LTC3805-5 Adjustable Frequency Current Mode Flyback/ Boost/SEPIC DC/DC Controller FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTION VIN and VOUT Limited Only by External Components 4.5V Undervoltage Lockout Threshold Adjustable Slope Compensation Adjustable Overcurrent Protection with Automatic Restart Adjustable Operating Frequency (70kHz to 700kHz) with One External Resistor Synchronizable to an External Clock ±1.5% Reference Accuracy Only 100mV Current Sense Voltage Drop RUN Pin with Precision Threshold and Adjustable Hysteresis Programmable Soft-Start with One External Capacitor Low Quiescent Current: 360μA Small 10-Lead MSOP and 3mm × 3mm DFN APPLICATIONS ■ ■ ■ ■ ■ The LTC®3805-5 is a current mode DC/DC controller designed to drive an N-channel MOSFET in flyback, boost and SEPIC converter applications. Operating frequency and slope compensation can be programmed by external resistors. Programmable overcurrent sensing protects the converter from overload and short-circuit conditions. Soft-start can be programmed using an external capacitor and the soft-start capacitor also programs an automatic restart feature. The LTC3805-5 provides ±1.5% output voltage accuracy and consumes only 360μA of quiescent current during normal operation and only 40μA during micropower startup. Using a 9.5V internal shunt regulator, the LTC3805-5 can be powered from a high VIN through a resistor or it can be powered directly from a low impedance DC voltage from 4.7V to 8.8V. The LTC3805-5 is available in the 10-lead MSOP package and the 3mm × 3mm DFN package. Automotive Power Supplies Telecom Power Supplies Isolated Electronic Equipment Auxiliary/Housekeeping Power Supplies Power over Ethernet , LT, LTC and LTM are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. TYPICAL APPLICATION Efficiency and Power Loss vs Load Current 100 5V to 12V/1A Boost Converter VCC 100μF GATE LTC3805-5 ITH 470pF 0.1μF 118k OC SSFLT ISENSE FS 1.33k 3k GND 95 2.0 5V EFFICIENCY 8.5V EFFICIENCY 1.5 90 8.5V LOSS 85 5V LOSS 1 191k 80 FB SYNC VOUT 12V 1A EFFICIENCY (%) 22μF ×2 RUN 20k UPS840 4.3μH 8mΩ 13.7k 38055 TA01 75 0.01 POWER LOSS (W) VIN 5V 2.5 0.5 1 0.1 LOAD CURRENT (A) 10 0 38055TA01b 38055fa 1 LTC3805-5 ABSOLUTE MAXIMUM RATINGS (Note 1) VCC to GND Low Impedance Source ........................ –0.3V to 8.8V Current Fed ........................................25mA into VCC* SYNC ........................................................... –0.3V to 6V SSFLT........................................................... –0.3V to 5V FB, ITH, FS ................................................. –0.3V to 3.5V RUN ........................................................... –0.3V to 18V OC, ISENSE .................................................... –0.3V to 1V Operating Temperature Range E-Grade ................................................–40°C to 85°C I-Grade............................................... –40°C to 125°C Junction Temperature ........................................... 125°C Storage Temperature Range...................–65°C to 125°C Lead Temperature (Soldering, 10 sec) LTC3805EMSE-5 Only....................................... 300°C *LTC3805-5 internal clamp circuit regulates VCC voltage to 9.5V PIN CONFIGURATION TOP VIEW SSFLT 1 ITH 2 TOP VIEW 10 GATE 11 SSFLT ITH FB RUN FS 9 VCC FB 3 RUN 4 7 ISENSE FS 5 6 SYNC 8 OC 1 2 3 4 5 10 9 8 7 6 11 GATE VCC OC ISENSE SYNC MSE PACKAGE 10-LEAD PLASTIC MSOP DD PACKAGE 10-LEAD (3mm s 3mm) PLASTIC DFN TJMAX = 125°C, θJA = 45°C/W EXPOSED PAD (PIN 11) IS GND, MUST BE CONNECTED TO GND TJMAX = 125°C, θJA = 45°C/W EXPOSED PAD (PIN 11) IS GND, MUST BE CONNECTED TO GND ORDER INFORMATION Lead Free Finish TAPE AND REEL (MINI) TAPE AND REEL PART MARKING* PACKAGE DESCRIPTION TEMPERATURE RANGE LTC3805EMSE-5#TRMPBF LTC3805EMSE-5#TRPBF LTDGX 10-Lead Plastic MSOP LTC3805IMSE-5#TRMPBF LTC3805IMSE-5#TRPBF LTDGX 10-Lead Plastic MSOP LTC3805EDD-5#TRMPBF LTC3805EDD-5#TRPBF LDHB 10-Lead (3mm × 3mm) Plastic DFN LTC3805IDD-5#TRMPBF LTC3805IDD-5#TRPBF LDHB 10-Lead (3mm × 3mm) Plastic DFN TRM = 500 pieces. *Temperature grades are identified by a label on the shipping container. Consult LTC Marketing for parts specified with wider operating temperature ranges. Consult LTC Marketing for information on lead based finish parts. For more information on lead free part marking, go to: http://www.linear.com/leadfree/ For more information on tape and reel specifications, go to: http://www.linear.com/tapeandreel/ –40°C to 85°C –40°C to 125°C –40°C to 85°C –40°C to 125°C ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C, VCC = 5V, unless otherwise noted (Note 2). SYMBOL PARAMETER CONDITIONS MIN TYP MAX UNITS VTURNON VCC Turn-On Voltage VCC Rising ● 4.3 4.5 4.7 V VTURNOFF VCC Turn-Off Voltage VCC Falling ● 3.75 3.95 4.15 V VHYST VCC Hysteresis VCLAMP1mA VCC Shunt Regulator Voltage ICC = 1mA, VRUN = 0 ● 8.8 9.25 9.65 V VCLAMP25mA VCC Shunt Regulator Voltage ICC = 25mA, VRUN = 0 ● 8.9 9.5 9.9 V 0.55 V 38055fa 2 LTC3805-5 ELECTRICAL CHARACTERISTICS The ● denotes the specifications which apply over the full operating temperature range, otherwise specifications are at TA = 25°C, VCC = 5V, unless otherwise noted (Note 2). SYMBOL PARAMETER CONDITIONS ICC Input DC Supply Current Normal Operation (fOSC = 200kHz) (Note 4) MIN TYP MAX 360 VRUN < VRUNON or VCC < VTURNON – 100mV (Micropower Start-Up) ● UNITS μA 40 90 μA VRUNON RUN Turn-On Voltage VCC = VTURNON + 100mV ● 1.122 1.207 1.292 V VRUNOFF RUN Turn-Off Voltage VCC = VTURNON + 100mV ● 1.092 1.170 1.248 V ● 4 5 5.8 μA ● ● 0.788 0.780 0.780 0.800 0.800 0.800 0.812 0.812 0.812 V V V IRUN(HYST) RUN Hysteresis Current VFB Regulated Feedback Voltage IFB VFB Input Current gm Error Amplifier Transconductance