LTC3803IS6-3

LTC3803-3 Constant Frequency Current Mode Flyback DC/DC Controller in ThinSOT FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ■ DESCRIPTIO VIN and VOUT Limited Only by External Components Adjustable Slope Compensation Internal Soft-Start –40°C to 125°C Operating Temperature Range Constant Frequency 300kHz Operation ±1.5% Reference Accuracy Current Mode Operation for Excellent Line and Load Transient Response No Minimum Load Requirement Low Quiescent Current: 240μA Low Profile (1mm) SOT-23 Package The LTC®3803-3 is a constant frequency current mode flyback controller optimized for driving 6V-rated N-channel MOSFETs in high input voltage applications. Constant frequency operation is maintained down to very light loads, resulting in less low frequency noise generation over a wide range of load currents. Slope compensation can be programmed with an external resistor. The LTC3803-3 provides ±1.5% output voltage accuracy and consumes only 240μA of quiescent current. Groundreferenced current sensing allows LTC3803-3-based converters to accept input supplies beyond the LTC3803-3’s absolute maximum VCC. A micropower hysteretic start-up feature allows efficient operation at high input voltages. For simplicity, the LTC3803-3 can also be powered from a high VIN through a resistor, due to its internal 9.4V shunt regulator. An internal undervoltage lockout shuts down the LTC3803-3 when the input voltage falls below 4.4V, guaranteeing at least 4.4V of gate drive to the external MOSFET. The LTC3803-3 is available in a low profile (1mm) 6-lead SOT-23 (ThinSOTTM) package. APPLICATIO S ■ ■ ■ ■ Telecom Power Supplies 42V and 12V Automotive Power Supplies Auxiliary/Housekeeping Power Supplies Power Over Ethernet Powered Devices , LT, LTC and LTM are registered trademarks of Linear Technology Corporation. ThinSOT is a trademark of Linear Technology Corporation. All other trademarks are the property of their respective owners. TYPICAL APPLICATIO VIN 36V TO 72V 5V Output Nonisolated Telecom Housekeeping Power Supply UPS840 T1 10k 1μF 10V X5R 82k 470pF 1μF 100V X5R 100 VOUT 5V 2A MAX EFFICIENCY (%) 95 90 85 80 75 70 65 60 55 • • FDC2512 300μF* 6.3V X5R VCC ITH/RUN NGATE LTC3803-3 GND VFB 20k 105k 38033 TA01 4.7k 68mΩ SENSE 150pF 200V 220Ω 50 250 T1: COOPER CTX02-15242 *THREE 100μF UNITS IN PARALLEL U Efficiency vs Load Current VOUT = 5V VIN = 36V VIN = 48V VIN = 60V VIN = 72V 500 750 1000 1250 1500 1750 2000 LOAD CURRENT (mA) 38033 TA02 U U 38033fa 1 LTC3803-3 ABSOLUTE MAXIMUM RATINGS (Note 1) PACKAGE/ORDER INFORMATION TOP VIEW ITH/RUN 1 GND 2 VFB 3 6 NGATE 5 VCC 4 SENSE VCC to GND Low Impedance Source .......................... – 0.3V to 8V Current Fed ...................................... 25mA into VCC* NGATE Voltage ......................................... – 0.3V to VCC VFB, ITH/RUN Voltages ..............................– 0.3V to 3.5V SENSE Voltage ........................................... – 0.3V to 1V NGATE Peak Output Current ( VITHSHDN V VITH/RUN < VITHSHDN ITH/RUN AND VCC > VTURNON (NOMINALLY 0.28V) (NOMINALLY 8.7V) An internal soft-start feature is enabled whenever the LTC3803-3 comes out of shutdown. Specifically, the ITH/RUN voltage is clamped and is prevented from reaching maximum until roughly 1.4ms has passed. This allows the input and output currents of LTC3803-3based power supplies to rise in a smooth and controlled manner on start-up. 38033fa LTC3803-3 OPERATIO Powering the LTC3803-3 In the simplest case, the LTC3803-3 can be powered from a high voltage supply through a resistor. A built-in shunt regulator from the VCC pin to GND will draw as much current as needed through this resistor to regulate the VCC voltage to around 9.5V as long as the VCC pin is not forced to sink more than 25mA. This shunt regulator is always active, even when the LTC3803-3 is in shutdown, since it serves the vital function of protecting the VCC pin from seeing too much voltage. For higher efficiency or for wide VIN range applications, flyback controllers are typically powered through a separate bias winding on the flyback transformer. The LTC3803-3 has a wide UVLO hysteresis (1V min) and small VCC supply current draw (