AN-825: Power Supply Considerations in iCoupler® Isolation Products (Rev. 0) PDF

AN-825
APPLICATION NOTE
One Technology Way • P.O. Box 9106 • Norwood, MA 02062-9106, U.S.A. • Tel: 781.329.4700 • Fax: 781.461.3113 • www.analog.com
Power Supply Considerations in iCoupler® Isolation Products
by Rich Ghiorse
INTRODUCTION
This application note is a guide to help users understand the
function of Analog Devices iCouplers under various power
supply conditions. Also discussed are the details of calculating
supply current consumption and power dissipation.
05784-001
iCoupler products offer an alternative isolation solution to
optocouplers with superior integration, performance, and
power consumption characteristics. An iCoupler isolation
channel consists of CMOS input and output circuits and a chipscale transformer (Figure 1). In all applications, an iCoupler is
powered by two separate sources which do not share a common
ground. Various scenarios must be considered during design to
ensure that all powered states are understood.
Figure 1. ADuM140x Quad Isolator
Rev. 0 | Page 1 of 8
AN-825
TABLE OF CONTENTS
Introduction ...................................................................................... 1
iCoupler Channels During Power Supply Transitions .................4
Revision History ............................................................................... 2
Calculating Supply Currents ............................................................5
iCoupler Power Supply Basics......................................................... 3
Power Dissipation Considerations ..................................................7
Inside the iCoupler ........................................................................... 4
Conclusion..........................................................................................7
REVISION HISTORY
2/06—Revision 0: Initial Version
Rev. 0 | Page 2 of 8
AN-825
iCOUPLER POWER SUPPLY BASICS
Figure 2 shows a simple diagram of a powered iCoupler. It is
helpful to think of the iCoupler as having two separate circuits:
Side1 and Side2. To use the iCoupler as an isolator, VDD1 and
VDD2 must be isolated from each other. This brings to light
several key points:
• Side1 is powered solely by VDD1 while Side2 is powered solely
by VDD2.
• VDD1 and VDD2 are referenced to GND1 and GND2,
respectively.
• Due to the isolation barrier, VDD1 and VDD2 have no
reference point to each other.
• Supply currents IDD1 and IDD2 are confined to their
respective sides.
Figure 3 shows an example of voltage measurements referenced
to different grounds. In this case, the iCoupler is powered with
VDD1 = 5V, VDD2 = +3V, and there is a common-mode voltage of
400 V across the isolation barrier (VCM). The voltages in regular
font are referenced to a common system ground (chosen to be
GND1), while the voltages in quotes are referenced to local
grounds GND1 and GND2. Even though the voltage values are
different, they are valid for this example because they are
measured from different reference points.
This example stresses two important points:
•
Always consider the reference point in all iCoupler voltage
measurements.
•
All iCoupler voltages are referenced to their respective
grounds (GND1 or GND2).
i Coupler
iCoupler
VDD1
VDD2
+
–
SIDE1
GND1
403V "3V"
+
SIDE2
–
+
–
VDD2
GND2
+
–
SIDE2
GND1
GND2
–
400V "0V"
+
ISOLATION BARRIER MEANS NO REFERENCE POINT OR
CURRENT PATH BETWEEN SIDE 1 AND SIDE 2 CIRCUITS
"0V" 0V
SIDE1
VCM
Figure 2. Basic Diagram of a Powered iCoupler
Figure 3. Example of iCoupler Showing Measurements
Referenced to Different Grounds
Rev. 0 | Page 3 of 8
05784-003
VDD1
VDD2
IDD2
05784-002
IDD1
VDD1
"5V" 5V
AN-825
INSIDE THE iCOUPLER
Figure 4 shows a detailed block diagram of the ADuM1201
dual-channel iCoupler. The ADuM1201 has an input and
output channel on each side of the isolation barrier. The
channels are identical, with the only difference being the
direction of data flow. Each iCoupler channel consists of a
cascade of circuits: an input buffer, an encoder (with refresh
generator), an isolation transformer, a decoder (with watchdog
timer), and an output buffer.
