary switch to be on for an absolute minimum time per each oscillator cycle. If the output load is less than VFLBK = ( VOUT + ISEC • ESR + RDS(ON) ) NSF NSF = transformer effective secondary-to-feedback winding turns ratio (i.e., NS/NFLBK) The flyback voltage is scaled by an external resistive divider R1/R2 and presented at the FB pin. The feedback amplifier 3525fa 10 LT3825 OPERATION that developed under these conditions, forced continuous operation normally occurs. See Applications Information for further details. Enable Delay (ENDLY) The flyback pulse appears when the primary side switch shuts off. However, it takes a finite time until the transformer primary side voltage waveform represents the output voltage. This is partly due to rise time on the primary side MOSFET drain node but, more importantly, is due to transformer leakage inductance. The latter causes a voltage spike on the primary side, not directly related to output voltage. Some time is also required for internal settling of the feedback amplifier circuitry. In order to maintain immunity to these phenomena, a fixed delay is introduced between the switch turn-off command and the enabling of the feedback amplifier. This is termed “enable delay.” In certain cases where the leakage spike is not sufficiently settled by the end of the enable delay period, regulation error may result. See Applications Information for further details. Collapse Detect Once the feedback amplifier is enabled, some mechanism is then required to disable it. This is accomplished by a collapse detect comparator, which compares the flyback voltage (FB referred) to a fixed reference, nominally 80% of VFB. When the flyback waveform drops below this level, the feedback amplifier is disabled. Minimum Enable Time The feedback amplifier, once enabled, stays enabled for a fixed minimum time period termed “minimum enable time.” This prevents lockup, especially when the output voltage is abnormally low; e.g., during start-up. The minimum enable time period ensures that the VC node is able to “pump up” and increase the current mode trip point to the level where the collapse detect system exhibits proper operation. This time is set internally. Effects of Variable Enable Period The feedback amplifier is enabled during only a portion of the cycle time. This can vary from the fixed minimum enable time described to a maximum of roughly the “off” switch time minus the enable delay time. Certain parameters of feedback amp behavior are directly affected by the variable enable period. These include effective transconductance and VC node slew rate. Load Compensation Theory The LT3825 uses the flyback pulse to obtain information about the isolated output voltage. An error source is caused by transformer secondary current flow through the synchronous MOSFET RDS(ON) and real life nonzero impedances of the transformer secondary and output capacitor. This was represented previously by the expression “ISEC • (ESR + RDS(ON)).” However, it is generally more useful to convert this expression to effective output impedance. Because the secondary current only flows during the off portion of the duty cycle (DC), the effective output impedance equals the lumped secondary impedance divided by OFF time DC. Since the OFF time duty cycle is equal to 1 – DC then: RS(OUT) = ESR + RDS(ON) 1 – DC where: RS(OUT) = effective supply output impedance DC = duty cycle RDS(ON) and ESR are as defined previously This impedance error may be judged acceptable in less critical applications, or if the output load current remains relatively constant. In these cases the external FB resistive divider is adjusted to compensate for nominal expected error. In more demanding applications, output impedance error is minimized by the use of the load compensation function. Figure 1 shows the Block Diagram of the load compensation function. Switch current is converted to a voltage by the external sense resistor, averaged and lowpass filtered by the internal 50k resistor RCMPF and the external capacitor on CCMP . This voltage is impressed across the external RCMP resistor by op amp A1 and transistor Q3 producing a current at the collector of Q3 that is subtracted from the 3825fa 11 LT3825 OPERATION VFLBK R1 • FB Q1 Q2 8 IIN = K1 • IOUT VFB VIN R2 Average primary side current is expressed in terms of output current as follows: T1 LOAD COMP I where: • • MP K1 = + Q3 A1 – RCMPF + 50k SENSE 12 VOUT VIN • Eff So the effective change in VOUT target is: ΔVOUT = K1• ΔIOUT • 14 RCMP 13 CCMP RSENSE 3825 F01 Figure 1. Load Compensation Diagram RSENSE • R1• NSF RCMP thus : R ΔVOUT = K1• SENSE • R1• NSF ΔIOUT RCMP where: FB node. This effectively increases the voltage required at the top of the R1/R2 feedback divider to achieve equilibrium. The average primary side switch current increases to maintain output voltage regulation as output loading increases. The increase in average current increases the RCMP resistor current which affects a corresponding increase in sensed output voltage, compensating for the IR drops. Assuming a relatively fixed power supply efficiency, Eff, power balance gives: POUT = Eff • PIN VOUT • IOUT = Eff • VIN • IIN K1 = dimensionless variable related to VIN, VOUT and efficiency as explained above RSENSE = external sense resistor Nominal output impedance cancellation is obtained by equating this expression with RS(OUT): K1 • ESR + RDS(ON) RSENSE • R1 • NSF = RCMP 1 – DC Solving for RCMP gives: RCMP = K1 • RSENSE • (1 – DC) • R1 • NSF ESR + RDS(ON) The practical aspects of applying this equation to determine an appropriate value for the RCMP resistor are found in the Applications Information. 