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| 型号： | EVAL-ADM2491EEBZ |
| 厂商： | Analog Devices Inc |
| 文件页数： | 5/16页 |
| 文件大小： | 0K |
| 描述： | BOARD EVAL FOR ADM2491 |
| 标准包装： | 1 |
| 主要目的： | 接口，收发器，RS-485 |
| 已用 IC / 零件： | ADM2491 |
| 已供物品： | 板 |
ADM2491E
Rev. B | Page 13 of 16
THERMAL SHUTDOWN
The ADM2491E 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.
FAIL-SAFE RECEIVER INPUTS
The receiver inputs include a fail-safe feature that guarantees a
logic high on the RxD pin when the A and B inputs are floating
or open circuited.
MAGNETIC FIELD IMMUNITY
Because iCoupler devices use a coreless technology, no magnetic
components are present and the problem of magnetic saturation
of the core material does not exist. Therefore, iCoupler devices
have essentially infinite dc field immunity. The following analysis
defines the conditions under which this may occur. The 3 V
operating condition of the ADM2491E is examined because it
represents the most susceptible mode of operation.
The limitation on the ac magnetic field immunity of the iCoupler
is set by the condition that induced an error voltage in the
receiving coil (the bottom coil in this case) that was large to
either falsely set or reset the decoder. The voltage induced
across the bottom coil is given by
∑ π
=
2
n
r
dt
dβ
V
;
N
n
,
..
.
,
2
,1
=
where (if the pulses at the transformer output are greater than
1.0 V in amplitude):
β is the magnetic flux density (gauss).
N is the number of turns in the receiving coil.
rn is the radius of the nth turn in the 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 26.
MAGNETIC FIELD FREQUENCY (Hz)
1k
10k
100k
100M
1M
10M
100
10
1
0.1
0.01
0.001
MA
XI
MU
M
A
LLO
W
ABL
E
M
AG
NE
T
IC
F
L
UX
DE
NS
IT
Y
(
kG
AUS
S
)
06985-
026
Figure 26. 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 27 shows the magnetic flux density values in terms of
more familiar quantities, such as maximum allowable current
flow, at given distances away from the ADM2491E transformers.
MAGNETIC FIELD FREQUENCY (Hz)
1k
10k
100k
100M
1M
10M
DISTANCE = 1m
DISTANCE = 100mm
DISTANCE = 5mm
1000
100
0.1
1
10
0.01
MA
XI
MU
M
A
LLO
W
ABL
E
CURRE
NT
(
kA)
06985-
027
Figure 27. Maximum Allowable Current for
Various Current-to-ADM2491E Spacings
With combinations of strong magnetic field and high frequency,
any loops formed by printed circuit board traces can induce
error voltages large enough 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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