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| 型号： | EVAL-ADM2487EEB5Z |
| 厂商： | Analog Devices Inc |
| 文件页数： | 7/20页 |
| 文件大小： | 0K |
| 描述： | BOARD EVAL FOR ADM2487 |
| 标准包装： | 1 |
| 主要目的： | 接口，收发器，RS-485 |
| 已用 IC / 零件： | ADM2487 |
| 已供物品： | 板 |
ADM2482E/ADM2487E
Rev. A | Page 15 of 20
THERMAL SHUTDOWN
The ADM2482E/ADM2487E contain thermal shutdown
circuitry that protects the parts from excessive power dissipa-
tion 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.
TRUE FAIL-SAFE RECEIVER INPUTS
The receiver inputs have a true fail-safe feature that ensures
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 true 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 thre-
shold means that when the voltage between A and B decays to
0 V, the receiver output is guaranteed to be high.
MAGNETIC FIELD IMMUNITY
The limitation on the magnetic field immunity of the iCoupler
is set by the condition in which an induced voltage in the
receiving coil of the transformer is large enough to either
falsely set or reset the decoder. The following analysis defines
the conditions under which this may occur. The 3 V operating
condition of the ADM2482E/ADM2487E is examined because
it represents the most susceptible mode of operation.
The pulses at the transformer output have an amplitude greater
than 1 V. The decoder has a sensing threshold of about 0.5 V,
thus establishing a 0.5 V margin in which induced voltages can
be tolerated.
The voltage induced across the receiving coil is given by
N
n
r
dt
dβ
V
n
,
2
,
1
;
2
K
=
π
=
∑
where:
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).
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 can be determined using Figure 31.
MAGNETIC FIELD FREQUENCY (Hz)
1k
10k
100k
100M
1M
10M
100
10
1
0.1
0.01
0.001
M
A
XI
M
U
M
A
L
O
W
A
B
L
E
MA
G
N
ET
IC
F
L
U
X
DE
NS
IT
Y
(
kG
AUS
S
)
07
37
9-
0
23
Figure 31. 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 32 shows the magnetic flux density values in terms of
more familiar quantities, such as maximum allowable current
flow at given distances away from the ADM2482E/ADM2487E
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
M
AX
IM
U
M
AL
L
O
W
A
BL
E
CURR
E
NT
(
kA)
07
37
9-
02
4
Figure 32. Maximum Allowable Current for
Various Current-to-ADM2482E/ADM2487E Spacings
With combinations of strong magnetic field and high frequency,
any loops formed by PCB 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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