3525fa 12 LT3825 APPLICATIONS INFORMATION Transformer Design Transformer design/specification is the most critical part of a successful application of the LT3825. The following sections provide basic information about designing the transformer and potential tradeoffs. If you need help, the LTC Applications group is available to assist in the choice and/or design of the transformer. Turns Ratios The design of the transformer starts with determining duty cycle (DC). DC impacts the current and voltage stress on the power switches, input and output capacitor RMS currents and transformer utilization (size vs power). The ideal turns ratio is: NIDEAL = VOUT 1 – DC • VIN DC Avoid extreme duty cycles as they, in general, increase current stresses. A reasonable target for duty cycle is 50% at nominal input voltage. For instance, if we wanted a 48V to 5V converter at 50% DC then: NIDEAL = 5 1 – 0.5 1 • = 48 0.5 9.6 In general, better performance is obtained with a lower turns ratio. A DC of 45.5% yields a 1:8 ratio. Note the use of the external feedback resistive divider ratio to set output voltage provides the user additional freedom in selecting a suitable transformer turns ratio. Turns ratios that are the simple ratios of small integers; e.g., 1:1, 2:1, 3:2 help facilitate transformer construction and improve performance. When building a supply with multiple outputs derived through a multiple winding transformer, lower duty cycle can improve cross regulation by keeping the synchronous rectifier on longer, and thus, keep secondary windings coupled longer. For a multiple output transformer, the turns ratio between output windings is critical and affects the accuracy of the voltages. The ratio between two output voltages is set with the formula VOUT2 = VOUT1 • N21 where N21 is the turns ratio between the two windings. Also keep the secondary MOSFET RDS(ON) small to improve cross regulation. The feedback winding usually provides both the feedback voltage and power for the LT3825. So set the turns ratio between the output and feedback winding to provide a rectified voltage that under worst-case conditions is greater than the 11V maximum VCC turn-off voltage. NSF > VOUT 11+ VF For our example: NSF > 1 5 = 11+ 0.7 2.34 1 3 Leakage Inductance We will choose Transformer leakage inductance (on either the primary or secondary) causes a spike after the primary side switch turn-off. This is increasingly prominent at higher load currents, where more stored energy is dissipated. Higher flyback voltage may break down the MOSFET switch if it has too low a BVDSS rating. One solution to reducing this spike is to use a snubber circuit to suppress the voltage excursion. However, suppressing the voltage extends the flyback pulse width. If the flyback pulse extends beyond the enable delay time, output voltage regulation is affected. The feedback system has a deliberately limited input range, roughly ±50mV referred to the FB node. This rejects higher voltage leakage spikes because once a leakage spike is several volts in amplitude, a further increase in amplitude has little effect on the feedback system. Therefore, it is advisable to arrange the snubber circuit to clamp at as high a voltage as possible, observing MOSFET breakdown, such that leakage spike duration is as short as possible. Application Note 19 provides a good reference on snubber design. 3825fa 13 LT3825 APPLICATIONS INFORMATION As a rough guide, leakage inductance of several percent (of mutual inductance) or less may require a snubber, but exhibit little to no regulation error due to leakage spike behavior. Inductances from several percent up to perhaps ten percent cause increasing regulation error. Avoid double digit percentage leakage inductances as there is a potential for abrupt loss of control at high load current. This curious condition potentially occurs when the leakage spike becomes such a large portion of the flyback waveform that the processing circuitry is fooled into thinking that the leakage spike itself is the real flyback signal! It then reverts to a potentially stable state whereby the top of the leakage spike is the control point, and the trailing edge of the leakage spike triggers the collapse detect circuitry. This typically reduces the output voltage abruptly to a fraction, roughly one-third to two-thirds of its correct value. Once load current is reduced sufficiently, the system snaps back to normal operation. When using transformers with considerable leakage inductance, exercise this worst-case check for potential bistability: 1. Operate the prototype supply at maximum expected load current. 2. Temporarily short circuit the output. 3. Observe that normal operation is restored. If the output voltage is found to hang up at an abnormally low value, the system has a problem. This is usually evident by simultaneously viewing the primary side MOSFET drain voltage to observe firsthand the leakage spike behavior. A final note—the susceptibility of the system to bistable behavior is somewhat a function of the load current/voltage characteristics. A load with resistive—i.e., I = V/R behavior—is the most apt to be bistable. Capacitive loads that exhibit I = V2/R behavior are less susceptible. Secondary Leakage Inductance Leakage inductance on the secondary forms an inductive divider on the transformer secondary, reducing the size of the feedback flyback pulse. This increases the output voltage target by a similar percentage. Note that unlike leakage spike behavior, this phenomenon is independent of load. Since the secondary leakage inductance is a constant percentage of mutual inductance (within manufacturing variations), the solution is to adjust the feedback resistive divider ratio to compensate. Winding Resistance Effects Primary or secondary winding resistance acts to reduce overall efficiency (POUT/PIN). Secondary winding resistance increases effective output impedance degrading load regulation. Load compensation can mitigate this to some extent but a good design keeps parasitic resistances low. Bifilar Winding A bifilar or similar winding is a good way to minimize troublesome leakage inductances. Bifilar windings also improve coupling coefficients and thus improve cross regulation in multiple winding transformers. However, tight coupling usually increases primary-to-secondary capacitance and limits the primary-to-secondary breakdown voltage, so it isn’t always practical. Primary Inductance The transformer primary inductance, LP , is selected based on the peak-to-peak ripple current ratio (X) in the transformer relative to its maximum value. As a general rule, keep X in the range of 20% to 40% ripple current (i.e., X = 0.2 to 0.4). Higher values of ripple will increase conduction losses, while lower values will require larger cores. 