ITH Pin Load = ±5μA (Note 5) 333 μA/V ΔVO(LINE) Output Voltage Line Regulation VTURNOFF < VCC < VCLAMP1mA (Note 5) 0.05 mV/V ΔVO(LOAD) Output Voltage Load Regulation ITH Sinking 5μA (Note 5) ITH Sourcing 5μA (Note 5) 3 3 mV/μA mV/μA fOSC Oscillator Frequency RFS = 350k 70 kHz RFS = 36k 700 kHz 0°C ≤ TA ≤ 85°C (E-Grade) (Note 5) –40°C ≤ TA ≤ 85°C (E-Grade) (Note 5) –40°C ≤ TA ≤ 125°C (I-Grade) (Note 5) VITH = 1.3V (Note 5) 20 DCON(MIN) Minimum Switch-On Duty Cycle fOSC = 200kHz 6 DCON(MAX) Maximum Switch-On Duty Cycle fOSC = 200kHz 70 fSYNC As a Function of fOSC 70kHz < fOSC < 700kHz, 70kHz < fSYNC < 700kHz 67 VSYNC Minimum SYNC Amplitude ISS Soft-Start Current 80 nA 9 % 95 % 133 % 2.9 V –6 μA IFTO Fault Timeout Current 2 μA tSS(INT) Internal Soft-Start Time No External Capacitor on SSFLT 1.8 ms tFTO(INT) Internal Fault Timeout No External Capacitor on SSFLT 4.5 ms tRISE Gate Drive Rise Time CLOAD = 3000pF 30 ns tFALL Gate Drive Fall Time CLOAD = 3000pF 30 ns VI(MAX) Peak Current Sense Voltage RSL = 0 (Note 6) ISL(MAX) Peak Slope Compensation Output Current (Note 7) VOCT Overcurrent Threshold ROC = 0 (Note 8) IOC Overcurrent Threshold Adjust Current 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 LTC3805E-5 is guaranteed to meet specifications from 0°C to 85°C. Specifications over the –40°C to 85°C operating temperature range are assured by design, characterization and correlation with statistical process controls. The LTC3805I-5 is guaranteed to meet performance specifications over the –40°C to 125°C operating temperature range. Note 3: TJ is calculated from the ambient temperature TA and power dissipation PD according to the following formula: TJ = TA + (PD • 45°C/W) ● 85 100 115 10 ● 85 100 10 mV μA 115 mV μA Note 4: Dynamic supply current is higher due to the gate charge being delivered at the switching frequency. Note 5: The LTC3805-5 is tested in a feedback loop that servos VFB to the output of the error amplifier while maintaining ITH at the midpoint of the current limit range. Note 6: Peak current sense voltage is reduced dependent on duty cycle and an optional external resistor in series with the SENSE pin. For details, refer to Programmable Slope Compensation in the Applications Information section. Note 7: Guaranteed by design. Note 8: Overcurrent threshold voltage is reduced dependent on an optional external resistor in series with the OC pin. For details, refer to Programmable Overcurrent in the Applications Information section. 38055fa 3 LTC3805-5 TYPICAL PERFORMANCE CHARACTERISTICS Reference Voltage vs Supply Voltage Oscillator Frequency vs RFS 812 0.800300 800 808 0.800200 700 804 0.800100 800 fOSC (kHz) 600 VFB (V) VFB VOLTAGE (mV) Reference Voltage vs Temperature 0.800000 796 0.799900 792 0.799800 788 –50 0.799700 500 400 300 200 75 0 25 50 TEMPERATURE (°C) –25 100 125 100 4 5 6 7 VCC (V) 8 9 200 199 198 197 4 5 6 7 8 9 1.205 RFS = 124kΩ 200 199 198 –50 –25 75 0 25 50 TEMPERATURE (°C) 38055 G04 1.190 1.185 VRUN(OFF) 1.180 100 125 1.170 –50 –25 75 0 25 50 TEMPERATURE (°C) 38055 G05 RUN Hysteresis Current vs Temperature 100 125 38055 G06 VCC Undervoltage Lockout Thresholds vs Temperature 5.0 VCC UNDERVOLTAGE LOCKOUT (V) 5.20 5.10 IRUN(HYST) 1.195 1.175 VCC (V) 5.00 4.90 4.80 4.70 –50 VRUN(ON) 1.200 201 RUN VOLTAGE (V) OSCILLATOR FREQUENCY (kHz) fOSC (kHz) RUN Undervoltage Lockout Thresholds vs Temperature 202 RFS = 124kΩ 400 300 38055 G03 Oscillator Frequency vs Temperature 203 201 200 100 38055 G02 Oscillator Frequency vs Supply Voltage 202 0 RFS (kΩ) 38055 G01 196 0 10 –25 75 0 25 50 TEMPERATURE (°C) 100 125 38055 G07 VTURN(ON) 4.5 VTURN(OFF) 4.0 3.5 3.0 –50 –25 75 0 25 50 TEMPERATURE (°C) 100 125 38055 G08 38055fa 4 LTC3805-5 TYPICAL PERFORMANCE CHARACTERISTICS Start-Up ICC Supply Current vs Temperature ICC Supply Current vs Temperature 350 9.60 48 9.55 46 340 44 42 40 38 36 34 330 320 9.45 9.40 9.35 9.30 310 VCLAMP1mA 9.25 32 30 –50 –25 0 25 50 75 TEMPERATURE (°C) 100 300 –50 125 75 0 25 50 TEMPERATURE (°C) –25 100 38055 G09 100.2 100.0 99.8 99.6 99.4 99.2 0 25 50 75 TEMPERATURE (°C) 100 125 102 100 98 96 94 –50 75 0 25 50 TEMPERATURE (°C) –25 100 External Soft-Start Current vs Temperature 125 1.65 1.60 1.55 1.50 1.45 –50 –25 75 0 25 50 TEMPERATURE (°C) 100 125 38055 G14 External Timeout Current vs Temperature 5.3 IFTO EXTERNAL TIMEOUT CURRENT (μA) 2.1 5.2 2.0 5.1 1.9 5.0 4.9 1.8 4.8 1.7 4.7 1.6 4.6 4.5 –50 125 1.70 38055 G13 38055 G12 ISS SOFT-START CURRENT (μA) –25 100 1.75 INTERNAL SOFT-START TIME (ms) OVERCURRENT THRESHOLD (mV) 100.4 0 25 50 75 TEMPERATURE (°C) Internal Soft-Start Time vs Temperature 104 100.6 –25 38055 G11 Overcurrent Threshold vs Temperature 100.8 99.0 –50 9.20 –50 125 38055 G10 Peak Current Sense Voltage vs Temperature PEAK CURRENT SENSE VOLTAGE (mV) VCLAMP25mA 9.50 VCLAMP (V) SUPPLY CURRENT (μA) START-UP SUPPLY CURRENT (μA) 50 VCC Shunt Regulator Voltage vs Temperature –25 0 25 50 75 TEMPERATURE (°C) 100 125 38055 G15 1.5 –50 –25 75 0 25 50 TEMPERATURE (°C) 100 125 38055 G16 38055fa 5 LTC3805-5 PIN FUNCTIONS SSFLT (Pin 1): Soft-Start Pin. A capacitor placed from this pin to GND (Exposed Pad) controls the rate of rise of converter output voltage during start-up. This capacitor is also used for time out after a fault prior to restart. ITH (Pin 2): Error Amplifier Compensation Point. Normal operating voltage range is clamped between 0.7V and 1.9V. FB (Pin 3): Receives the feedback voltage from an external resistor divider across the output. RUN (Pin 4): An external resistor divider connects this pin to VIN and sets the thresholds for converter operation. FS (Pin 5): A resistor connected from this pin to ground sets the frequency of operation. SYNC (Pin 6): Input to