ADM2682EBRIZ datasheet(16/24 Pages) AD

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ADM2682E/ADM2687E

Rev. 0 | Page 16 of 24

CIRCUIT DESCRIPTION

SIGNAL ISOLATION

The ADM2682E/ADM2687E signal isolation of 5 kV rms is

implemented on the logic side of the interface. The part achieves

signal isolation by having a digital isolation section and a trans-

ceiver section (see Figure 1). Data applied to the TxD and DE

pins and referenced to logic ground (GND1) are coupled across

an isolation barrier to appear at the transceiver section referenced

to isolated ground (GND2). Similarly, the single-ended receiver

output signal, referenced to isolated ground in the transceiver

section, is coupled across the isolation barrier to appear at the

RxD pin referenced to logic ground.

POWER ISOLATION

The ADM2682E/ADM2687E power isolation of 5 kV rms is

implemented using an isoPower integrated isolated dc-to-dc

converter. The dc-to-dc converter section of the ADM2682E/

ADM2687E works on principles that are common to most

modern power supplies. It is a secondary side controller

architecture with isolated pulse-width modulation (PWM)

feedback. VCC power is supplied to an oscillating circuit that

switches current into a chip-scale air core transformer. Power

transferred to the secondary side is rectified and regulated to

3.3 V. The secondary (VISO) side controller regulates the output

by creating a PWM control signal that is sent to the primary

(VCC) side by a dedicated iCoupler (5 kV rms signal isolated)

data channel. The PWM modulates the oscillator circuit to

control the power being sent to the secondary side. Feedback

allows for significantly higher power and efficiency.

TRUTH TABLES

The truth tables in this section use the abbreviations found in

Table 11.

Table 11. Truth Table Abbreviations

Letter

Description

High level

Low level

Don’t care

Indeterminate

High impedance (off )

Disconnected

Table 12. Transmitting (see Table 11 for Abbreviations)

Inputs

Outputs

TxD

Table 13. Receiving (see Table 11 for Abbreviations)

Inputs

Output

A − B

RxD

≥ −0.03 V

L or NC

≤ −0.2 V

L or NC

−0.2 V < A − B < −0.03 V

L or NC

Inputs open

L or NC

THERMAL SHUTDOWN

The ADM2682E/ADM2687E contain thermal shutdown circuitry

that protects the parts from excessive power dissipation during

fault conditions. Shorting the driver outputs to a low impedance

source can result in high driver currents. The thermal sensing

circuitry detects the increase in die temperature under this

condition and disables the driver outputs. This circuitry is

designed to disable the driver outputs when a die temperature

of 150°C is reached. As the device cools, the drivers are reenabled

at a temperature of 140°C.

OPEN- AND SHORT-CIRCUIT, FAIL-SAFE RECEIVER

INPUTS

The receiver inputs have open- and short-circuit, fail-safe features

that ensure that the receiver output is high when the inputs are

open or shorted. During line-idle conditions, when no driver on

the bus is enabled, the voltage across a terminating resistance at

the receiver input decays to 0 V. With traditional transceivers,

receiver input thresholds specified between −200 mV and

+200 mV mean that external bias resistors are required on the

A and B pins to ensure that the receiver outputs are in a known

state. The short-circuit, fail-safe receiver input feature eliminates

the need for bias resistors by specifying the receiver input

threshold between −30 mV and −200 mV. The guaranteed negative

threshold means that when the voltage between A and B decays

to 0 V, the receiver output is guaranteed to be high.

DC CORRECTNESS AND MAGNETIC FIELD IMMUNITY

The digital signals transmit across the isolation barrier using

Coupler technology. This technique uses chip-scale transformer

windings to couple the digital signals magnetically from one

side of the barrier to the other. Digital inputs are encoded into

waveforms that are capable of exciting the primary transformer

winding. At the secondary winding, the induced waveforms are

decoded into the binary value that was originally transmitted.

Positive and negative logic transitions at the isolator input cause

narrow (~1 ns) pulses to be sent to the decoder via the transformer.

The decoder is bistable and is, therefore, either set or reset by

the pulses, indicating input logic transitions. In the absence of

logic transitions at the input for more than 1 μs, periodic sets of

refresh pulses indicative of the correct input state are sent to

ensure dc correctness at the output. If the decoder receives no

internal pulses of more than approximately 5 μs, the input side

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