ADP1074ACCZ-R7 datasheet(16/32 Pages) AD

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ADP1074

Data Sheet

Rev. D | Page 16 of 32

THEORY OF OPERATION

The ADP1074 is a current mode, fixed frequency, active clamp,

synchronous forward controller designed for isolated dc to dc

power supplies. Analog Devices proprietary iCouplers are

integrated in the ADP1074 to eliminate the bulky signal trans-

formers and optocouplers that transmit signals over the isolation

boundary. Integrating the iCouplers reduces system design

complexity, cost, and component count and improves overall

system reliability. With the integrated isolators and MOSFET

drivers on both the primary and the secondary side, the ADP1074

offers a compact system level design and yields a higher efficiency

than a nonsynchronous forward converter at heavy loads.

Traditionally in a forward or flyback converter, a discrete opto-

coupler is used in the feedback path to transmit the signal from

the secondary to the primary side, and an external transformer

is used for transmitting the PWM signal from the primary to

the secondary side for synchronous rectification. However, the

current transfer ratio (CTR) of the optocouplers degrades over

time and over temperature and so the optocoupler must be

replaced every five to ten years, depending on the manufacturing

quality and optocoupler grade that determines the initial CTR. The

ADP1074 eliminates the use of optocouplers and signal

transformers, thus reducing system cost, PCB area, and

complexity while improving system reliability, without the

issue of CTR degradation of the optocouplers.

The ADP1074 controller offers a complete solution for an isolated

dc to dc power supply by integrating the 5 kV isolators and the

primary and secondary control circuitries in one package.

The PWM controls are performed on the primary side by

sensing the input peak current cycle by cycle with a sense resistor

at the source of the main switching MOSFET. The output of the

converter is sensed by the secondary circuitry, which sends the

feedback and PWM signals to the primary side via the 5 kV

integrated isolators for a complete control loop solution.

The primary circuitry in the ADP1074 includes an 8 V LDO,

input current sensing, bias circuit, and MOSFET drivers

including an active clamp reset driver, slope compensation,

external frequency synchronization, PWM generator, and a

programmable maximum duty cycle setting. The primary side

also has pins for differential sensing of the current sense signal.

The secondary circuitry includes the feedback compensation, a

5 V LDO regulator, an internal reference, two MOSFET drivers for

synchronous rectification, and a dedicated pin for overvoltage

protection. Additionally, the secondary side features differential

output voltage sensing and power good pins, and a program-

mable light load mode setting.

The integrated iCouplers carry out the communications

between the primary and secondary sides by transmitting the

feedback signal and the PWMs over the isolation barrier.

The feedback signal and timing of synchronous rectifier PWMs

are transmitted between the primary and the secondary sides,

or between the secondary and primary sides, through the

Couplers using a proprietary transmission scheme.

The ADP1074 also offers features such as input current protection,

UVLO, precision enable with adjustable hysteresis, OTP, LLM,

and tracking.

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