synchronize the oscillator to an external source. ISENSE (Pin 7): Performs two functions: for current mode control, it monitors the switch current, using the voltage across an external current sense resistor. Pin 7 also injects a current ramp that develops slope compensation voltage across an optional external programming resistor. OC (Pin 8): Overcurrent Pin. Connect this pin to the external switch current sense resistor. An additional resistor programs the overcurrent trip level. VCC (Pin 9): Supply Pin. A capacitor must closely decouple VCC to GND (Exposed Pad). GATE (Pin 10): Gate Drive for the External N-Channel MOSFET. This pin swings from GND to VCC. Exposed Pad (Pin 11): Ground. A capacitor must closely decouple GND to VCC (Pin 9). Must be soldered to electrical ground on PCB. 38055fa 6 LTC3805-5 BLOCK DIAGRAM 9 4 RUN VCC SOFT-START RAMP 800mV REFERENCE 1 SSFLT OC SHUTDOWN SOFT-START FAULT OVERCURRENT COMPARATOR 10μA 8 UNDERVOLTAGE LOCKOUT + – 100mV + – ERROR AMPLIFIER CURRENT COMPARATOR R + Q S 3 FB GATE DRIVER GATE 10 – SHUTDOWN SLOPE COMP CURRENT RAMP 20mV OSCILLATOR ITH CLAMPS 11 SWITCHING LOGIC AND BLANKING CIRCUIT GND ISENSE 1.2V 2 ITH 5 FS 6 7 SYNC 38055 BD 38055fa 7 LTC3805-5 OPERATION The LTC3805-5 is a programmable-frequency current mode controller for flyback, boost and SEPIC DC/DC converters. The LTC3805-5 is designed so that none of its pins need to come in contact with the input or output voltages of the power supply circuit of which it is a part, allowing the conversion of voltages well beyond the LTC3805-5’s absolute maximum ratings. Main Control Loop Please refer to the Block Diagram of this data sheet and the Typical Application shown on the front page. An external resistive voltage divider presents a fraction of the output voltage to the FB pin. The divider is designed so that when the output is at the desired voltage, the FB pin voltage equals the 800mV internal reference voltage. If the load current increases, the output voltage decreases slightly, causing the FB pin voltage to fall below the 800mV reference. The error amplifier responds by feeding current into the ITH pin causing its voltage to rise. Conversely, if the load current decreases, the FB voltage rises above the 800mV reference and the error amplifier sinks current away from the ITH pin causing its voltage to fall. The voltage at the ITH pin controls the pulse-width modulator formed by the oscillator, current comparator and SR latch. Specifically, the voltage at the ITH pin sets the current comparator’s trip threshold. The current comparator’s ISENSE input monitors the voltage across an external current sense resistor in series with the source of the external MOSFET. At the start of a cycle, the LTC3805-5’s oscillator sets the SR latch and turns on the external power MOSFET. The current through the external power MOSFET rises as does the voltage on the ISENSE pin. The LTC3805-5’s current comparator trips when the voltage on the ISENSE pin exceeds a voltage proportional to the voltage on the ITH pin. This resets the SR latch and turns off the external power MOSFET. In this way, the peak current levels through the external MOSFET and the flyback transformer’s primary and secondary windings are controlled by the voltage on the ITH pin. If the current comparator does not trip, the LTC3805-5 automatically limits the duty cycle to 80%, resets the SR latch, and turns off the external MOSFET. The path from the FB pin, through the error amplifier, current comparator and the SR latch implements the closed-loop current-mode control required to regulate the output voltage against changes in input voltage or output current. For example, if the load current increases, the output voltage decreases slightly, and sensing this, the error amplifier sources current from the ITH pin, raising the current comparator threshold, thus increasing the peak currents through the transformer primary and secondary. This delivers more current to the load and restores the output voltage to the desired level. The ITH pin serves as the compensation point for the control loop. Typically, an external series RC network is connected from ITH to ground and is chosen for optimal response to load and line transients. The impedance of this RC network converts the output current of the error amplifier to the ITH voltage which sets the current comparator threshold and commands considerable influence over the dynamics of the voltage regulation loop. 38055fa 8 LTC3805-5 OPERATION Start-Up/Shutdown Setting the Oscillator Frequency The LTC3805-5 has two shutdown mechanisms to disable and enable operation: an undervoltage lockout on the VCC supply pin voltage, and a precision-threshold RUN pin. The voltage on both pins must exceed the appropriate threshold before operation is enabled. The LTC3805-5 transitions into and out of shutdown according to the state diagram shown in Figure 1. Operation in fault timeout is discussed in a subsequent section. During shutdown the LTC3805-5 draws only a small 40μA current. Connect a frequency set resistor RFS from the FS pin to ground to set the oscillator frequency over a range from 70kHz to 700kHz. The oscillator frequency is calculated from: 24 • 10 9 fOSC = RFS − 1500 The undervoltage lockout (UVLO) mechanism prevents the LTC3805-5 from trying to drive the external MOSFET gate with insufficient voltage on the VCC pin. The voltage at the VCC pin must initially exceed VTURNON = 4.5V to enable LTC3805-5 operation. After operation is enabled, the voltage on the VCC pin may fall as low as VTURNOFF = 4V before undervoltage lockout disables the LTC3805-5. See the Applications Information section for more detail. The RUN pin is connected to the input voltage