High Speed Digital Isolators Using Microscale On-Chip Transformers

High Speed Digital Isolators Using
Microscale On-Chip Transformers
By Baoxing Chen, John Wynne, and Ronn Kliger
This is the English version of an article
that appeared in the July 22, 2003, issue
of Elektronik magazine. Visit the
magazine site at www.elektroniknet.de.
2
Introduction
In many industrial applications, such as process control systems or data acquisition and control systems, digital signals must be
transmitted from various sensors to a central controller for processing and analysis. The controller then needs to transmit commands as a
result of the analysis performed, coupled with user inputs to various actuators, to achieve certain operations. To maintain safety voltage at
the user interface and to prevent transients from being transmitted from the sources, galvanic isolation is required. There are three
commonly known classes of isolation devices: optocouplers, capacitively coupled isolators, and transformer based isolators.
Optocouplers rely on light emitting diodes to convert the electrical signals to light signals and on photo detectors to convert the light signals
back to electrical signals. The intrinsic low conversion efficiencies for electrical light conversion and slow response photo detectors lead to
optocoupler limitations in terms of lifetime, speed, and power assumption. The capacitively coupled isolators have limitations in their size and
ability to reject common-mode voltage transients, while the traditional transformer assembly based isolators are bulky and expensive. All these
isolators are restricted, moreover, because of integrated circuit integration limitations and the fact that they often need hybrid packaging.
Recently iCoupler ®, a new isolation technology based on chip scale transformers, was developed by Analog Devices. The first product, the
ADuM1100 single-channel digital isolator, is in volume production. iCoupler technology leverages thick-film processing techniques to build
microscale on-chip transformers and achieves thousands of volts of isolation on a chip. iCoupler isolated transformers can be monolithically
integrated with standard silicon ICs and can be fabricated in single- or multichannel configurations. The bidirectional nature of inductive
coupling further facilitates bidirectional signal transfer. The combination of high bandwidth for these on-chip transformers and fine scale
CMOS circuitry leads to isolators of unmatched performance characteristics in power, speed, timing accuracy, and ease of use.
ADuM1100 Architecture: A Single-Channel
Digital Isolator
The ADuM1100 is a single-channel 100 Mbps
digital isolator. It has two ICs packaged in an
8-lead SOIC package. A cross-section view of the
ADuM1100 is shown in Figure 1. There are two
lead frame paddles inside the package, with gap
between them of about 0.4 mm. The molding
compound has breakdown strength over 25 kV/mm,
so the 0.4 mm gap filled with molding compound
provides greater than 10 kV insulation between the
substrates of two IC chips.
TOP COIL
BOTTOM COIL
POLYIMIDE
LAYERS
(a)
(b)
The driver chip sitting on the left paddle takes the
input digital signal, encodes it, and drives the
Figure 1a. Cross-sectional view of ADuM1100 in an 8-lead SOIC package; Figure 1b. Cross-sectional view of
encoded differential signal through bond wires to
top coil and polyimide layers
the top coils of the transformers built on top of the
receiver chip sitting on the right paddle. The driver
die is a standard CMOS chip, and the receiver die, shown in Figure 2,
is a CMOS chip with the additional structures of two polyimide layers
and transformer primary coil fabricated on top of the passivation. The
polyimide between the top and bottom coils is about 20 mm thick.
Characteristics
Some primary characteristics of this polyimide are shown in Table 1.
The breakdown strength of the cured polyimide film is greater than
Weight Loss = 5% at T = (˚C) 500
300 V/m, so 20 m of polyimide provides greater than 6 kV of
CTE (ppm)
4050
insulation between a given transformer’s coils. This provides a
Tensile Strength (Mpa)
150
comfortable margin over the production test voltage of 3 kVRMS.
Because of the structural quality of these wafer processed polyimide
Mechanical Elongation (%)
50
films, no partial discharge over 5 pC can be detected, even at 3 kVRMS.
Young’s Modulus (Gpa)
3.3
The top coil is gold plated, with a 4 m thick layer, and the coil track
width and spacing between the turns are all 4 m. The polyimide
layers have good mechanical elongation and tensile strength, which
also helps the adhesion between the polyimide layers or between
polyimide layer and deposited metal layer. The minimum interaction
between the gold film and the polyimide film, coupled with high
temperature stability of the polyimide film, results in a system that
provides reliable insulation when subjected to various types of
environmental stresses.
Electrical Dielectric Constant
3.3
Moisture Uptake (%)
0.8
Breakdown Voltage
300 V/m
Table 1. Polyimide properties
3
In addition to the fact that thousands of volts of isolation can be
achieved on-chip, the ADuM110 also makes it possible to transmit
very high bandwidth signals very efficiently, accurately, and reliably.
Figure 3 is a simplified schematic of the ADuM1100. To guarantee
input stability, the front glitch filter filters out pulses narrower than a
pulse width of approximately 2 ns. Upon the receipt of a signal edge,
a 1 ns pulse is sent to either Coil 1 or Coil 2. (For a leading edge
signal it is sent to Coil 1, and for a falling edge signal to Coil 2.)
