Preliminary Technical Data

ADM2485

100

THERMAL SHUTDOWN

The ADM2485 contains thermal shutdown circuitry that

10

0.001

1k

10k

100k

1M

10M

100M

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

1

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

0.1

0.01

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

MAGNETIC FIELD FREQUENCY (Hz)

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 ADM2485’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

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—still 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 ADM2485 transformers.

1000

DISTANCE = 1m

DISTANCE = 5mm

DISTANCE = 100mm

induced across the bottom coil is given by

V =

⎜

⎛ − dβ

⎟

n

2

;

n = 1, 2, . . . , N

⎝ dt ⎠

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

1.0 V in amplitude:

N = number of turns in receiving coil

100

10

1

0.1

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

0.01

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

1k

10k

100k

1M

10M

100M

MAGNETIC FIELD FREQUENCY (Hz)

tolerated.

Figure 28. Maximum Allowable Current for

Given the geometry of the receiving coil and an imposed

Various Current-to-ADM2485 Spacings

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

At combinations of strong magnetic field and high frequency,

0.5 V margin at the decoder, a maximum allowable magnetic

any loops formed by printed circuit board traces could induce

field is calculated, as shown in Figure 27.

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.

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