ADP1071-1ACCZ-R7 datasheet(14/27 Pages) AD

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ADP1071-1/ADP1071-2

Data Sheet

Rev. B | Page 14 of 27

THEORY OF OPERATION

The ADP1071-1/ADP1071-2 are PWM, current mode, fixed

frequency, synchronous flyback controllers designed for isolated

dc-to-dc power supplies. Analog Devices iCouplers are integrated

in the ADP1071-1/ADP1071-2 to eliminate the bulky signal

transformers 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

ADP1071-1/ADP1071-2 offer a compact system level design

and yield a higher efficiency than a diode rectified flyback

converter at heavy loads.

Traditionally in a forward or flyback converter, a discrete

optocoupler is used in the feedback path to transmit the signal

from the secondary to the primary side, and an external trans-

former 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 5 to 10 years, depending on the manu-

facturing quality and optocoupler grade that determines the

initial CTR. The ADP1071-1/ADP1071-2 eliminate the use of

an optocoupler and signal transformer, thus reducing system

cost, PCB area, and complexity, while improving system reliability

without the issue of CTR degradation of the optocouplers.

The ADP1071-1/ADP1071-2 controllers offer 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 ADP1071-1/ADP1071-2 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 a programmable LLM 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 ADP1071-1/ADP1071-2 also offer features such as input

current protection, OVP, UVLO, precision enable with adjustable

hysteresis, OTP, LLM, and tracking.

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