3525fa 14 LT3825 APPLICATIONS INFORMATION Ripple current and percentage ripple is largest at minimum duty cycle; in other words, at the highest input voltage. LP is calculated from: LP 2 2 VIN(MAX ) • DCMIN ) VIN(MAX ) • DCMIN ) • Eff ( ( = = fOSC • XMAX • PIN fOSC • XMAX • POUT where: fOSC is the oscillator frequency DCMIN is the DC at maximum input voltage XMAX is ripple current ratio at maximum input voltage For a 48V (VIN = 36V to 72V) to 5V/8A converter with 90% efficiency, POUT = 40W and PIN = 44.44W. Using X = 0.4 and fOSC = 200kHz: DCMIN = 1+ LP = 1 = N • VIN(MAX) VOUT (72V • 0.357) 1 = 35.7% 1 72 1+ • 8 5 The main design goals for core selection are reducing copper losses and preventing saturation. Ferrite core material saturates hard, rapidly reducing inductance when the peak design current is exceeded. This results in an abrupt increase in inductor ripple current and, consequently, output voltage ripple. Do not allow the core to saturate! The maximum peak primary current occurs at minimum VIN: X PIN • 1+ MIN VIN(MIN) • DCMAX 2 IPK = now : DCMAX = 1+ XMIN 1 = N • VIN(MIN) VOUT ( VIN(MIN) • DCMAX ) = fOSC • LP • PIN 1 = 52.6% 1 36 1+ • 8 5 2 = (36 • 52.6%) 2 200kHz • 186μH • 44.44 = 0.202 2 200kHz • 0.4 • 44.44W = 186μH Optimization might show that a more efficient solution is obtained at higher peak current but lower inductance and the associated winding series resistance. A simple spreadsheet program is useful for looking at tradeoffs. Transformer Core Selection Once LP is known, the type of transformer is selected. High efficiency converters use ferrite cores to minimize core loss. Actual core loss is independent of core size for a fixed inductance, but decreases as inductance increases. Since increased inductance is accomplished through more turns of wire, copper losses increase. Thus transformer design balances core and copper losses. Remember that increased winding resistance will degrade cross regulation and increase the amount of load compensation required. Using the example numbers leads to: IPK = 44.44W 0.202 • 1+ = 2.58A 36 • 0.526 2 Multiple Outputs One advantage that the flyback topology offers is that additional output voltages can be obtained simply by adding windings. Designing a transformer for such a situation is beyond the scope of this document. For multiple windings, realize that the flyback winding signal is a combination of activity on all the secondary windings. Thus load regulation is affected by each windings load. Take care to minimize cross regulation effects. 3825fa 15 LT3825 APPLICATIONS INFORMATION Setting Feedback Resistive Divider The expression for VOUT developed in the Operation section is rearranged to yield the following expression for the feedback resistors: V OUT +ISEC • ESR + RDS(ON) R1= R2 –1 VFB • NSF ( ) Continuing the example, if ESR + RDS(ON) = 8mΩ, R2 = 3.32k, then: 5 + 8 • 0.008 R1= 3.4k – 1 = 37.6k 1.232 • 1/ 3 choose 37.4k. It is recommended that the Thevenin impedance of the resistive divider (R1||R2) is roughly 3k for bias current cancellation and other reasons. Current Sense Resistor Considerations The external current sense resistor is used to control peak primary switch current, which controls a number of key converter characteristics including maximum power and external component ratings. Use a noninductive current sense resistor (no wire-wound resistors). Mounting the resistor directly above an unbroken ground plane connected with wide and short traces keeps stray resistance and inductance low. The dual sense pins allow for a fully Kelvined connection. Make sure that SENSE+ and SENSE– are isolated and connect close to the sense resistor to preserve this. Peak current occurs at 98mV of sense voltage VSENSE. So the nominal sense resistor is VSENSE/IPK. For example, a peak switch current of 10A requires a nominal sense resistor of 0.010Ω. Note that the instantaneous peak power in the sense resistor is 1W, and that it is rated accordingly. The use of parallel resistors can help achieve low resistance, low parasitic inductance and increased power capability. Size RSENSE using worst-case conditions, minimum LP , VSENSE and maximum VIN. Continuing the example, let us assume that our worst-case conditions yield an IPK 40% above nominal so IPK = 3.64A . If there is a 10% tolerance on RSENSE and minimum VSENSE = 80mV, then RSENSE • 110% = 80mV/3.64A and nominal RSENSE = 20mΩ. Round to the nearest available lower value. Selecting the Load Compensation Resistor The expression for RCMP was derived in the Operation section as: RCMP = K1 • RSENSE • (1 – DC) • R1 • NSF = RS(OUT) ESR + RDS(ON) Continuing the example: V 5 K1= OUT = = 0.116 VIN • Eff 48 • 90% If ESR + RDS(ON) = 8m RCMP = 0.116 • 20m • (1– 0.455) 8m • 37.4k = 1.96k This value for RCMP is a good starting point, but empirical methods are required for producing the best results. This is because several of the required input variables are difficult to estimate precisely. For instance, the ESR term above includes that of the transformer secondary, but its effective ESR value depends on high frequency behavior, not simply DC winding resistance. Similarly, K1 appears as a simple ratio of VIN to VOUT times (differential) efficiency, but theoretically estimating efficiency is not a simple calculation. The suggested empirical method is as follows: 1. Build a prototype of the desired supply including the actual secondary components. 2. Temporarily ground the CCMP pin to disable the load compensation function. Measure output voltage while sweeping output current over the expected range. Approximate the voltage variation as a straight line, ΔVOUT/ΔIOUT = RS(OUT). 3. Calculate a value for the K1 constant based on VIN, VOUT and the measured efficiency. 3525fa 16 LT3825 APPLICATIONS INFORMATION 4. Compute: RSENSE • R1 • NSF RS(OUT) 5. Verify this result by connecting a resistor of this value from the RCMP pin to ground. 6. Disconnect the ground short to CCMP and connect a 0.1μF filter capacitor to ground. Measure the output impedance RS(OUT) = ΔVOUT/ΔIOUT with the new compensation in place. RS(OUT) should have decreased significantly. Fine tuning is accomplished experimentally by slightly altering RCMP . A revised estimate for RCMP is: R S(OUT)CMP RCMP = RCMP • 1+ R S(OUT) 200 fOSC (kHz) RCMP = K1 • 300 100 50 30 100 COSC (pF) 200 3825 F02 Figure 2. fOSC vs OSC Capacitor Values occur with slope compensation and system stability. Keep the sync pulse width greater than 500ns. Selecting Timing Resistors where RʹCMP is the new value for the load compensation resistor, RS(OUT)CMP is the output impedance with RCMP in place and RS(OUT) is the output impedance with no load compensation (from step 2). Setting Frequency The switching frequency of the LT3825 is set by an external capacitor connected between the OSC pin and ground. Recommended values are between 200pF and 33pF, yielding switching frequencies between 50kHz and 250kHz. Figure 2 shows the nominal relationship between external capacitance and switching frequency. Place the capacitor as close as possible to the IC and minimize OSC trace length and area to minimize stray capacitance and potential noise pickup. You can synchronize the oscillator frequency to an external