using a voltage divider. Converter operation is enabled when the voltage on the RUN pin exceeds VRUNON = 1.207V and disabled when the voltage falls below VRUNOFF = 1.170V. Additional hysteresis is added by a 5μA current source acting on the voltage divider’s Thevenin resistance. Setting the input voltage range and hysteresis is further discussed in the Applications Information section. The oscillator may be synchronized to an external clock using the SYNC input. The rising edge of the external clock on the SYNC pin triggers the beginning of a switching period, i.e., the GATE pin going high. The pulse width of the external clock is quite flexible. The clock must stay high only for about 200ns to trigger the start of a new switching period. Conversely, the pulse width can be increased to a duty cycle not greater than 55%. Overcurrent Protection With the OC pin connected to the external MOSFET’s current sense resistor, the converter is protected in the event of an overload or short-circuit on the output. During normal operation the peak value of current in the external MOSFET, as measured by the current sense resistor (plus any adjustment for slope compensation), is set by the voltage on the ITH pin operating through the current comparator. As the output current increases, so does the voltage on the ITH pin and so does the peak MOSFET current. VRUN > VRUNON AND VCC > VTURNON LTC3805-5 SHUTDOWN VRUN < VRUNOFF LTC3805-5 ENABLED VCC < VTURNOFF VSSFLT < 0.7V LTC3805-5 FAULT TIMEOUT VOC > 100mV 38055 F01 Figure 1. Start-Up/Shutdown State Diagram 38055fa 9 LTC3805-5 OPERATION First, consider operation without overcurrent protection. For some maximum converter output current, the voltage on the ITH pin rises to and is clamped at approximately 1.9V. This corresponds to a 100mV limit on the voltage at the ISENSE pin. As the output current is further increased, the duty cycle is reduced as the output voltage sags. However, the peak current in the external MOSFET is limited by the 100mV threshold at the ISENSE pin. As the output current is increased further, eventually, the duty cycle is reduced to the 6% minimum. Since the external MOSFET is always turned on for this minimum amount of time, the current comparator no longer limits the current through the external MOSFET based on the 100mV threshold. If the output current continues to increase, the current through the MOSFET could rise to a level that would damage the converter. To prevent damage, the overcurrent pin OC is also connected to the current sense resistor, and a fault is triggered if the voltage on the OC pin exceeds 100mV. To protect itself, the converter stops operating as described in the next section. External resistors can be used to adjust the overcurrent threshold to voltages higher or lower than 100mV as described in the Applications Information section. Soft-Start and Fault Timeout Operation The soft-start and fault timeout of the LTC3805-5 uses either a fixed internal timer or an external timer programmed by a capacitor from the SSFLT pin to GND. The internal soft-start and fault timeout times are minimums and can be increased by placing a capacitor from the SSFLT pin to GND. Operation is shown in Figure 1. Leave the SSFLT pin open to use the internal soft-start and fault timeout. The internal soft-start is complete in about 1.8ms. In the event of an overcurrent as detected by the OC pin exceeding 100mV, the LTC3805-5 shuts down and an internal timing circuit waits for a fault timeout of about 4.25ms and then restarts the converter. Add a capacitor CSS from the SSFLT pin to GND to increase both the soft-start time and the time for fault timeout. During soft-start, CSS is charged with a 6μA current. When the LTC3805-5 comes out of shutdown, the LTC3805-5 quickly charges CSS to about 0.7V at which point GATE begins switching. From that point, GATE continues switching with increasing duty cycle until the SSFLT pin reaches about 2.25V at which point soft-start is over and closed-loop regulation begins. The voltage on the SSFLT pin additionally further charges to about 4.75V. CSS also performs the timeout function in the event of a fault. After a fault, CSS is slowly discharged from about 4.75V to about 0.7V by a 2μA current. When the voltage on the SSFLT pin reaches 0.7V the converter attempts to restart. More detail on programming the external soft-start fault timeout is described in the Applications Information section. Powering the LTC3805-5 A built-in shunt regulator from the VCC pin to GND limits the voltage on the VCC pin to approximately 9.5V as long as the shunt regulator is not forced to sink more than 25mA. The shunt regulator is always active, even when the LTC3805-5 is in shutdown, since it serves the vital function of protecting the VCC pin from overvoltage. The shunt regulator permits the use of a wide variety of powering schemes for the LTC3805-5 even from high voltage sources that exceed the LTC3805-5’s absolute maximum ratings. Further details on powering schemes are described in the Applications Information section. Adjustable Slope Compensation The LTC3805-5 injects a 10μA peak current ramp out of its ISENSE pin which can be used, in conjunction with an external resistor, for slope compensation in designs that require it. This current ramp is approximately linear and begins at zero current at 6% duty cycle, reaching peak current at 80% duty cycle. Additional details are provided in the Applications Information section. 