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參數(shù)資料
| 型號： | ADM3252EABCZ |
| 廠商： | Analog Devices Inc |
| 文件頁數(shù)： | 5/16頁 |
| 文件大?。?/td> | 0K |
| 描述： | IC LINE DRIVER/RECEIVER |
| 標準包裝： | 1 |
| 系列： | * |
Data Sheet
ADM3252E
Rev. A | Page 13 of 16
DC CORRECTNESS AND MAGNETIC FIELD
IMMUNITY
Positive and negative logic transitions at the isolator input cause
narrow (~1 ns) pulses to be sent to the decoder via the trans-
former. 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 s, 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 for more than approximately
5 s, the input side is assumed to be unpowered or nonfunctional,
in which case the isolator output is forced to a default state by
the watchdog timer circuit. This situation should occur in the
ADM3252E during power-up and power-down operations only.
The limitation on the ADM3252E magnetic field immunity is
set by the condition in which induced voltage in the receiving
coil of the transformer is sufficiently large to falsely set or reset
the decoder. The following analysis defines the conditions
under which this can occur.
The pulses at the transformer output have an amplitude of >1.0 V.
The decoder has a sensing threshold of about 0.5 V, thus estab-
lishing a 0.5 V margin in which induced voltages can be tolerated.
The voltage induced across the receiving coil is given by
V = (dβ/dt)
∑πrn2; n = 1, 2, … , N
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 internally and an
imposed requirement that the induced voltage be, at most, 50%
of the 0.5 V margin at the decoder, a maximum allowable
magnetic field is calculated, as shown in Figure 21.
Figure 21. 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 approximately
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 of the worst-case polarity), the received pulse is
reduced from >1.0 V to 0.75 V, which is still well above the
0.5 V sensing threshold of the decoder.
The preceding magnetic flux density values correspond to
specific current magnitudes at given distances from the trans-
formers. Figure 22 expresses these allowable current magnitudes
as a function of frequency for selected distances. As shown in
Figure 22, the ADM3252E is extremely immune and can be
affected only by extremely large currents operated at high
frequency very close to the component. For example, at a
magnetic field frequency of 1 MHz, a 0.5 kA current placed
5 mm away from the ADM3252E is required to affect the
operation of the component.
Figure 22. Maximum Allowable Current
for Various Current-to-ADM3252E Spacings
Note that in the presence of strong magnetic fields and high
frequencies, any loops formed by PCB traces may induce error
voltages sufficiently large to trigger the thresholds of succeeding
circuitry. Exercise care in the layout of such traces to avoid this
possibility.
POWER CONSIDERATIONS
The ADM3252E power input, data input channels on the primary
side, and data channels on the secondary side are all protected from
premature operation by undervoltage lockout (UVLO) circuitry.
Below the minimum operating voltage, the power converter holds
its oscillator inactive and all input channel drivers and refresh cir-
cuits are idle. Outputs remain in a high impedance state to prevent
transmission of undefined states during power-up and power-
down operations.
MAGNETIC FIELD FREQUENCY (Hz)
100
M
A
X
IM
U
M
A
LLOWA
B
LE
M
A
GN
E
TIC
FLU
X
DE
NS
IT
Y
(
kg
au
ss)
0.001
1M
10
0.01
1k
10k
10M
0.1
1
100M
100k
10515-
008
MAGNETIC FIELD FREQUENCY (Hz)
M
AX
IM
UM
AL
L
O
W
ABL
E
CURRE
NT
(
kA)
1K
100
10
1
0.1
0.01
1k
10k
100M
100k
1M
10M
DISTANCE = 5mm
DISTANCE = 1m
DISTANCE = 100mm
10515-
009
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