Input and output channel parameters are designated by a
subscript I and a subscript O, where the subscript I designates
an input supply value and the subscript O designates an output
supply value. Some examples include: IDDI (input supply
current) and VDDO (output supply voltage). Following this
convention and referring to Figure 4, VDD1 can be considered
both a VDDO and a VDDI because Channel A’s output and Channel
B’s input reside on the VDD1 side of the iCoupler. This same
reasoning applies to the other side of the iCoupler with VDD2
considered a VDDI to Channel A and a VDDO to Channel B.
The iCoupler uses chip-scale transformers to isolate digital
signals. Edge information from the input signal is encoded and
applied to Isolation Transformer T1 and Isolation Transformer
T2 in the form of 1 ns wide pulses, as shown at the output of the
encoders in Figure 4 Two pulses indicate an input signal with a
rising edge and one pulse indicates an input signal with a falling
edge. These pulses are coupled through T1 and T2 and decoded
on the other side of the barrier for reconstruction at the output.
The refresh generator outputs a pulse every 1 μs guaranteeing
the DC correctness at the output. The watchdog timer
automatically forces the output to a high state if the decoder has
not seen a pulse within approximately 2 μs as is the case in the
event of lost input side power, or if the device is damaged.
iCOUPLER CHANNELS DURING POWER SUPPLY
TRANSITIONS
When considering iCoupler operation in various power states,
it helps to consider individual channels instead of an entire
device. There are four power states for an iCoupler channel, as
given in Table 1. State 0 and State 3 are normal conditions; the
channel is either completely off or completely on. State 1 and
State 2 present special conditions where the channel is partially
powered. These states represent situations seen during power
supply transitions, or in fault conditions.
Table 1. The Four Power States of an iCoupler Channel
State
0
1
VDDI
Off
Off
VDDO
Off
On
2
On
Off
3
On
On
Comments
Entire channel off, normal condition
Input side off; output side on, special
condition
Output side off; input side on, special
condition
Entire channel on, normal condition
In real terms, iCoupler supplies are considered off for values
below 2.7 V. Given that supplies have finite rise times, a subtle
point is raised: at some value of supply voltage below 2.7 V, a
channel may start to operate, albeit not predictably. For the
ADuM1xxx series of iCouplers, this wake-up value for the
supplies is ~1.8 V.
ADuM1201
ENCODED PULSES
VDD1
CHANNEL A
VOA
VDD2
WATCHDOG
2μS
REFRESH
DECODER
ENCODER
VIA
T1
T2
GND1
ENCODER
DECODER
REFRESH
WATCHDOG
2μS
Figure 4. Block Diagram of ADuM1201 Showing Internal Circuitry
Rev. 0 | Page 4 of 8
VOB
GND2
05784-004
VIB
CHANNEL B
AN-825
Figure 5 illustrates how an iCoupler output reacts during
various power supply states. Indeterminate operation exists
for VDDI in the region from 1.8 V to 2.7 V; this is eliminated by
having supply rise times >0.1 V/μs. In the case where
unpowered outputs or inputs are connected to other circuits
that are powered, ensure that voltages applied to the iCoupler
are kept within the absolute maximum ratings.
STATE 0
STATE 1
STATE 2
CALCULATING SUPPLY CURRENTS
Supply currents for iCoupler are impacted by the values of
supply voltage, output load, and data rate of the isolation
channels. IDD1 and IDD2 are determined by performing separate
calculations for each channel and summing the results. To
facilitate calculations for IDD1 and IDD2 in multichannel
iCouplers, a design tool is provided on the Analog Devices
website at www.analog.com/iCoupler.
STATE 3
VDDO
OUTPUT SUPPLY
2.7V
1.8V
The values for IDDO and IDDI for a given channel are calculated
using Equation 1 and Equation 2 1.
VDDI
2.7V
VOX
FULLY OPERATIONAL
INDETERMINATE
(LOGIC LOW)
05784-005
INPUT SUPPLY
•
In unpowered states, voltages applied to the iCoupler
should not exceed absolute maximum ratings.
Supply rise times are <0.1 V/μs.
•
Supplies are excessively noisy.
