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LT1681E

Dual Transistor Synchronous Forward Controller

Linear Technology 

LT1681

APPLICATIO S I FOR ATIO

Overview

The LT1681 is a high voltage, high current synchronous

regulator controller, optimized for use with dual transistor

forward topologies. The IC uses a constant frequency,

current mode architecture with internal logic that prevents

operation over 50% duty cycle. A unique synchronization

scheme allows the system clock to be synchronized up to

an operational frequency of 350kHz, along with phase

control for easy integration of multicontroller systems. A

local precision 5V supply is available for external support

circuitry and can be loaded up to 20mA.

Internal fault detection circuitry disables switching when

a variety of system faults are detected such as: input

supply overvoltage or undervoltage faults, excessive sys-

tem temperature, transformer primary-side saturation and

local supply overcurrent conditions. The LT1681 has a

current limit soft-start feature that gradually increases the

current drive capability of a converter system to yield a

smooth start-up with minimal overshoot. The soft-start

circuitry is also used for smooth recoveries from system

fault conditions.

External FET switches are employed for the switch ele-

ments, and hearty switch drivers allow implementation of

high current designs. An adaptive blanking scheme built

into the LT1681 allows for correct current-sense blanking

regardless of switch size and even while using external

switch drive buffers. The LT1681 employs a voltage output

error amplifier, providing superior integrator linearity and

allowing easy high bandwidth integration of optocoupler

feedback for fully isolated solutions.

Theory of Operation (See Block Diagram)

The LT1681 senses the output voltage of its associated

converter via the VFB pin. The difference between the

voltage on this pin and an internal 1.25V reference is

amplified to generate an error voltage on the VC pin, which

is used as a threshold for the current sense comparator.

The current sense comparator gets its information from

the SENSE pin, which monitors the voltage drop across an

external current sense resistor. When the detected switch

current increases to the level corresponding to the error

voltage on the VC pin, the switches are disabled until the

next switch cycle.

During normal operation, the LT1681 internal oscillator

runs at twice the switching frequency. The oscillator

output toggles a T flip-flop, generating a 50% duty cycle

pulse that is used internally as the system clock for the IC.

When the output of this flip-flop transitions high, the

primary switches are enabled. The primary-side switches

stay enabled until the transformer primary current, sensed

via the SENSE pin, connected to a ground-referenced

resistor in series with the bottom-side switch FET, is

sufficient to trip the current sense comparator and, in turn,

reset the RS latch. When the RS latch resets, the primary

switches are disabled and the synchronous switch is

enabled. The adaptive blanking circuit senses the bottom-

side gate voltage via the BLKSENS pin and prevents

current sensing until the FET is fully enabled, preventing

false triggering due to a turn-on transition glitch. If the

current comparator threshold is not obtained when the

flip-flop output transitions low, the RS latch is bypassed

and the primary switches are disabled until the next flip-

flop output transition, forcing a maximum switch duty

cycle less than 50%.

System Fault Detection—The General Fault Condition

(GFC)

The LT1681 contains circuitry for detecting internal and

system faults. Detection of a fault triggers a “general fault

condition” or GFC. When a GFC is detected, the LT1681

disables switching and discharges the soft-start capaci-

tor. When the GFC subsides, the LT1681 initiates a start-

up cycle via the soft-start circuitry to assure a graceful

recovery. Recovery from a GFC is gated by the soft-start

capacitor discharge. The capacitor must be discharged to

a threshold of 225mV before the GFC can be concluded. As

the zero output current threshold of the SS pin is typically

a transistor VBE, or 0.7V, latching the GFC until a 225mV

threshold is achieved assures a zero output current state

is obtained in the event of a short-duration fault. A GFC is

also triggered during a system state change event, such as

entering shutdown mode, to prevent any mode transition

abnormalities.

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