38055fa 10 LTC3805-5 APPLICATIONS INFORMATION Many LTC3805-5 application circuits can be derived from the topologies shown on the first page or in the Typical Applications section of this data sheet. The LTC3805-5 itself imposes no limits on allowed input voltage VIN or output voltage VOUT. These are all determined by the ratings of the external power components. In Figure 8, the factors are: Q1 maximum drain-source voltage (BVDSS), on-resistance (RDS(ON)) and maximum drain current, T1 saturation flux level and winding insulation breakdown voltages, CIN and COUT maximum working voltage, equivalent series resistance (ESR), and maximum ripple current ratings, and D1 and RSENSE power ratings. VCC Bias Power The VCC pin must be bypassed to the GND pin with a minimum 1μF ceramic or tantalum capacitor located immediately adjacent to the two pins. Proper supply bypassing is necessary to supply the high transient currents required by the MOSFET gate driver. For maximum flexibility, the LTC3805-5 is designed so that it can be operated from voltages well beyond the LTC3805-5’s absolute maximum ratings. Figure 2 shows the simplest case, in which the LTC3805-5 is powered with a resistor RVCC connected between the input voltage and VCC. The built-in shunt regulator limits the voltage on the VCC pin to around 9.5V as long as the internal shunt regulator is not forced to sink more than 25mA. This powering scheme has the drawback that the power loss in the resistor reduces converter efficiency and the 25mA shunt regulator maximum may limit the maximumto-minimum range of input voltage. The typical application circuit in Figure 9 shows a different flyback converter bias power strategy for a case in which neither the input or output voltage is suitable for providing bias power to the LTC3805-5. A small NPN preregulator transistor and a zener diode are used to accelerate the rise of VCC and reduce the value of the VCC bias capacitor. The flyback transformer has an additional bias winding to provide bias power. Note that this topology is very powerful because, by appropriate choice of transformer turns ratio, the output voltage can be chosen without regard to the value of the input voltage or the VCC bias power for the LTC3805-5. The number of turns in the bias winding is chosen according to NBIAS = NSEC VCC + VD2 VOUT + VD1 where NBIAS is the number of turns in the bias winding, NSEC is the number of turns in the secondary winding, VCC is the desired voltage to power the LTC3805-5, VOUT is the converter output voltage, VD1 is the forward voltage drop of D1 and VD2 is the forward voltage drop of D2. Note that since VOUT is regulated by the converter control loop, VCC is also regulated although not as precisely. If an “off-the-shelf” transformer with excessive bias windings is used, the resistor, RBIAS in Figure 9, can be added to limit the current. VIN RVCC LTC3805-5 VCC CVCC GND 38055 F02 Figure 2. Powering the LTC3805-5 via the Internal Shunt Regulator 38055fa 11 LTC3805-5 APPLICATIONS INFORMATION Transformer Design Considerations Transformer specification and design is perhaps the most critical part of applying the LTC3805-5 successfully. In addition to the usual list of caveats dealing with high frequency power transformer design, the following should prove useful. Turns Ratios Due to the use of the external feedback resistor divider ratio to set output voltage, the user has relative freedom in selecting transformer turns ratio to suit a given application. Simple ratios of small integers, e.g., 1:1, 2:1, 3:2, etc. can be employed which yield more freedom in setting total turns and transformer inductance. Simple integer turns ratios also facilitate the use of “off-the-shelf” configurable transformers. Turns ratio can be chosen on the basis of desired duty cycle. However, remember that the input supply voltage plus the secondary-to-primary referred version of the flyback pulse (including leakage spike) must not exceed the allowed external MOSFET breakdown rating. Leakage Inductance Transformer leakage inductance (on either the primary or secondary) causes a voltage spike to occur after the turn off of MOSFET (Q1) in Figure 8. This is increasingly prominent at higher load currents, where more stored energy must be dissipated. In some cases an RC “snubber” circuit will be required to avoid overvoltage breakdown at the MOSFET’s drain node. Application Note 19 is a good reference on snubber design. A bifilar or similar winding technique is a good way to minimize troublesome leakage inductances. However, remember that this will limit the primary-to-secondary breakdown voltage, so bifilar winding is not always practical. Setting Undervoltage and Hysteresis on VIN The RUN pin is connected to a resistive voltage divider connected to VIN as shown in Figure 3. The voltage threshold for the RUN pin is VRUNON rising and VRUNOFF falling. Note that VRUNON – VRUNOFF = 35mV of built-in voltage hysteresis that helps eliminate false trips. To introduce further user-programmable hysteresis, the LTC3805-5 sources 5μA out of the RUN pin when operation of LTC3805-5 is enabled. As a result, the falling threshold for the RUN pin also depends on the value of R1 and can be programmed by the user. The falling threshold for VIN is therefore VIN(RUN,FALLING) = VRUNOFF • R1+ R2 − R1 • 5μ A R2 where R1(5μA) is the additional hysteresis introduced by the 5μA current sourced by the RUN pin. When in shutdown, the RUN pin does not source the 5μA current and the rising threshold for VIN is simply VIN(RUN,RISING) = VRUNON • R1+ R2 R2 Note that for some applications the RUN pin can be connected to VCC in which case the VCC thresholds, VTURNON and VTURNOFF, control operation. 