ADM2486BRWZ 查看數據表（PDF） - Analog Devices

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ADM2486BRWZ

High Speed, Half-Duplex iCoupler Isolated RS-485 Transceiver

Analog Devices 

THERMAL SHUTDOWN

The ADM2486 contains thermal shutdown circuitry that

protects the part 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 re-enabled

at a temperature of 140°C.

RECEIVER FAIL-SAFE INPUTS

The receiver input includes a fail-safe feature that guarantees a

logic high RxD output when the A and B inputs are floating or

open-circuited.

MAGNETIC FIELD IMMUNITY

Because iCouplers use a coreless technology, no magnetic

components are present, and the problem of magnetic

saturation of the core material does not exist. Therefore,

iCouplers have essentially infinite dc field immunity. The

analysis below defines the conditions under which this may

occur. The ADM2486’s 3 V operating condition is examined

because it represents the most susceptible mode of operation.

The limitation on the iCoupler’s ac magnetic field immunity is

set by the condition in which the induced error voltage in the

receiving coil (the bottom coil in this case) is made sufficiently

large, either to falsely set or reset the decoder. The voltage

induced across the bottom coil is given by

V

=

⎜⎛

⎝

−dβ

dt

⎟⎞∑

⎠

πrn2

;

n

= 1, 2, . . . ,

N

where, if the pulses at the transformer output are greater than

1.0 V in amplitude:

β = magnetic flux density (gauss)

N = number of turns in receiving coil

rn = radius of nth turn in receiving coil (cm)

The decoder has a sensing threshold of about 0.5 V; therefore,

there is a 0.5 V margin in which induced voltages can be

tolerated.

Given the geometry of the receiving coil and an imposed

requirement that the induced voltage is, at most, 50% of the

0.5 V margin at the decoder, a maximum allowable magnetic

field is calculated, as shown in Figure 27.

ADM2486

100.000

10.000

1.000

0.100

0.010

0.001

1k

10k

100k

1M

10M

MAGNETIC FIELD FREQUENCY (Hz)

100M

Figure 27. Maximum Allowable External Magnetic Flux Density

For example, at a magnetic field frequency of 1 MHz, the

maximum allowable magnetic field of 0.2 kGauss induces a

voltage of 0.25 V at the receiving coil. This is about 50% of the

sensing threshold and does not cause a faulty output transition.

Similarly, if such an event occurs during a transmitted pulse and

is the worst-case polarity, it reduces the received pulse from

>1.0 V to 0.75 V. This is well above the 0.5 V sensing threshold

of the decoder.

Figure 28 shows the magnetic flux density values in terms of

more familiar quantities such as maximum allowable current

flow at given distances away from the ADM2486 transformers.

1000.00

100.00

DISTANCE = 1m

DISTANCE = 5mm

10.00

DISTANCE = 100mm

1.00

0.10

0.01

1k

10k

100k

1M

10M

MAGNETIC FIELD FREQUENCY (Hz)

100M

Figure 28. Maximum Allowable Current for

Various Current-to-ADM2486 Spacings

At combinations of strong magnetic field and high frequency,

any loops formed by printed circuit board traces could induce

sufficiently large error voltages to trigger the thresholds of

succeeding circuitry. Care should be taken in the layout of

such traces to avoid this possibility.

Rev. C | Page 15 of 20

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