Once the short pulses are transmitted to the secondary coils (the
bottom coils in this case), they are amplified and the input signal is
reconstructed through an SR flip-flop to appear as an isolated
output. The wide bandwidth of these microscale transformers and
high speed CMOS make the transmission of these short nanosecond
pulses possible. Since only signal edges are being used, this
transmission scheme is very power efficient. With a very energetic
pulse having a current ramping to 100 mA within 1 ns, the average
current for a 1 Mbps input signal is only 50 A. Some additional
power is dissipated by the switching of the surrounded CMOS gates.
Figure 2. ADuM1100 receiver chip
At 5 V, an additional 50 A/Mbps is needed if the total capacitance of the CMOS gates
is 20 pF. The typical optocoupler, on the other hand, dissipates over 10 mA, even
operating at 1 Mbps. This represents two orders of magnitude (100 ) improvement
in power dissipation provided by iCoupler isolators.
If there is no input change for a certain period of time, approximately 1 s, the
monostable generates a 1 ns pulse and sends it to Coil 1 or Coil 2, depending on the
input logic level. The 1 ns refreshing pulse is sent to Coil 1 if input is high and is sent
to Coil 2 if input is low. This helps maintain dc correctness for the isolator because
normally pulses are transmitted only on reception of a signal edge. The receiver
includes a watchdog circuit that will timeout at 2 s if it is not reset by an incoming
pulse. If a timeout happens, the receiver output will return to a default safe level (logic
high in the ADuM1100). The combination of refresh and watchdog functions provides the
additional advantage of detecting the failure of any field device on the system side. With
other isolators, this would ordinarily require the use of an extra isolated data channel.
1ns
SET_HI
+
DIFF
SPULE 1
INPUT
VSS1
VSS2
1ns
GLITCH
FILTER
S
SET
1s
ASTABLE
Q
+
DIFF
Q
REFRESH
R
R
OUTPUT
1ns
TRANSFORMER
1ns
+
DIFF
Figure 3. ADuM1100 circuit diagram
SET_LO
SET
2s
WATCHDOG
TIME OUT
4
The bandwidth of the isolator is dependent on the input filter
bandwidth within. For example, 500 Mbps can be achieved with a
2 ns input filter. For the ADuM1100, we chose a signal bandwidth
of 100 MBd, still 2 faster than the fastest optocouplers. Very tight
edge symmetry between input and output logic signals is also
preserved due to the instantaneous nature of the inductive coupling
between these microscale on-chip coils. The ADuM1100 has edge
symmetry of better than 2 ns for 5 V operation. As the bandwidth of
isolation systems continues to expand, the iCoupler technology will
be capable of meeting the challenge while optocoupler technology
is likely to struggle. Table 2 summarizes the existing performance
characteristics of the iCoupler technology as provided by the
ADuM1100 digital isolator.
In the ADuM1100, the capacitance between the top (input) coil and
the bottom (receiver) coil is only 0.2 pF, while the bottom coil has a
resistance of 80 . Thus the error signal induced on the bottom coil
by a 25 kV/s transient on the top coil is only 0.4 V, much less than
the receiver detection threshold. The transient immunity of iCoupler
isolators can be optimized through careful selection of the decoder
detection threshold, the resistance of the receiving coil, and, of
course, the capacitance between the top and bottom coils.
One recurring question about transformer based isolators involves
their magnetic immunity capability. Since iCouplers use air core
technology, no magnetic components are present and the problem
of magnetic saturation for the core material does not exist.
Therefore, iCouplers have essentially infinite dc field immunity.
The limitation on the ADuM1100’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 induced across the
bottom coil is given by:
Parameter
ADuM1100AR/BR
Data Rate (Mbps, min)
25/100
Supply Current at 10 Mbps, max (mA,)
2.0
Propagation Delay, max (ns)
18
Pulse Width Distortion, max (ns)
2
where:
Propagation Delay Skew, max (ns)
6
Common-Mode Transient Immunity,
min (kV/s)
25
= magnetic flux density (Gauss)
N = number of turns in receiving coil
rn = radius of nth turn in receiving coil (cm)
Isolation Rating (V)
2500
Temperature Range (°C)
–40 to +125
V = ( − dβ /dt ) Σπ rn ; n = 1,2,..., N
2
Table 2. ADuM1100 characteristics
In addition to the improvements in efficiency and bandwidth iCoupler
technology provides, it also offers a more robust and reliable isolation
solution than competitive offerings. Because high voltage transients
are present in many data acquisition and control systems, the ability
of the isolator to prevent transients from affecting the logic controller
is very important. High performance optocouplers have transient
immunity of less than 10 kV/s, while the ADuM1100 has a transient
immunity better than 25 kV/s. The induced error voltage at the
receiver input induced by an input-output transient is given by:
V = CR dV/dt
Because of the very small geometry of the receiving coil in the
ADuM1100, even a wire carrying 1000 A at 1 MHz and positioned
only 1 cm away from the ADuM1100 would not induce an error
voltage large enough to falsely trigger the decoder. Note that at
combinations of strong magnetic field and high frequency, any
loops formed by printed circuit board traces could induce error
voltages sufficiently large to trigger the thresholds of succeeding
circuitry. Typically the PC board design rather than the isolator itself is
the limiting factor in the presence of such big magnetic transients.