38055fa 12 LTC3805-5 APPLICATIONS INFORMATION External Run/Stop Control Feedback in Isolated Applications To implement external run control, place a small N-channel MOSFET from the RUN pin to GND as shown in Figure 3. Drive the gate of this MOSFET high to pull the RUN pin to ground and prevent converter operation. Isolated applications do not use the FB pin and error amplifier but control the ITH pin directly using an optoisolator driven on the other side of the isolation barrier as shown in Figure 4. For isolated converters, the FB pin is grounded which provides pull-up on the ITH pin. This pull-up is not enough to properly bias the optoisolator which is typically biased using a resistor to VCC. Since the ITH pin cannot sink the optoisolator bias current, a diode is required to block it from the ITH pin. A Schottky diode should be used to ensure that the optoisolator is able to pull ITH down to its lower clamp. Selecting Feedback Resistor Divider Values The regulated output voltage is determined by the resistor divider across VOUT (R3 and R4 in Figure 8). The ratio of R4 to R3 needed to produce a desired VOUT can be calculated: R3 = VOUT − 0 . 8 V R4 0 . 8V Oscillator Synchronization Choose resistance values for R3 and R4 to be as large as possible in order to minimize any efficiency loss due to the static current drawn from VOUT, but just small enough so that when VOUT is in regulation the input current to the VFB pin is less than 1% of the current through R3 and R4. A good rule of thumb is to choose R4 to be less than 80k. VIN Current Sense Resistor Considerations R1 RUN LTC3805-5 R2 GND The oscillator may be synchronized to an external clock by connecting the synchronization signal to the SYNC pin. The LTC3805-5 oscillator and turn-on of the switch are synchronized to the rising edge of the external clock. The frequency of the external sync signal must be ±33% with respect to fOSC (as programmed by RFS). Additionally, the value of fSYNC must be between 70kHz and 700kHz. RUN/STOP CONTROL (OPTIONAL) 38055 F03 Figure 3. Setting RUN Pin Voltage and Run/Stop Control The external current sense resistor (RSENSE in Figure 8) allows the user to optimize the current limit behavior for the particular application. As the current sense resistor is varied from several ohms down to tens of milliohms, peak switch current goes from a fraction of an ampere to several amperes. Care must be taken to ensure proper circuit operation, especially with small current sense resistor values. 38055fa 13 LTC3805-5 APPLICATIONS INFORMATION For example, with the peak current sense voltage of 100mV on the ISENSE pin, a peak switch current of 5A requires a sense resistor of 0.020Ω. Note that the instantaneous peak power in the sense resistor is 0.5W and it must be rated accordingly. The LTC3805-5 has only a single sense line to this resistor. Therefore, any parasitic resistance in the ground side connection of the sense resistor will increase its apparent value. In the case of a 0.020Ω sense resistor, one milliohm of parasitic resistance will cause a 5% reduction in peak switch current. So the resistance of printed circuit copper traces and vias cannot necessarily be ignored. Note: LTC3805-5 enforces 6% < Duty Cycle < 80%. A good starting value for RSLOPE is 3k, which gives a 30mV drop in current comparator threshold at 80% duty cycle. Designs that do not operate at greater than 50% duty cycle do not need slope compensation and may replace RSLOPE with a direct connection. Programmable Slope Compensation Overcurrent Threshold Adjustment The LTC3805-5 injects a ramping current through its ISENSE pin into an external slope compensation resistor RSLOPE. This current ramp starts at zero right after the GATE pin has been high for the LTC3805-5’s minimum duty cycle of 6%. The current rises linearly towards a peak of 10μA at the maximum duty cycle of 80%, shutting off once the GATE pin goes low. A series resistor RSLOPE connecting the ISENSE pin to the current sense resistor RSENSE develops a ramping voltage drop. From the perspective of the ISENSE pin, this ramping voltage adds to the voltage across the sense resistor, effectively reducing the current comparator threshold in proportion to duty cycle. This stabilizes the Figure 5 shows the connection of the overcurrent pin OC along with the ISENSE pin and the current sense resistor RSENSE located in the source circuit of the power NMOS which is driven by the GATE pin. The internal overcurrent threshold on the OC pin is set at VOCT = 100 mV which is the same as the peak current sense voltage VI(MAX) = 100 mV on the ISENSE pin. The role of the slope compensation adjustment resistor RSLOPE and the slope compensation current ISLOPE is discussed in the prior section. In combination with the overcurrent threshold adjust current IOC = 10μA, an external resistor ROC can be used to lower the overcurrent control loop against subharmonic oscillation. The amount of reduction in the current comparator threshold (ΔVSENSE) can be calculated using the following equation: ΔVSENSE = DutyCycle − 6 % 10μ A • R SLOPE 80 % ISOLATION BARRIER VCC GATE LTC3805-5 LTC3805-5 ISENSE ITH OC RSLOPE ISLOPE ROC IOC = 10μA FB GND RSENSE 38055 F04 Figure 4. Circuit for Isolated Feedback GND 38055 F05 Figure 5. Circuit to Decrease Overcurrent Threshold 38055fa 14 LTC3805-5 APPLICATIONS INFORMATION trip threshold from 100mV. This section describes how to pick ROC to achieve the desired performance. In the discussion that follows be careful to distinguish between “current limit” where the converter continues to run with the ISENSE pin limiting current on a cycle-by-cycle basis while the output voltage falls below the regulation point and “overcurrent protection” where the OC pin senses an overcurrent and shuts down the converter for a timeout period before attempting an automatic restart. One overcurrent protection strategy is for the converter to never enter current limit but to