•
Problems occur with latch-up and EOS/ESD during
system-level testing.
(2)
where:
IDDI(D), IDDO(D) are the dynamic input and output supply current
per channel (mA/Mbps).
fr is the input stage refresh rate (Mbps).
IDDI(Q) and IDDO(Q) are the input and output quiescent supply
currents (mA).
The ADuM3xxx series of iCouplers are ESD-hardened products
that carry the same functional specifications as the ADuM1xxx
series of iCouplers. While the ADuM3xxx series was developed
to provide more robust ESD/latch-up immunity, it also
addresses the power-up and power-down problems. The
ADuM3xxx series does this with under-voltage lockout
circuitry that eliminates indeterminate operation at all supply
voltages. Use of the ADuM3xxx series should be considered in
applications where:
•
IDDI = (IDDI (D)) × (2f-fr) + IDDI (Q) (mA); f > 0.5 × fr
f is the input logic frequency (MHz, half of input data rate, NRZ
signaling).
The key points taken from this example:
Rise times for supplies should be >0.1 V/μs.
(1)
CL is the output load capacitance (pF).
Figure 5. iCoupler Output During Various Power Supply States
•
IDDO = (IDDO (D) + (0.5 × 10 - 3) × CL × VDDO) × (2f-fr) + IDDO (Q) (mA);
f > 0.5 × fr
VDDO is the output supply value (V).
1
The ADuM1100 and ADuM3100 are single-channel isolators and use a
different set of equations for calculating IDDO and IDDI. These models specify
input and output dynamic power dissipation capacitance, CPD1 and CPD2, and
use the following equations:
IDD1 = CPD1 × VDD1 × f + IDD1Q.
IDD2 = (CPD2 + CL) × VDD2 × f + IDD2Q, where CL is load capacitance.
Rev. 0 | Page 5 of 8
AN-825
Figure 6 shows an example using the ADuM1401 quad
iCoupler. The operating conditions are: VDD1 = +5V, VDD2= +3V,
CL = 15 pF, f = 40 Mbps (f = 20 MHz). The total IDD1 and IDD2
currents are the sum of the appropriate IDDI and IDDO for each of
the four channels.
For Channel A, Channel B, and Channel C:
IDDO = (0.03 + 0.0005 × 15 × 3) × (2 × 20 − 1.1) + 0.11 = 2.2 mA
IDDI = (0.19) × (2 × 20 − 1.1) + 0.50 = 7.9 mA
For Channel D:
IDDO = (0.05 + 0.0005 × 15 × 5) × (2 × 20 − 1.1) + 0.11 = 3.5 mA
The first step is to identify that VDD1 powers three input
channels (the A, B, and C channels) and one output channel
(Channel D). Conversely, VDD2 powers one input channel
(Channel D) and three output channels (the A, B, and C
channels). Therefore, IDD1 and IDD2 are given by Equation 3
and Equation 4:
IDDI = (0.1) × (2 × 20 −1.1) + 0.26 = 4.2 mA
Finally, the values of IDD1 and IDD2 are calculated using
Equation 3 and Equation 4:
IDD1= IDDI (ChA) + IDDI (ChB) + IDDI (ChC) + IDDO (ChD) (mA)
(3)
IDD2= IDDO (ChA) + IDDO (ChB) + IDDO (ChC) + IDDI (ChD) (mA)
(4)
IDD1 = 3.5 + 7.9 + 7.9 + 7.9 = 27.2 mA
(3)
IDD2 = 4.2 + 2.2 + 2.2 + 2.2 = 10.8 mA
(4)
Looking at the values in Table 2, note that input current values
are higher than output current values. Input channels see higher
loads because they have to provide drive current for the
isolation transformers. The amount of current drawn by an
iCoupler is frequency dependent, represented by the term IDDI (D)
(dynamic input current) in Equation 2. Output channels also
have a frequency dependent term, represented by IDDO (D)
(dynamic output current) in Equation 1.
Next, calculate values for IDDO and IDDI using Equation 1 and
Equation 2. In the example, there are total of eight intermediate
calculations. Table 2 helps to organize the results of these
calculations. In theory, there are 16 possible calculations, but
8 are listed as not applicable (NA) because on a given side of the
isolator a channel is either an input or an output, never both.
