Datasheet ADP1074 (Analog Devices) - 16

ADP1074 Data Sheet THEORY OF OPERATION
The ADP1074 is a current mode, fixed frequency, active clamp, The PWM controls are performed on the primary side by synchronous forward control er designed for isolated dc to dc sensing the input peak current cycle by cycle with a sense resistor power supplies. Analog Devices proprietary iCouplers are at the source of the main switching MOSFET. The output of the integrated in the ADP1074 to eliminate the bulky signal trans- converter is sensed by the secondary circuitry, which sends the formers and optocouplers that transmit signals over the isolation feedback and PWM signals to the primary side via the 5 kV boundary. Integrating the iCouplers reduces system design integrated isolators for a complete control loop solution. complexity, cost, and component count and improves overal The primary circuitry in the ADP1074 includes an 8 V LDO, system reliability. With the integrated isolators and MOSFET input current sensing, bias circuit, and MOSFET drivers drivers on both the primary and the secondary side, the ADP1074 including an active clamp reset driver, slope compensation, offers a compact system level design and yields a higher efficiency external frequency synchronization, PWM generator, and a than a nonsynchronous forward converter at heavy loads. programmable maximum duty cycle setting. The primary side Traditionally in a forward or flyback converter, a discrete opto- also has pins for differential sensing of the current sense signal. coupler is used in the feedback path to transmit the signal from The secondary circuitry includes the feedback compensation, a the secondary to the primary side, and an external transformer 5 V LDO regulator, an internal reference, two MOSFET drivers for is used for transmitting the PWM signal from the primary to synchronous rectification, and a dedicated pin for overvoltage the secondary side for synchronous rectification. However, the protection. Additionally, the secondary side features differential current transfer ratio (CTR) of the optocouplers degrades over output voltage sensing and power good pins, and a program- time and over temperature and so the optocoupler must be mable light load mode setting. replaced every five to ten years, depending on the manufacturing quality and optocoupler grade that determines the initial CTR. The The integrated iCouplers carry out the communications ADP1074 eliminates the use of optocouplers and signal between the primary and secondary sides by transmitting the transformers, thus reducing system cost, PCB area, and feedback signal and the PWMs over the isolation barrier. complexity while improving system reliability, without the The feedback signal and timing of synchronous rectifier PWMs issue of CTR degradation of the optocouplers. are transmitted between the primary and the secondary sides, The ADP1074 controller offers a complete solution for an isolated or between the secondary and primary sides, through the dc to dc power supply by integrating the 5 kV isolators and the iCouplers using a proprietary transmission scheme. primary and secondary control circuitries in one package. The ADP1074 also offers features such as input current protection, UVLO, precision enable with adjustable hysteresis, OTP, LLM, and tracking. Rev. C | Page 16 of 32 Document Outline FEATURES APPLICATIONS SIMPLIFIED BLOCK DIAGRAM TABLE OF CONTENTS REVISION HISTORY GENERAL DESCRIPTION SPECIFICATIONS INSULATION AND SAFETY RELATED SPECIFICATIONS REGULATORY INFORMATION DIN V VDE V 0884-10 (VDE V 0884-10) INSULATION CHARACTERISTICS DIN V VDE V 0884-10 (VDE V 0884-10) INSULATION CHARACTERISTICS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE ESD CAUTION PIN CONFIGURATIONS AND FUNCTION DESCRIPTIONS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION DETAILED BLOCK DIAGRAM PRIMARY SIDE SUPPLY, INPUT VOLTAGE, AND LDO SECONDARY SIDE SUPPLY AND LDO PRECISION ENABLE SOFT START PROCEDURE OUTPUT VOLTAGE SENSING AND FEEDBACK LOOP COMPENSATION AND STEADY STATE OPERATION SLOPE COMPENSATION INPUT/OUTPUT CURRENT-LIMIT PROTECTION TEMPERATURE SENSING FREQUENCY SETTING (RT PIN) MAXIMUM DUTY CYCLE FREQUENCY SYNCHRONIZATION SYNCHRONOUS RECTIFIER (SR) DRIVERS OUTPUT OVERVOLTAGE PROTECTION (OVP) ACTIVE CLAMP (PGATE) LEADING EDGE BLANKING GATE DELAY AND SR DEAD TIME LIGHT LOAD MODE (LLM) AND SR PHASE IN EXTERNAL START-UP CIRCUIT SOFT STOP POWER GOOD OCP/FEEDBACK RECOVERY OUTPUT VOLTAGE TRACKING REMOTE SYSTEM RESET OCP COUNTER INSULATION LIFETIME LAYOUT GUIDELINES TYPICAL APPLICATION CIRCUITS OUTLINE DIMENSIONS ORDERING GUIDE NOTES