maintain output voltage regulation up to the point of tripping the overcurrent protection. Operation at minimum input voltage VIN(MIN) hits current limiting for the smallest output current and is the design point for this strategy. First, for operation at VIN(MIN), calculate the duty cycle Duty Cycle VIN(MIN) using the appropriate formula depending on whether the converter is a boost, flyback or SEPIC. Then use Duty Cycle VIN(MIN) to calculate ΔVSENSE(VIN(MIN)) using the formula in the prior section. For overcurrent protection to trip at exactly the point where current limiting would begin set: Δ VSENSE ( VIN(MIN)) ROC(CRIT) = 10μ A To find the actual output current that trips overcurrent protection, calculate the peak switch current IPK(VIN(MIN)) from: 100mV − Δ VSENSE ( VIN(MIN)) IPK ( VIN(MIN)) = R SENSE Then calculate the converter output current that corresponds to IPK(VIN(MIN)). Again, the calculation depends both on converter type and the details of converter design including inductor current ripple. For minimum input voltage, ROC(CRIT) produces an overcurrent trip at an output current just before loss of output voltage regulation and the onset of current limiting. Note that the output current that causes an overcurrent trip is higher for higher input voltages but that an overcurrent trip will always occur before loss of output voltage regulation. If desired to meet a specific design target, an increase in ROC above ROC(CRIT) can be used to reduce the trip threshold and make the converter trip for a lower output current. This calculation is based on steady-state operation. Depending on design, overcurrent protection can also be triggered during a start up transient, particularly if large output filter capacitors are being charged as output voltage rises. If that is a problem, output capacitor charging can be slowed by using a larger value of SSFLT capacitor. It is also possible to trip overcurrent protection during a load step especially if the trip threshold is lowered by making ROC > ROC(CRIT). Another overcurrent protection strategy is keep the converter running as current limiting reduces the duty cycle and the output voltage sags. In this case, the goal is often keep the converter in normal operation over as wide a range as possible, including current limiting, and to trigger the overcurrent trip only to prevent damage. To implement this strategy use a value of ROC smaller than ROC(CRIT). This also reduces sensitivity to overcurrent trips caused by transient operation. In the limit, set ROC = 0 and connect the OC pin directly to RSENSE. This causes an overcurrent trip near minimum duty cycle or around 6%. In some cases it may be desirable to increase the trip threshold even further. In this strategy, the converter is allowed to operate all the way down to minimum duty cycle at which point the cycle-by-cycle current limit of 38055fa 15 LTC3805-5 APPLICATIONS INFORMATION the ISENSE pin is lost and switch current goes up proportionally to the output current. Figures 6 and 7 show two ways to do this. Figure 6 is for relatively low currents with relatively large values of RSENSE. Using this circuit the overcurrent trip threshold is increased from 100mV to: VOC = R SENSE1 + R SENSE2 100mV R SENSE1 where it is assumed that the values of RSENSE1 and RSENSE2 are so small that the IOC = 10μA threshold adjustment current produces a negligible change in VOC. For larger currents, values of the current sense resistors must be very small and the circuit of Figure 6 becomes impractical. The circuit of Figure 7 can be substituted and the current sense threshold is increased from 100mV to: R1+ R2 VOC = 100mV R1 where the values of R1 and R2 should be kept below 10Ω to prevent the IOC = 10μA threshold adjustment current from producing a shift in VOC. External Soft-Start Fault Timeout The external soft-start is programmed by a capacitor CSS from the SSFLT pin to GND. At the initiation of soft-start the voltage on the SSFLT pin is quickly charged to 0.7V at which point GATE begins switching. From that point, ISENSE OC 2 . 25 − 0 . 7 V 6μ A After soft-start is complete, the voltage on the SSFLT pin continues to charge to about a final value of 4.75V. Note that choosing a value of CSS less than 5.8nF has no effect since it would attempt to program an external soft-start time tSS(EXT) less than the mandatory minimum internal soft-start time tSS(IN) = 1.8ms. t SS(EXT) = C SS If there is an overcurrent fault detected on the OC pin, the LTC3805-5 enters a shutdown mode while a 2μA current discharges the voltage on the SSFLT pin from 4.75V to about 0.7V. The fault timeout tFTO(EXT) is therefore t FTO(EXT ) = C SS At this point, the LTC3805-5 attempts a restart. In the event of a persistent fault, such as a short-circuit on the converter output, the converter enters a “hiccup” mode where it continues to try and restart at repetition rate determined by CSS. If the fault is eventually removed the converter successfully restarts. RSLOPE LTC3805-5 ISLOPE IOC = 10μA ISENSE RSENSE2 OC 38055 F06 Figure 6. Circuit to Increase the Overcurrent Threshold for Small Switch Currents RSLOPE ISLOPE IOC = 10μA R2 R1 RSENSE1 GND 4 . 75V − 0 . 7 V 2μ A GATE GATE LTC3805-5 a 6μA current charges the voltage on the SSFLT pin until the voltage reaches about 2.25V at which point soft-start is over and the converter enters closed-loop regulation. The soft-start time tSS(EXT) as a function of the soft-start capacitor CSS is therefore GND RSENSE 38055 F07 Figure 7. Circuit to Increase the Overcurrent Threshold for Large Switch Currents 38055fa 16 LTC3805-5 TYPICAL APPLICATIONS 4.56μH BH510-1009 CIN 10μF 50V • BAS516 57.6k MMBT6428LT1 301W T1 10μF 50V • PDZ6.8B 0.1μF 10k 221k R4 7.15k 69.8k 80.6k D1 PDS760 COUT 47μF 16V 4.7μF 1 6.8nF 10 GATE SSFLT LTC3805-5 2 9 VCC ITH 3 8 OC FB 4 7 ISENSE RUN 5 6 SYNC FS GND 11 VOUT 12V 2A Q1 HAT2266 RSENSE 0.005Ω 3.01k R3 100k 38055 F08 Figure 8. 