In addition to magnetic immunity, the level of electromagnetic
radiation emitted from the iCoupler device is a concern. Using
far-field approximation:
P = 160π2 I2 Σ rn ; n = 1,2,..., N
4
where:
P = total radiated power
where:
C is the capacitance between the input coil and the receiver coil
R is the resistance of the bottom coil
dV/dt is the magnitude of the transient
I = coil loop current
Again, given the very small geometry of the coils, the total radiated
power is still less than 50 pW, even if the part is operating at 0.5 GHz.
5
ADuM130x/ADuM140x: Multichannel Products
In addition to the many performance improvements discussed previously, iCoupler
technology also offers tremendous advantages in terms of integration. The optical
interference makes the realization of multichannel optocouplers very difficult. Transformers
based on iCoupler technology can be easily integrated onto a single chip. Furthermore, one
data channel can transmit signals in one direction, say from the top coil to the bottom coil,
while the neighboring channel can transmit a signal in the other direction, from the bottom
coil to the top coil. The bidirectional nature of inductive coupling makes this possible.
Analog Devices is currently sampling the ADuM130x/ADuM140x family of multichannel
products, which consists of five 3-channel and 4-channel products covering all possible
channel directionality configurations. Besides providing flexible channel configurations,
they support both 3 V and 5 V operation at either side of the isolation barrier and
support the use of these isolators as level translators. One side could be at 2.7 V, for
example, while the other side could be at 5.5 V. The edge symmetry of 2 ns is preserved
over all possible supply configurations at all temperatures from –40˚C to 100˚C. The
ability to mix bidirectional channels of isolation in a single package enables users to
reduce the size and cost of their systems.
Figure 4. ADuM140x die photo
For the ADuM1100, two transformers are used to transmit a single channel of data.
One is dedicated to transmit pulses representing the signal’s leading edge or updating
input high, and the other is dedicated to transmit pulses representing the signal’s falling
edge or updating input low. For the ADuM130x/ADuM140x product family, a single
transformer is used for each data channel. The ADuM140x shown in Figure 4 has four
transformers in total. The leading edge and falling edge are encoded differently, and the
encoded pulses are combined in the same transformer; as a result, the receiver has
responsibility for decoding the pulses to see whether they are for leading edge or falling
edge. The output signal is then constructed correspondingly.
ADuM1401
+V2
+V1
0.1 F
0.1 F
VE1
VDD1
VDD2
VE2
68HC11
SCK
MOSI
P0.0
MISO
A/D CONVERTER
VIA
VIB
VIC
VOD
ENCODER
DECODER
ENCODER
DECODER
ENCODER
DECODER
ENCODER
DECODER
GND 1
VOA
VOB
VOC
VID
GND 2
ISOLATION BARRIER
Figure 5. An SPI interface implemented using an ADuM1401 quad-channel isolator
SCLK
DIN
START
CONVERSION
DOUT
Of course, there is a penalty for using a single transformer per data channel
rather than using two transformers per data channel. The propagation delay is
longer for the single transformer architecture because of the additional encode
and decode time needed. The penalty for bandwidth is hardly a factor, even at
input speed of 100 Mbps.
In contrast to the ADuM1100, the ADuM130x/ADuM140x uses a dedicated
transformer chip, separate from the receiver integrated circuit. This partitioning
exemplifies the ease of integration for iCoupler technology. Besides standalone
multichannel isolators, the iCoupler technology can be embedded with other data
acquisition and control ICs to make the use of isolation even more transparent.
Consequently, in the future, system designers will be able to devote their time to
improving system functionality, rather than worrying about isolation.
10mm
C1
Analog Devices, Inc.
Europe
c/o Analog Devices SA
17–19, rue Georges Besse
Parc de Haute
Technologie d’Antony
F-92182
Antony Cedex, France
Tel: 33.1.46.74.45.00
Fax: 33.1.46.74.45.01
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South Tower Building
1-16-1 Kaigan,
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Fax: 813.5402.1063
C2
ADuM1401
Worldwide
Headquarters
One Technology Way
P.O. Box 9106
Norwood, MA
02062-9106 U.S.A.
Tel: 781.329.4700,
(1.800.262.5643,
U.S.A. only)
Fax: 781.326.8703
ADC
16mm
Analog Devices, Inc.
Southeast Asia
Headquarters
4501 Nat West Tower
Times Square
1 Matheson Street
Causeway Bay
Hong Kong, PRC
Tel: 852.2.506.9336
Fax: 852.2.506.4755
Figure 6. With the 4-channel ADuM1401, isolating an SPI interface requires only three
components occupying 160 mm 2
Summary
The tremendous advantages of iCoupler products over traditional optocouplers
in terms of power consumption, signal bandwidth, robustness, and ease of
integration make them ideal choices for future demanding isolation applications.
© 2003 Analog Devices, Inc. All rights reserved.
Trademarks and registered trademarks are the property
of their respective owners.
Printed in the U.S.A.
G04484-2.6-9/03