frequency. This is done with a signal on the SYNC pin. Set the LT3825 frequency 10% slower than the desired external frequency using the OSC pin capacitor, then use a pulse on the SYNC pin of amplitude greater than 2V and with the desired frequency. The rising edge of the SYNC signal initiates an OSC capacitor discharge forcing primary MOSFET off (PG voltage goes low). If the oscillator frequency is much different from the sync frequency, problems may There are three internal “one-shot” times that are programmed by external application resistors: minimum on time, enable delay time and primary MOSFET turn-on delay. These are all part of the isolated flyback control technique, and their functions are previously outlined in the Theory of Operation section. The following information should help in selecting and/or optimizing these timing values. Minimum On Time (tON(MIN)) Minimum on time is the programmable period during which current limit is blanked (ignored) after the turn on of the primary side switch. This improves regulator performance by eliminating false tripping on the leading edge spike in the switch, especially at light loads. This spike is due to both the gate/source charging current and the discharge of drain capacitance. The isolated flyback sensing requires a pulse to sense the output. Minimum on time ensures that there is always a signal to close the loop. The LT3825 does not employ cycle skipping at light loads. Therefore, minimum on time along with synchronous rectification sets the switch over to forced continuous mode operation. 3825fa 17 LT3825 APPLICATIONS INFORMATION The tON(MIN) resistor is set with the following equation: RtON(MIN) (kΩ) = tON(MIN)(ns) – 104 1.063 Keep RtON(MIN) greater than 70k. A good starting value is 160k. Enable Delay Time (ENDLY) Enable delay time provides a programmable delay between turn-off of the primary gate drive node and the subsequent enabling of the feedback amplifier. As discussed earlier, this delay allows the feedback amplifier to ignore the leakage inductance voltage spike on the primary side. The worst-case leakage spike pulse width is at maximum load conditions. So set the enable delay time at these conditions. While the typical applications for this part use forced continuous operation, it is conceivable that a secondaryside controller might cause discontinuous operation at light loads. Under such conditions the amount of energy stored in the transformer is small. The flyback waveform becomes “lazy” and some time elapses before it indicates the actual secondary output voltage. The enable delay time should be made long enough to ignore the “irrelevant” portion of the flyback waveform at light load. Even though the LT3825 has a robust gate drive, the gate transition time slows with very large MOSFETs. Increase delay time as required when using such MOSFETs. The enable delay resistor is set with the following equation: RENDLY (kΩ) = tENDLY (ns) – 30 2.616 Keep RENDLY greater than 40k. A good starting point is 56k. Primary Gate Delay Time (PGDLY) Primary gate delay is the programmable time from the turn-off of the synchronous MOSFET to the turn-on of the primary side MOSFET. Correct setting eliminates overlap between the primary side switch and secondary side synchronous switch(es) and the subsequent current spike in the transformer. This spike will cause additional component stress and a loss in regulator efficiency. The primary gate delay resistor is set with the following equation: RPGDLY (kΩ) = tPGDLY (ns) + 47 9.01 A good starting point is 27k. Soft-Start Functions The LT3825 contains an optional soft-start function that is enabled by connecting an external capacitor between the SFST pin and ground. Internal circuitry prevents the control voltage at the VC pin from exceeding that on the SFST pin. There is an initial pull-up circuit to quickly bring the SFST voltage to approximately 0.8V. From there it charges to approximately 2.8V with a 20μA current source. The SFST node is discharged to 0.8V when a fault occurs. A fault is VCC too low (undervoltage lockout), current sense voltage greater than 200mV or the IC’s thermal (over temperature) shutdown is tripped. When SFST discharges, the VC node voltage is also pulled low to below the minimum current voltage. Once discharged and the fault removed, the SFST recharges up again. In this manner, switch currents are reduced and the stresses in the converter are reduced during fault conditions. The time it takes to fully charge soft-start is: t SS = CSFST • 1.4V = 70ms • CSFST (μF) 20μA UVLO Pin Function The UVLO pin provides a user programming undervoltage lockout. This is typically used to provide undervoltage lockout based on VIN. The gate drivers are disabled when UVLO is below the 1.24V UVLO threshold. An external resistive divider between the input supply and ground is used to set the turn-on voltage. 3525fa 18 LT3825 APPLICATIONS INFORMATION The bias current on this pin depends on the pin voltage and UVLO state. The change provides the user with adjustable UVLO hysteresis. When the pin rises above the UVLO threshold a small current is sourced out of the pin, increasing the voltage on the pin. As the pin voltage drops below this threshold, the current is stopped, further dropping the voltage on UVLO. In this manner, hysteresis is produced. Referring to Figure 3, the voltage hysteresis at VIN is equal to the change in bias current times RA. The design procedure is to select the desired VIN referred voltage hysteresis, VUVHYS. Then: RA = VUVHYS IUVLO where: IUVLO = IUVLOL – IUVLOH is approximately 3.4μA RB is then selected with the desired turn-on voltage: RB = RA VIN(ON) – 1 VUVLO If we wanted a VIN-referred trip point of 36V, with 1.8V (5%) of hysteresis (on at 36V, off at 34.2V): Even with good board layout, board noise may cause problems with UVLO. You can filter the divider but keep large capacitance off the UVLO node because it will slow the hysteresis produced from the change in bias current. Figure 3c shows an alternate method of filtering by splitting the RA resistor with the capacitor. The split should put more of the resistance on the UVLO side. Converter Start-Up The standard topology for the LT3825 utilizes a third transformer winding on the primary side that provides both feedback information and local VCC power for the LT3825 (see Figure 4). This power “bootstrapping” improves converter efficiency but is not inherently self-starting. Start-up is affected with an external “trickle-charge” resistor and the LT3825’s internal VCC undervoltage lockout