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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