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AND8027/D Zener Diode Based Integrated Passive Device Filters, An Alternative to Traditional I/O EMI Filter Devices http://onsemi.com APPLICATION NOTE Jim Lepkowski Senior Applications Engineer Phoenix Central Applications Laboratory Background devices have been used for a number of years to solve EMI Electromagnetic Compatibility (EMC) has become a problems; however, these devices tend to be relatively major design concern for all new designs. The designs that expensive and large in size. are manufactured today must function in close proximity to A brief discussion of the different filtering options a wide range of other electronic devices. These devices must available to a designer is given in the follow paragraphs. A be capable of operating without either becoming effected by summary of the advantages and disadvantages of the filter or adversely effecting the operation of neighboring units. In devices is shown in Table 1 on page 2. addition, most systems are connected through input/output (I/O) cables to other systems. Thus the I/O interface has Ferrite Beads become a major source and entry point for both conducted Ferrite beads are a series filter device that provides high and radiated Electromagnetic Interference (EMI) and frequency attenuation with a small resistive power loss at Electrostatic Discharge (ESD). DC and low frequencies. At low frequencies, the device Today’s advanced products are based on integrated functions as a resistor with a resistance that is typically equal devices that are faster and smaller and thus are more noise to 50 to 200 ohms. At high frequencies, the device functions sensitive than previous generation devices. Designers are as an inductor and has an impedance that increases with being challenged to build more complex units, while frequency. The equivalent model for a ferrite bead is shown reducing the size and cost of the design. In addition, the new in Figure 1. These devices are very effective for solving designs must be compliant with the revised EMI/ESD problems such as the “ringing” noise that often is imposed standards that are more stringent than previous standards. on high–speed digital signals. Traditional EMI I/O Filter Options There are several filter design choices available to LR attenuate the noise entering and exiting an I/O port, including ferrite beads, feed–through capacitors, filter connectors and Pi or Tee filters. These traditional filter Figure 1. Ferrite bead equivalent circuit Semiconductor Components Industries, LLC, 2001 1 Publication Order Number: June, 2001 – Rev. 2 AND8027/D AND8027/D Table 1. EMI Filter Device Options EMI Device Filtering Advantages Disadvantages Package Availability Mechanism Ferrite Beads Series • Low cost • RelativelRelativelyy largelarge in size • Discrete devices attenuation • “Slip–on”Sli on packageackage does not • Ferrite material saturates at high DC • “Slip–on”Sli on beads require PCB modification currents • Integrated package Feedthroughg Shunt • Signalg is filtered before PCB • Highg cost • Chassis mountingg CiCapacitors attenuationi entryt • RltilRelatively llarge iin size i • Small imimpedance edance at ggroundround • Difficult to use on PCB connection • Tee filter frequency characteristics are • Effective in segmented chassis dedependentpendent on source (cable) and load designs (receiver) impedances Filter Shunt • Signal is filtered before PCB • High cost • Chassis mounting Connectors attenuation entryentry • Connector size increases • Small impedance at ground connection • Effective in segmented chassis designs RC Filters Shunt • Current limiting via resistance • dV/dt limiting, but no voltage clamping • Discrete devices attenuation • RsRs are smallersmaller t thanhan L Lss forfor ESD • IntegratedIntegrated ppacackagekage • Filter circuit located on PCB • 1st order LPF with –20 20 dB/decade attenuation • RsRh have iinsertion til loss/power / dissipation LC Filters Shunt • 2nd order LPF with –4040 dB/ • Filter will amplifyam lify at self resonance • Discrete devices attenuation decade attenuation frequency • Integrated package • di/dt limiting via inductance • Filter circuit located on PCB • Ls have low insertion loss/ • Ls are bibiggergger than Rs power dissipation • ESD voltage is not clamped RC Zener Shunt • Low cost • PCB routing complexity increases with • SMT IC packages BBased d FiltFilters attenuationtt ti • Small IC packages mmulti–channel lti channel ICs • “Flip chips” • Minimal parasiticarasitic inductance • Frequency responseres onse is not adjustable • Filter response is “close” to • 1st order LPF with –20 dB/decade ideal resresponseponse attenuation • Voltage clamping for ESD • Filter circuit located on PCB • Rs have insertion loss/loss/powerpower dissipation http://onsemi.com 2 AND8027/D The demand of cost sensitive portable products such as cellular telephones has resulted in the development of integrated passive device (IPD) filters that are now available to replace low pass filters that have been implemented with discrete resistors, capacitors and diodes. The NZMM7V0T4 multiple channel filter array, as shown in Figure 2, is the first member of ON Semiconductor’s new family of IPD EMI filters that includes single, dual and multiple filter arrays. The schematics for the NZF220TT1 single channel and the NZF220DFT1 dual channel IPD EMI filters are shown in Figures 3 and 4. The zener diode based Pi filters are functionally equivalent to the resistor/capacitor filter shown in Figure 5. The Pi filter circuits are formed by a 100 Ω resistor and two zener diodes that have a junction capacitance of 22 pF. 19 20 21 22 23 24 18 1 17 2 NC 16 3 15 4 14 5 13 6 12 11 10 9 8 7 Figure 2. NZMM7V0T4 Device Schematic 1 6 1 3 2 3 4 2 Figure 3. NZF220DFT1 Device Schematic Figure 4. NZF220TT1 Device Schematic R1 100 Ω VIN VOUT C1 C2 22pF 22 pF Figure 5. Pi Filter Channel – Equivalent Circuit http://onsemi.com 3 AND8027/D Feed–through Capacitors and Filter Connectors Pi and Tee Filters Feed–through capacitors and filter connectors are shunt The two most popular bi–directional low pass filter filter devices that are typically mounted on a conductive configurations are Pi and Tee filters. Pi and Tee filters can be chassis or a shielded enclosure. The mechanical mounting constructed using discrete components, integrated discrete forms the ground connection and the high frequency noise components, or an IPD device that uses zener diodes as the is shunted to the chassis ground instead of signal ground. capacitive elements. These filters are typically mounted on Thus, the noise signal is filtered before the signal reaches the a PCB and attenuate the noise to signal ground, in contrast PCB. The effectiveness of the filter is usually very good to the enclosure mounted