circuit. The VCC undervoltage lockout has wide hysteresis to facilitate start-up. In operation, the “trickle charge” resistor RTR is connected to VIN and supplies a small current, typically on the order of 1mA to charge CTR. Initially the LT3825 is off and draws only its start-up current. When CTR reaches the VCC turn-on threshold voltage the LT3825 turns on abruptly and draws its normal supply current. VIN RA = 1.8V = 529k, use 523k 3.4μA RB = RTR • VIN + 523k = 18.5k, use 18.7k 36V – 1 1.23V • CTR IVCC LT3825 VIN IUVLO VIN IUVLO RB PG GND RA1 VIN RA2 RA • VCC UVLO LT3825 RA RB UVLO LT3825 CUVLO UVLO RB (3b) UV Turning OFF Figure 3 VVCC IVCC 0 3825 F03 (3a) UV Turning ON VON THRESHOLD (3c) UV Filtering VPG 3825 F04 Figure 4. Typical Power Bootstrapping 3825fa 19 LT3825 APPLICATIONS INFORMATION Switching action commences and the converter begins to deliver power to the output. Initially the output voltage is low and the flyback voltage is also low, so CTR supplies most of the LT3825 current (only a fraction comes from RTR.) VCC voltage continues to drop until after some time, typically tens of milliseconds, the output voltage approaches its desired value. The flyback winding then provides the LT3825 supply current and the VCC voltage stabilizes. If CTR is undersized, VCC reaches the VCC turn-off threshold before stabilization and the LT3825 turns off. The VCC node then begins to charge back up via RTR to the turnon threshold, where the part again turns on. Depending upon the circuit, this may result in either several on-off cycles before proper operation is reached, or permanent relaxation oscillation at the VCC node. RTR is selected to yield a worst-case minimum charging current greater than the maximum rated LT3825 start-up current, and a worst-case maximum charging current less than the minimum rated LT3825 supply current. RTR(MAX) < VIN(MIN) – VCC(ON _ MAX) ICC(ST _ MAX) and RTR(MIN) > VIN(MAX) – VCC(ON _ MIN) ICC(MIN) Make CTR large enough to avoid the relaxation oscillatory behavior described above. This is complicated to determine theoretically as it depends on the particulars of the secondary circuit and load behavior. Empirical testing is recommended. Note that the use of the optional soft-start function lengthens the power-up timing and requires a correspondingly larger value for CTR. If you have an available input voltage within the VCC range, the internal wide hysteresis range UVLO function becomes counterproductive. In such cases it is better to operate the LT3825 directly from the available supply. In this case, use the LT3837 which is identical to the LT3825 except that it lacks the internal VCC undervoltage lockout function. It is designed to operate directly from supplies in the range of 4.5V to 19V. See the LT3837 data sheet for further information. The LT3825 has an internal clamp on VCC of approximately 19.5V. This provides some protection for the part in the event that the switcher is off (UVLO low) and the VCC node is pulled high. If RTR is sized correctly the part should never attain this clamp voltage. Control Loop Compensation Loop frequency compensation is performed by connecting a capacitor network from the output of the feedback amplifier (VC pin) to ground as shown in Figure 5. Because of the sampling behavior of the feedback amplifier, compensation is different from traditional current mode switcher controllers. Normally only CVC is required. RVC can be used to add a “zero” but the phase margin improvement traditionally offered by this extra resistor is usually already accomplished by the nonzero secondary circuit impedance. CVC2 can be used to add an additional high frequency pole and is usually sized at 0.1 times CVC. VC 9 CVC2 RVC CVC 3825 F05 Figure 5. VC Compensation Network In further contrast to traditional current mode switchers, VC pin ripple is generally not an issue with the LT3825. The dynamic nature of the clamped feedback amplifier forms an effective track/hold type response, whereby the VC voltage changes during the flyback pulse, but is then “held” during the subsequent “switch on” portion of the 3525fa 20 LT3825 APPLICATIONS INFORMATION next cycle. This action naturally holds the VC voltage stable during the current comparator sense action (current mode switching). AN19 provides a method for empirically tweaking frequency compensation. Basically it involves introducing a load current step and monitoring the response. Slope Compensation This part incorporates current slope compensation. Slope compensation is required to ensure current loop stability when the DC is greater than 50%. In some switcher controllers, slope compensation reduces the maximum peak current at higher duty cycles. The LT3825 eliminates this problem by having circuitry that compensates for the slope compensation so that maximum current sense voltage is constant across all duty cycles. Minimum Load Considerations At light loads, the LT3825 derived regulator goes into forced continuous conduction mode. The primary side switch always turns on for a short time as set by the tON(MIN) resistor. If this produces more power than the load requires, power will flow back into the primary during the “off” period when the synchronization switch is on. This does not produce any inherently adverse problems, though light load efficiency is reduced. Maximum Load Considerations The current mode control uses the VC node voltage and amplified sense resistor voltage as inputs to the current comparator. When the amplified sense voltage exceeds the VC node voltage, the primary side switch is turned off. In normal use, the peak switch current increases while FB is below the internal reference. This continues until VC reaches its 2.56V clamp. At clamp, the primary side MOSFET will turn off at the rated 98mV VSENSE level. This repeats on the next cycle. It is possible for the peak primary switch currents as referred across RSENSE to exceed the max 98mV rating because of the minimum switch on time blanking. If the voltage on VSENSE exceeds 206mV after the minimum turn-on time, the SFST capacitor is discharged, causing the discharge of the VC capacitor. This then reduces the peak current on the next cycle and will reduce overall stress in the primary switch. Short-Circuit Conditions Loss of current limit is possible under certain conditions such as an output short circuit. If the duty cycle exhibited by the minimum on time is greater than the ratio of secondary winding voltage (referred-to-primary) divided by input voltage, then peak current is not controlled at the nominal value. It ratchets up cycle-by-cycle to some higher level. Expressed mathematically, the requirement to maintain short-circuit control is: DCMIN = tON(MIN) • fOSC < ( ISC • RSEC + RDS(ON) ) VIN • NSP where: tON(MIN) = primary side switch minimum on time ISC = short-circuit output current NSP = secondary-to-primary turns ratio (NSEC/NPRI) Other variables as previously defined Trouble is typically encountered only in applications with a relatively high product of input voltage times secondaryto-primary turns ratio and/or a relatively long minimum switch on time. Additionally, several real world effects such as transformer leakage inductance, AC winding losses, and output switch voltage drop combine to make this simple theoretical calculation a conservative estimate. Prudent design evaluates the switcher for short-circuit protection and adds any additional circuitry to prevent destruction. 