The intermediate calculations using Equation 1 and Equation 2
and typical values from the ADuM1401 data sheet follow. For
simplicity, the data rates and loads for all the channels are
assumed to be the same. This may not always be the case.
Key points for this example:
•
Separate calculations are required for each channel to
determine IDDO and IDDI values.
•
Final values for supply currents IDD1 and IDD2 are calculated
by summing individual IDDO and IDDI values.
•
Supply currents increase with higher capacitive loads,
higher logic frequencies, and higher supply voltages.
Table 2. Supply Current Calculations for Figure 6
IDD1 (mA)
IDD2 (mA)
IDDO (mA)
IDDI (mA)
IDDO (mA)
IDDI (mA)
Channel A
N/A
7.9
2.2
N/A
Channel B
N/A
7.9
2.2
N/A
Channel C
N/A
7.9
2.2
N/A
Channel D
3.5
N/A
N/A
4.2
0.1μF
5V
VDD2
VDD1
3V
+
+
–
0.1μF
IDD1
–
IDD2
VIA
ENCODE
DECODE
VIA
15pF
T1
40Mbps = 20MHz
T2
VIB
DECODE
ENCODE
VOB
15pF
40Mbps = 20MHz
VIC
ENCODE
DECODE
VOC
15pF
40Mbps = 20MHz
VOD
ENCODE
DECODE
VID
15pF
GND2
GND1
ADuM1401
Figure 6. Supply Current Calculation Example Using the ADuM1401
Rev. 0 | Page 6 of 8
05784-006
40Mbps = 20MHz
AN-825
POWER DISSIPATION CONSIDERATIONS
CONCLUSION
Total power dissipation PD is the sum of Side1 power and Side2
power, P1 and P2, as shown in Equation 5 and Equation 6:
The unique nature of the iCoupler as an isolation device gives
rise to the need for detailed understanding of power supply
conditions, power supply transition, power supply currents,
and power dissipation. Topics discussed in this note help give
the user a clearer understanding of power supply subtleties seen
in iCoupler applications. This allows the user to make more
informed decisions on power supply requirements, power supply
current consumption and power dissipation for iCouplers.
PD = P1 + P2 (W)
(5)
PD = VDD1 × IDD1 + VDD2 × IDD2 (W)
(6)
Equation 7 is used to calculate the total package temperature
rise. Because the internal construction of the iCoupler is slightly
different, Side1 and Side2 have different thermal resistances
given by θJCI and θJCO.
TRISE = θJCI × VDD1 × IDD1 + θJCO × VDD2 × IDD2 (°C)
(7)
Knowing TRISE and TAMAX and using Equation 8, the user makes
a calculation to verify that the maximum junction temperature,
TMAX is not exceeded:
TAMAX + TRISE ≤ TMAX (°C)
(8)
The following is an example calculation for an ADuM1401 with
worst-case conditions from a power dissipation viewpoint:
f = 90 Mbps, CL = 15 pF, VDD1 = VDD2 = 5.5 V, IDD1 = 82 mA,
IDD2 = 43 mA, θJCI = 33°C⁄W, θJCI = 28°C∕W, and
TAMAX = +105°C.
TMAX is calculated as follows:
P1 = 5.5 V × .082 A = 0.45 W
P2 =5.5 V × .043 A = 0.23 W
TRISE = (33 × 0.451 + 28 × 0.237) = 21.5°C
TMAX = 105°C + 21.5°C = 126.5°C (well below 150°C limit)
In applications where design criteria require a maximum
junction temperature below 150°C, the maximum safe ambient
temperature is determined by working the previous calculation
backwards. The result of this calculation gives a new TAMAX for a
different value of TMAX, at given supply values and data rates.
This is the case in a design that must follow set reliability
guidelines as required in military, aerospace, or other high
reliability applications.
Rev. 0 | Page 7 of 8
AN-825
NOTES
©2006 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
AN05784-0-2/06(0)
Rev. 0 | Page 8 of 8
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