ADuM1100 Datasheet, PDF (11/16 Pages) Analog Devices – iCoupler Digital Isolator

4
3
5V INPUT SIGNAL
2
1
3.3V INPUT SIGNAL
0
1
2
3
4
5
6
7
8
9 10
INPUT RISE TIME (10%–90%, ns)
Figure 5. Typical Propagation Delay Change due to
Input Rise Time Variation (for V_DD1 = 3.3 V and 5 V)
0
–1
5V INPUT SIGNAL
–2
3.3V INPUT SIGNAL
–3
–4
1
2
3
4
5
6
7
8
9 10
INPUT RISE TIME (10%–90%, ns)
Figure 6. Typical Propagation Delay Change due to
Input Fall Time Variation (for V_DD1 = 3.3 V and 5 V)
The impact of the slower input edge rates can also affect the
measured pulse width distortion as based on the input 50% level.
This impact may either increase or decrease the apparent pulse
width distortion depending on the relative magnitudes of t_PHL,
t_PLH, and PWD. The case of interest here is the condition
that leads to the largest increase in pulse width distortion. The
change in this case is given by
∆_PWD = PWD′ – PWD = ∆_LH – ∆_HL =
( )_{( )} ( ) ^{t /0.8V}₁ ^{V –V}_{ITH(L–H)} ^–V_{ITH(H–L)} ^{, for t = t}_r ^{= t} _f
where:
PWD = t_PLH – t_PHL
PWD′ = t ′_PLH – t ′_PHL
This adjustment in pulse width distortion is plotted as a func-
tion of input rise/fall time in Figure 7.
ADuM1100
6
5
4
5V INPUT SIGNAL
3
3.3V INPUT SIGNAL
2
1
0
1
2
3
4
5
6
7
8
9 10
INPUT RISE/FALL TIME (10%–90%, ns)
Figure 7. Typical Pulse Width Distortion Adjustment due
to Input Rise/Fall Time Variation (at V_DD1 = 3.3 V and 5 V)
Method of Operation, DC Correctness, and
Magnetic Field Immunity
Referring to the functional block diagram, the two coils act as a
pulse transformer. Positive and negative logic transitions at the
isolator input cause narrow (2 ns) pulses to be sent via the trans-
former to the decoder. The decoder is bistable and therefore
either set or reset by the pulses indicating input logic transitions.
In the absence of logic transitions at the input for more than 2 µs,
a periodic update pulse of the appropriate polarity is sent to ensure
dc correctness at the output. If the decoder receives none of
these update pulses for more than about 5 µs, the input side is
assumed to be unpowered or nonfunctional, in which case the
isolator output is forced to a logic high state by the watchdog
timer circuit.
The limitation on the ADuM1100’s magnetic field immunity is set
by the condition in which induced voltage in the transformer’s
receiving coil is sufficiently large to either falsely set or reset the
decoder. The analysis that follows defines the conditions under
which this may occur. The ADuM1100’s 3.3 V operating condi-
tion is examined because it represents the most susceptible mode
of operation.
The pulses at the transformer output are greater than 1.0 V in
amplitude. The decoder has sensing thresholds at about 0.5 V,
therefore establishing a 0.5 V margin in which induced voltages can
be tolerated. The induced voltage induced across the receiving
coil is given by
V = ₍– dβ /dt₎ Σπ r_n²; n = 1, 2, . . . . , N
where:
β = magnetic flux density (Gauss).
N = number of turns in receiving coil.
r_n = radius of nth turn in receiving coil (cm).
REV. E
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