3825fa 21 LT3825 APPLICATIONS INFORMATION Output Voltage Error Sources The LT3825’s feedback sensing introduces additional sources of errors. The following is a summary list. For the primary-side power MOSFET, the peak current is: IPK(PRI) = X PIN • 1+ MIN VIN(MIN) • DCMAX 2 The internal bandgap voltage reference sets the reference voltage for the feedback amplifier. The specifications detail its variation. where XMIN is peak-to-peak current ratio as defined earlier. The external feedback resistive divider ratio proportional directly affects regulated voltage. Use 1% components. For each secondary-side power MOSFET, the peak current is: Leakage inductance on the transformer secondary reduces the effective secondary-to-feedback winding turns ratio (NS/NF) from its ideal value. This increases the output voltage target by a similar percentage. Since secondary leakage inductance is constant from part to part (with a tolerance) adjust the feedback resistor ratio to compensate. The transformer secondary current flows through the impedances of the winding resistance, synchronous MOSFET RDS(ON) and output capacitor ESR. The DC equivalent current for these errors is higher than the load current because conduction occurs only during the converter’s “off” time. So divide the load current by (1 – DC). If the output load current is relatively constant, the feedback resistive divider is used to compensate for these losses. Otherwise, use the LT3825 load compensation circuitry (see Load Compensation). If multiple output windings are used, the flyback winding will have a signal that represents an amalgamation of all these windings impedances. Take care that you examine worstcase loading conditions when tweaking the voltages. Power MOSFET Selection The power MOSFETs are selected primarily on the criteria of “on” resistance RDS(ON), input capacitance, drain-tosource breakdown voltage (BVDSS), maximum gate voltage (VGS) and maximum drain current (ID(MAX)). IPK(SEC) = IOUT 1– DCMAX X • 1+ MIN 2 Select a primary-side power MOSFET with a BVDSS greater than: BVDSS ≥IPK V LLKG + VIN(MAX) + OUT(MAX) CP NSP where NSP reflects the turns ratio of that secondary-to-primary winding. LLKG is the primary-side leakage inductance and CP is the primary-side capacitance (mostly from the COSS of the primary-side power MOSFET). A snubber may be added to reduce the leakage inductance as discussed earlier. For each secondary-side power MOSFET, the BVDSS should be greater than: BVDSS ≥ VOUT + VIN(MAX) • NSP Choose the primary side MOSFET RDS(ON) at the nominal gate drive voltage (7.5V). The secondary side MOSFET gate drive voltage depends on the gate drive method. Primary side power MOSFET RMS current is given by: IRMS(PRI) = PIN VIN(MIN) DCMAX 3525fa 22 LT3825 APPLICATIONS INFORMATION For each secondary-side power MOSFET RMS current is given by: IRMS(SEC) = IOUT 1 – DCMAX fOSC is the operating frequency CMILLER is calculated from the gate charge curve included on most MOSFET data sheets (Figure 6). b QA QB GATE CHARGE (QG) 3825 F06 Figure 6. Gate Charge Curve The flat portion of the curve is the result of the Miller (gate-to-drain) capacitance as the drain voltage drops. The Miller capacitance is computed as: CMILLER = QB – Q A VDS With CMILLER determined, calculate the primary-side power MOSFET power dissipation: PDPRI = IRMS(PRI)2 • RDS(ON) (1+ δ) + PIN(MAX) DCMIN (1 + δ) is generally given for a MOSFET in the form of a normalized RDS(ON) vs temperature curve. If you don’t have a curve, use δ = 0.005/°C • ΔT for low voltage MOSFETs. The secondary-side power MOSFETs typically operate at substantially lower VDS, so you can neglect transition losses. The dissipation is calculated using: With power dissipation known, the MOSFETs’ junction temperatures are obtained from the equation: TJ = TA + PD • θJA where TA is the ambient temperature and θJA is the MOSFET junction to ambient thermal resistance. Once you have TJ, iterate your calculations recomputing δ and power dissipations until convergence. Gate Drive Node Consideration The curve is done for a given VDS. The Miller capacitance for different VDS voltages are estimated by multiplying the computed CMILLER by the ratio of the application VDS to the curve specified VDS. VIN(MAX) • VGATE(MAX) = 7.5V for this part PD(SEC) = IRMS(SEC)2 • RDS(ON)(1 + δ) MILLER EFFECT a RDR is the gate driver resistance (≈10Ω) VTH is the MOSFET gate threshold voltage Calculate MOSFET power dissipation next. Because the primary-side power MOSFET operates at high VDS, a transition power loss term is included for accuracy. CMILLER is the most critical parameter in determining the transition loss, but is not directly specified on the data sheets. VGS where: • RDR • CMILLER • fOSC VGATE(MAX) – VTH The PG and SG gate drivers are strong drives to minimize gate drive rise and fall times. This improves efficiency but the high frequency components of these signals can cause problems. Keep the traces short and wide to reduce parasitic inductance. The parasitic inductance creates an LC tank with the MOSFET gate capacitance. In less than ideal layouts, a series resistance of 5Ω or more may help to dampen the ringing at the expense of slightly slower rise and fall times and efficiency. The LT3825 gate drives will clamp the max gate voltage to roughly 7.5V, so you can safely use MOSFETs with max VGS of 10V or larger. 