5.5V-40V to 12V/2A SEPIC Converter Efficiency and Power Loss vs Load Current 3.5 100 90 80 3.0 EFFICIENCY 70 2.5 60 2.0 50 40 POWER LOSS 30 5.5V 10V 20V 30V 40V 20 10 0 0.01 1.5 0.1 1 LOAD CURRENT (A) 1.0 POWER LOSS (W) EFFICIENCY (%) VIN 5.5V T0 40V 0.5 10 0 38055 TA04b 38055fa 17 LTC3805-5 TYPICAL APPLICATIONS VIN+ 18V TO 72V 2.2μF 100V 2.2μF 100V 221k PA1277NL 4 5 6 7 3 8 2 221k MMBTA42 PDZ6.8B 6.8V D2 BAS516 VIN– 150pF 200V VOUT+ 3.3V 3A 100μF 6.3V D1 PDS1040 RBIAS 68Ω 402Ω 1 100μF 6.3V VOUT– BAS516 10W FDC2512 0.04Ω VCC BAT760 100μF 6.3V BAS516 BA760 274Ω 100k 47pF 6.8k 0.022μF VIN 1 2 3 4 5 221k 15.8k 75k U1 LTC3805-5 GATE SSFLT ITH VCC FB OC ISENSE RUN FS GND SYNC 11 22.1k PS2801-1-K VCC 4.7μF 10 9 8 7 6 U2 LT4430 1μF 0.47μF 2200pF 250VAC 3.01k 1 VIN 2 GND 3 OC 6 OPTO 5 COMP 4 FB 2.2nF 56k 38055 F09 Figure 9. Isolated Telecom Supply: 18V-72V Input to 3.3V/3A Output Efficiency and Power Loss vs Load Current and VIN 100 90 3.5 3.0 70 60 2.5 EFFICIENCY 2.0 50 1.5 40 1.0 30 20 0.01 0.5 POWER LOSS 0.1 1 LOAD CURRENT (A) POWER LOSS (W) EFFICIENCY (%) 80 4.0 18V 36V 48V 60V 72V 10 0 38055 TA03b 38055fa 18 LTC3805-5 PACKAGE DESCRIPTION DD Package 10-Lead Plastic DFN (3mm × 3mm) (Reference LTC DWG # 05-08-1699) R = 0.115 TYP 0.38 p 0.10 6 10 5 1 0.675 p0.05 3.50 p0.05 1.65 p0.05 2.15 p0.05 (2 SIDES) 3.00 p0.10 (4 SIDES) 1.65 p0.10 (2 SIDES) PIN 1 PACKAGE TOP MARK OUTLINE (SEE NOTE 6) 0.25 p 0.05 0.75 p0.05 0.200 REF 0.50 BSC 2.38 p0.05 (2 SIDES) 0.00 – 0.05 (DD) DFN 1103 0.25 p 0.05 0.50 BSC 2.38 p0.10 (2 SIDES) BOTTOM VIEW—EXPOSED PAD RECOMMENDED SOLDER PAD PITCH AND DIMENSIONS NOTE: 1. DRAWING TO BE MADE A JEDEC PACKAGE OUTLINE M0-229 VARIATION OF (WEED-2). CHECK THE LTC WEBSITE DATA SHEET FOR CURRENT STATUS OF VARIATION ASSIGNMENT 2. DRAWING NOT TO SCALE 3. ALL DIMENSIONS ARE IN MILLIMETERS 4. DIMENSIONS OF EXPOSED PAD ON BOTTOM OF PACKAGE DO NOT INCLUDE MOLD FLASH. MOLD FLASH, IF PRESENT, SHALL NOT EXCEED 0.15mm ON ANY SIDE 5. EXPOSED PAD SHALL BE SOLDER PLATED 6. SHADED AREA IS ONLY A REFERENCE FOR PIN 1 LOCATION ON THE TOP AND BOTTOM OF PACKAGE MSE Package 10-Lead Plastic MSOP (Reference LTC DWG # 05-08-1664 Rev B) BOTTOM VIEW OF EXPOSED PAD OPTION 2.794 p 0.102 (.110 p .004) 5.23 (.206) MIN 0.889 p 0.127 (.035 p .005) 1 2.06 p 0.102 (.081 p .004) 1.83 p 0.102 (.072 p .004) 2.083 p 0.102 3.20 – 3.45 (.082 p .004) (.126 – .136) 10 0.50 0.305 p 0.038 (.0197) (.0120 p .0015) BSC TYP RECOMMENDED SOLDER PAD LAYOUT 3.00 p 0.102 (.118 p .004) (NOTE 3) 10 9 8 7 6 3.00 p 0.102 (.118 p .004) (NOTE 4) 4.90 p 0.152 (.193 p .006) 0.254 (.010) DETAIL “A” 0o – 6o TYP 1 2 3 4 5 GAUGE PLANE 0.53 p 0.152 (.021 p .006) DETAIL “A” 0.18 (.007) 0.497 p 0.076 (.0196 p .003) REF SEATING PLANE 0.86 (.034) REF 1.10 (.043) MAX 0.17 – 0.27 (.007 – .011) TYP 0.50 (.0197) BSC 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 0.1016 p 0.0508 (.004 p .002) MSOP (MSE) 0307 REV B? 38055fa 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. 19 LTC3805-5 TYPICAL APPLICATION 5V-40V to 12V/1A Nonisolated Flyback Converter 2.2μF 100V 2.2μF 100V 10k MMBTA42 6.8V PDZ6.8B D2 BAS516 D1 UPS840 680Ω CSS 0.1μF 20k 1 2 3 4 5 VCC 75k U1 LTC3805-5 GATE SSFLT ITH VCC FB OC ISENSE RUN FS GND SYNC 11 220Ω 47pF 51.1k 90 8 80 7 70 6 60 5 50 4 3 40 5.5V 10V 20V 30V 40V 30 RSENSE 0.005Ω 4.7μF 10 9 8 7 6 100μF 16V FDC2512 VCC 470pF 100μF 16V Efficiency and Power Loss vs Load Current 3.65k 20 10 0.01 0.1 1 LOAD CURRENT (A) POWER LOSS (W) 150pF 200V 100pF VOUT 12V 1A 4 T3772 7 8 10 9 1 EFFICIENCY (%) VIN 5V TO 40V 2 1 10 0 38055 TA02b 3.01k 38055 TA02 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 LT1725 General Purpose Isolated Flyback Controller Suitable for Telecom 36V to 75V Inputs TM LTC1871 Wide Input Range, No RSENSE Current Mode Boost, Flyback and SEPIC Controller Programmable Frequency from 50kHz to 1MHz in MSOP-10 Package. LT1950 Single Switch PWM Controller with Auxiliary Boost Converter Wide Input Range Forward, Flyback, Boost or SEPIC Controller Suitable for 36V to 72V Inputs LT1952/LT1952-1 Single Switch Synchronous Forward Controllers Ideal for High Power 48V Input Applications LTC3706/LTC3705 Isolated Synchronous Forward Converter Chip Set with PolyPhase® Capability Ideal for High Power 48V Input Applications LTC3726/LTC3725 Isolated Synchronous Forward Converter Chip Set Ideal for High Power 48V Input Applications LTC3803 LTC3803-3 LTC3803-5 Constant-Frequency Current Mode Flyback DC/DC Controllers in ThinSOT Wide Input Range Flyback, Boost and SEPIC Controller. High Temperature Grade Available LTC3805 Adjustable Frequency Current Mode DC/DC Controller Wide Input Range Flyback, Boost and SEPIC Controller with Programmable Frequency, Run and Soft-Start LTC3806 Synchronous Flyback DC/DC Controller Current Mode Flyback Controller with Synchronous Gate Drive LT3825 Isolated No-OPTO Synchronous Flyback Controller with Wide Input Supply Range Input Voltage Limited Only by External Components, Ideal for 48V Input Applications LT3837 Isolated No-OPTO Synchronous Flyback Controller Suitable for Industrial 9V to 36V Inputs LTC3873 LTC3873-5 No RSENSE Constant Frequency Current Mode Boost, Flyback and SEPIC DC/DC Controllers Programmable Soft-Start, Adjustable Current Limit, 2mm × 3mm DFN or 8-Lead TSOT-23 Packages PolyPhase is a registrated treademark of Linear Technology Corporation. No RSENSE is a Trademark of Linear Technology Corporation. 38055fa 20 Linear Technology Corporation LT 0408 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 2008