Datasheet LT3781 (Analog Devices) - 10

LT3781
U U W U APPLICATIO S I FOR ATIO Overview
During normal operation, the LT3781 internal oscillator runs at twice the switching frequency. The oscillator The LT3781 is a high voltage, high current synchronous output toggles a T flip-flop, generating a 50% duty cycle regulator controller, optimized for use with dual transistor pulse that is used internally as the system clock for the IC. forward topologies. The IC uses a constant frequency, When the output of this flip-flop transitions high, the current mode architecture, with internal logic that pre- primary switches are enabled. The primary side switches vents operation over 50% duty cycle. A unique synchroni- stay enabled until the transformer primary current, sensed zation scheme allows the system clock to be synchronized via the SENSE pin connected to a ground-referenced up to an operational frequency of 350kHz, along with resistor in series with the bottom side switch FET, is phase control for easy integration of multicontroller sys- sufficient to trip the current sense comparator and, in turn, tems. A local precision 5V supply rail is available for reset the RS latch. When the RS latch resets, the primary external support circuitry and can be loaded up to 20mA. switches are disabled and the synchronous switch is Internal fault detection circuitry disables switching when enabled. The adaptive blanking circuit senses the bottom a variety of system faults are detected such as: input side gate voltage and prevents current sensing until the supply overvoltage or undervoltage faults, excessive sys- FET is fully enabled, preventing false triggering due to a tem temperature, and local supply overcurrent conditions. turn-on transition glitch. If the current comparator thresh- The LT3781 has a current-limit soft-start feature, which old is not obtained when the flip-flop output transitions gradually increases the current drive capability of a con- low, the RS latch is bypassed and the primary switches are verter system to yield a smooth start-up with minimal disabled until the next flip-flop output transition, forcing a overshoot. The soft-start circuitry is also used for smooth maximum switch duty cycle less than 50%. recoveries from system fault conditions.
System Fault Detection-The General Fault Condition
External FET switches are employed for the switch ele-
(GFC)
ments, and hearty switch drivers allow implementation of high current designs. An adaptive blanking scheme built The LT3781 contains circuitry for detecting internal and into the LT3781 allows for correct current-sense blanking system faults. Detection of a fault triggers a “general fault regardless of switch size. The LT3781 employs a voltage condition”, or GFC. When a GFC is detected, the LT3781 output error amplifier, providing superior integrator lin- disables switching and discharges the soft-start capaci- earity and allowing easy high bandwidth integration of tor. When the GFC subsides, the LT3781 initiates a start- optocoupler feedback for fully isolated solutions. up cycle via the soft-start circuitry to assure a graceful recovery. Recovery from a GFC is gated by the soft-start
Theory of Operation (See Block Diagram)
capacitor discharge. The capacitor must be discharged to The LT3781 senses the output voltage of its associated a threshold of 225mV before the GFC can be concluded. As converter via the V the zero output current threshold of the SS pin is typically FB pin. The difference between the voltage on this pin and an internal 1.25V reference is a transistor VBE, or 0.7V, latching the GFC until a 225mV amplified to generate an error voltage on the V threshold is achieved assures a zero output current state C pin, which is used as a threshold for the current sense comparator. in the event of a short-duration fault. A GFC is also The current sense comparator gets its information from triggered during system state change event, such as the SENSE pin, which monitors the voltage drop across an entering shutdown mode, to prevent any mode transition external current sense resistor. When the detected switch abnormalities. current increases to the level corresponding to the error voltage on the VC pin, the switches are disabled until the next switch cycle. 3781f 10