3825fa 23 LT3825 APPLICATIONS INFORMATION Synchronous Gate Drive There are several different ways to drive the synchronous gate MOSFET. Full converter isolation requires the synchronous gate drive to be isolated. This is usually accomplished by way of a pulse transformer. Usually the pulse driver is used to drive a buffer on the secondary as shown in the application on the front page of this data sheet. However, other schemes are possible. There are gate drivers and secondary side synchronous controllers available that provide the buffer function as well as additional features. Keep input capacitor series resistance (ESR) and inductance (ESL) small, as they affect electromagnetic interference suppression. In some instances, high ESR can also produce stability problems because flyback converters exhibit a negative input resistance characteristic. Refer to Application Note 19 for more information. The output capacitor is sized to handle the ripple current and to ensure acceptable output voltage ripple. The output capacitor should have an RMS current rating greater than: IRMS = IOUT Capacitor Selection In a flyback converter, the input and output current flows in pulses, placing severe demands on the input and output filter capacitors. The input and output filter capacitors are selected based on RMS current ratings and ripple voltage. Select an input capacitor with a ripple current rating greater than: IRMS = PIN VIN(MIN) 1 – DCMAX DCMAX 44 . 4W 1 – 52 . 6 % = 1 . 17 A 36 V 52 . 6 % PRIMARY CURRENT IRMS = 8 A 52.6% = 8.43A 1 – 52.6% This is calculated for each output in a multiple winding application. ESR and ESL along with bulk capacitance directly affect the output voltage ripple. The waveforms for a typical flyback converter are illustrated in Figure 7. IPRI SECONDARY CURRENT IPRI N RINGING DUE TO ESL $VCOUT OUTPUT VOLTAGE RIPPLE WAVEFORM Continuing the example:: The maximum acceptable ripple voltage (expressed as a percentage of the output voltage) is used to establish a starting point for the capacitor values. For the purpose of simplicity we will choose 2% for the maximum output ripple, divided equally between the ESR step and the Continuing the example: IRMS = DCMAX 1– DCMAX $VESR 3825 F07 Figure 7. Typical Flyback Converter Waveforms 3525fa 24 LT3825 APPLICATIONS INFORMATION charging/discharging ΔV. This percentage ripple changes, depending on the requirements of the application. You can modify the equations below. For a 1% contribution to the total ripple voltage, the ESR of the output capacitor is determined by: ESRCOUT ≤1% • VOUT • (1– DCMAX ) IOUT The other 1% is due to the bulk C component, so use: COUT ≥ IOUT 1% • VOUT • fOSC In many applications the output capacitor is created from multiple capacitors to achieve desired voltage ripple, reliability and cost goals. For example, a low ESR ceramic capacitor can minimize the ESR step, while an electrolytic capacitor satisfies the required bulk C. Continuing our example, the output capacitor needs: ESRCOUT ≤ 1% • COUT ≥ 5V • (1– 49%) 8A These electrical characteristics require paralleling several low ESR capacitors possibly of mixed type. Most capacitor ripple current ratings are based on 2000 hour life. This makes it advisable to derate the capacitor or to choose a capacitor rated at a higher temperature than required. One way to reduce cost and improve output ripple is to use a simple LC filter. Figure 8 shows an example of the filter. FROM SECONDARY WINDING C1 47μF s3 COUT2 1μF Take care to ensure that the LT3825 junction temperature does not exceed 125°C. Power is computed from the average supply current, the sum of quiescent supply current (ICC in the specifications) plus gate drive currents. The primary gate drive current is computed as: fOSC • QG where QG is the total gate charge at max VGS (obtained from the gate charge curve) and f is the switching frequency. Since the synchronous driver is usually driving a capacitive load, the synchronous gate drive power dissipation is: fOSC • CS • VSGMAX where CS is the SG capacitive load and VSGMAX is the SG pin max voltage. VOUT COUT 470μF Circuit simulation is a way to optimize output capacitance and filters, just make sure to include the component parasitics. LTC SwitcherCAD™ is a terrific free circuit simulation tool that is available at www.linear.com. Final optimization of output ripple must be done on a dedicated PC board. Parasitic inductance due to poor layout can significantly impact ripple. Refer to the PC Board Layout section for more details. IC Thermal Considerations = 3mΩ 8A = 800μF 1% • 5 • 200kHz L1 0.1μH The design of the filter is beyond the scope of this data sheet. However, as a starting point, use these general guide lines. Start with a COUT 1/4 the size of the nonfilter solution. Make C1 1/4 of COUT to make the second filter pole independent of COUT . C1 may be best implemented with multiple ceramic capacitors. Make L1 smaller than the output inductance of the transformer. In general, a 0.1μH filter inductor is sufficient. Add a small ceramic capacitor (COUT2) for high frequency noise on VOUT. For those interested in more details refer to “Second-Stage LC Filter Design,” Ridley, Switching Power Magazine, July 2000, p8-10. RLOAD SwitcherCAD is a trademark of Linear Technology Corporation. 3825 F08 Figure 8 3825fa 25 LT3825 APPLICATIONS INFORMATION The total IC dissipation is computed as: PD(TOTAL) = VCC • (ICC + fOSC • (QGPRI + CS • VSGMAX)) VCC is the worst-case LT3825 supply voltage. adjacent to the VCC and GND pins on the IC package. This capacitor carries high di/dt MOSFET gate drive currents. Use a low ESR ceramic capacitor. Take care in PCB layout to keep the traces that conduct high switching currents short, wide and with minimal overall loop area. These are typically the traces associated with the switches. This reduces the parasitic inductance and also minimizes magnetic field radiation. Figure 9 outlines the critical paths. Junction temperature is computed as: TJ = TA + PD • θJA where: TA is the ambient temperature θJA is the FE16 package junction-to-ambient thermal impedance (40°C/W). PC Board Layout Considerations In order to minimize switching noise and improve output load regulation, connect the GND pin of the LT3825 directly to the ground terminal of the VCC decoupling capacitor, the bottom terminal of the current sense resistor, the ground terminal of the input capacitor, and the ground plane (multiple vias). Place the VCC capacitor immediately Keep electric field radiation low by minimizing the length and area of traces (keep stray capacitances low). The drain of the primary side MOSFET is the worst offender in this category. Always use a ground plane under the switcher circuitry to prevent coupling between PCB planes. Check that the maximum BVDSS ratings of the MOSFETs are not exceeded due to inductive ringing. This is done by viewing the MOSFET node voltages with an oscilloscope. If it is breaking down either choose a higher voltage device, add a snubber or specify an avalanche-rated MOSFET. T1 VCC • VIN CVCC • GATE TURN-ON • LT3825 VCC PG GND MP CVIN GATE TURN-OFF OUT RSENSE VCC COUT CR LT3825 VCC T2 SG • GND Q4 GATE TURN-ON MS • Q3 GATE TURN-OFF 3825 F09 Figure 9. High Current Paths 3525fa 26 LT3825 APPLICATIONS INFORMATION Place the small-signal components away from high frequency switching nodes. This allows the use of a pseudo-Kelvin connection for the signal ground, where high di/dt gate driver currents flow out of the IC ground pin in one direction (to the bottom plate of the VCC decoupling capacitor) and small-signal currents flow in the other direction. Keep the trace from the feedback divider tap to the FB pin short to preclude inadvertent pickup. For applications with multiple switching power converters connected to the same input supply, make sure that the input filter capacitor for the LT3825 is not shared with other converters. AC input current from another converter could cause substantial input voltage ripple and this could interfere with the LT3825 operation. A few inches of PC trace or wire (L ≅ 100nH) between the CIN of the LT3825 and the actual source VIN is sufficient to prevent current sharing problems. 3825fa 27 LT3825 TYPICAL APPLICATIONS 48V to 5V at 8A Isolated Supply T1 4 7 8 9 • 5 + VIN+ 36V TO 72V 2.2μF 100V 1 47k 1/4W + BAS21 402k 1% 29.4k 1% P6SMB100A 47μF 20V • 3 MBRS1100 0.1μF R7 20Ω SG PG SENSE+ SG LT3825 3.01k 1% 15k 1% PGDLY tON SYNC RCMP ENDLY 12k 2.1k 1% 100k 2.2nF 250V Si7336ADP ×2 SENSE– OSC GND SFST 47Ω 1μF 2.2nF 0.1μF CCMP 10nF 680pF 8 0.22μF • PA0184 100k 0.1μF 1 15Ω •5 10k BAT54 4 ALL CAPACITORS 25V UNLESS OTHERWISE NOTED T1: EDFD25-3F3 GAP FOR LP = 200μH (i.e., AL = 200nH/T2) 3825 TA02a PINS 1 TO 3, 32T OF 2 × 32AWG PINS 4 TO 5, 11T OF 1 × 32AWG PINS 1 TO 3, 32T OF 2 × 32AWG PINS 10, 11, 12 TO PINS 7, 8, 9, 4T OF 5 MIL COPPER FOIL PINS 1 TO 3, 32T OF 2 × 32AWG 2 MIL POLYESTER FILM Efficiency vs Load Current Output Regulation vs Load Current 5.25 94 5.20 36VIN 92 5.15 90 5.10 48VIN 88 OUTPUT (V) EFFICIENCY (%) B0540W FMMT718 FMMT618 SG 0.03Ω 1W 330Ω VC 100k 47pF 0.1μF 10Ω 1/4W FB UVLO 10 11 12 1nF Si4490DY VCC 470μF 6TPE470MI ×4 • VOUT+ 5V 8A 72VIN 86 84 48VIN 5.05 36VIN 5.00 72VIN 4.95 4.90 82 4.85 80 4.80 78 1 2 6 5 4 LOAD CURRENT (A) 3 7 8 3825 TA02b 4.75 1 2 5 4 3 6 LOAD CURRENT (A) 7 8 3825 TA02c 3525fa 28 LT3825 TYPICAL APPLICATIONS 48V to 3.3V at 6A Isolated Supply T1 PULSE PB2134 4 7 • 8 9 5 + VIN+ 36V TO 72V 0.82μF 100V 47k 1/4W + BAS21 402k 1% 1 26.1k 1% • 3 47μF 20V 0.1μF 10 11 12 Si4490DY SG 0.04Ω 1W 330Ω FB LT3825 UVLO 15k 1% 3.01k 1% PGDLY tON SYNC RCMP 15k 866Ω 1% 100k SENSE– VC ENDLY 62k 47pF OSC GND SFST 1μF FMMT718 SENSE+ SG 47Ω FMMT618 Si7892DP SG PG 0.1μF 2.2nF 250V B0540W R7 20Ω VCC 150μF 6TPB150ML ×3 • VOUT+ 3.3V 6A CCMP 3.3nF 680pF 8 0.22μF • • 5 10k BAT54 PA0184 10k 0.1μF 15Ω 2.2nF 0.1μF 1 4 3825 TA03a Efficiency vs Load Current Output Regulation vs Load Current 3.43 90 36VIN 3.38 72VIN 48VIN 86 3.33 OUTPUT (V) EFFICIENCY (%) 88 84 36VIN 3.28 72VIN 48VIN 3.23 82 80 3.18 3.13 1 2 4 3 LOAD CURRENT (A) 5 6 1 2 3 4 5 6 LOAD CURRENT (A) 3825 TA03b 3825 TA03c 3825fa 29 LT3825 TYPICAL APPLICATIONS 48V to 5V at 12A Isolated Supply L1 0.1μH + VIN 36V TO 72V T1 28.7k 1% BAS21 T1 • 3.01k 1% 47k 1/4W 20Ω 47μF FB 2.2μF 100V SG PG LT3825 12k Si7336ADP Q2 SENSE+ SG 0.02Ω 1/2W PGDLY tON SYNC RCMP ENDLY 100k 750Ω 2.2μF 250V 47Ω Q1 UVLO 15k 1% C5 470μF Si4490DY SG VCC B0540W • 20Ω + 402k 1% C1 TO C4 47μF ×3 • 100pF 250V VOUT+ 3.3V 12A 330Ω SENSE– VC CCMP OSC GND SFST 150k 47pF 1μF 0.01μF 0.1μF 15Ω 2.2nF 0.1μF 10nF • 10k • 10k PA0184 BAT54 1nF 3825 TA04a C1 TO C4: TDK C3225X5R0J476M C5: SANYO 6TPD470M L1: VISHAY IHLP2525CZERR10M Q1: ZETEX FMMT618 Q2: ZETEX FMMT718 T1: PULSE PA1477NL Efficiency vs Load Current Output Regulation vs Load Current 3.43 92 90 3.38 36VIN 36VIN 3.33 48VIN OUTPUT (V) EFFICIENCY (%) 88 86 72VIN 84 48VIN 3.28 72VIN 3.23 82 3.18 80 78 2 3 4 9 5 6 7 8 LOAD CURRENT (A) 10 11 12 3825 TA04b 3.13 2 4 6 8 LOAD CURRENT (A) 10 12 3825 TA04c 3525fa 30 LT3825 PACKAGE DESCRIPTION FE Package 16-Lead Plastic TSSOP (4.4mm) (Reference LTC DWG # 05-08-1663) Exposed Pad Variation BC 4.90 – 5.10* (.193 – .201) 3.58 (.141) 3.58 (.141) 16 1514 13 12 1110 6.60 ±0.10 9 2.94 (.116) 4.50 ±0.10 6.40 2.94 (.252) (.116) BSC SEE NOTE 4 0.45 ±0.05 1.05 ±0.10 0.65 BSC 1 2 3 4 5 6 7 8 RECOMMENDED SOLDER PAD LAYOUT 4.30 – 4.50* (.169 – .177) 0.09 – 0.20 (.0035 – .0079) 0.50 – 0.75 (.020 – .030) NOTE: 1. CONTROLLING DIMENSION: MILLIMETERS MILLIMETERS 2. DIMENSIONS ARE IN (INCHES) 3. DRAWING NOT TO SCALE 0.25 REF 1.10 (.0433) MAX 0° – 8° 0.65 (.0256) BSC 0.195 – 0.30 (.0077 – .0118) TYP 0.05 – 0.15 (.002 – .006) FE16 (BC) TSSOP 0204 4. RECOMMENDED MINIMUM PCB METAL SIZE FOR EXPOSED PAD ATTACHMENT *DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH SHALL NOT EXCEED 0.150mm (.006") PER SIDE 3825fa 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. 31 LT3825 RELATED PARTS PART NUMBER DESCRIPTION COMMENTS LT1424-5 Isolated Flyback Switching Regulator 5V Output Voltage, No Optoisolator Required LT1424-9 Isolated Flyback Switching Regulator 9V Output Voltage, Regulation Maintained Under Light Loads LT1425 Isolated Flyback Switching Regulator No Third Winding or Optoisolator Required LTC1698 Isolated Secondary Synchronous Rectifier Controller Isolated Power Supplies, Contains Voltage Merging, Optocoupler Driver, Primary Synchronization Circuit LT1725 General Purpose High Power Isolated Flyback Controller Suitable for Telecom or Offline Input Voltage LT1737 High Power Isolated Flyback Controller Powered from a DC Supply Voltage LTC1871 Wide Input Range Current Mode No RSENSE™ Controller 50kHz to 1MHz, Boost, Flyback and SEPIC Topology LTC3710 Secondary Side Synchronous Post Regulator Generates a Regulated Auxiliary Output in Isolated DC/DC Converters. Dual N-Channel MOSFET Synchronous Drivers LT3781/LT1698 36V to 72V Input Isolated DC/DC Converter Chipset Synchronous Operation: Overvoltage/Undervoltage Protection, 10W to 100W Power Supply, 1/2 to 1/4 Brick Footprint LTC3803 SOT-23 Flyback Controller Adjustable Slope Compensation, Internal Soft-Start, 200kHz LTC3806 Synchronous Flyback DC/DC Controller Medium Power Multiple Outputs, 250kHz Soft-Start No RSENSE is a trademark of Linear Technology Corporation. 3525fa 32 Linear Technology Corporation LT 0108 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 2007