AND8200/D Design Considerations for ESD/EMI …

1 Semiconductor Components Industries, LLC, 2005 June, 2005 Rev. 11 Publication Order Number: and8200 /DAND8200/DDesign Considerations forESD/EMI Filters: I(Almost Everything You Wanted to KnowAbout EMI Filters and Were Afraid to Ask)Prepared by: Ryan HurleyApplications EngineerON SemiconductorBackgroundWith data transfer rates and clock frequencies everincreasing and encroaching on radio frequencies, the needfor EMI filters has increased dramatically. At the same time,the portability of electronic devices such as PDAs, laptopcomputers, MP3 players and so on, the need to protect thosedevices from ESD has also increased because use ofperipheral interfaces.

2 Some time ago these two functionswere independent of each other with discrete componentsmaking the EMI filter and a separate device to provide theESD protection. As consumer electronics continue to getsmaller, the ESD and EMI filter functions were graduallyintegrated ESD protection, a TVS diode would typically be diodes are nothing more than Zener diodes that havebeen modified to handle large currents for very brief periodsof time while clamping at some voltage slightly over thepeak operating voltage. A TVS diode with a clampingvoltage of V would be used in applications where themaximum operating voltage was V.

3 There is a limithowever, to the TVS s ability to clamp. That is a given TVScan only dissipate so much power in those short periods oftime. A way to improve this is to increase the size of theTVS. A larger device can handle more current than a similarbut smaller device, but at a cost, more filters have in the past been RC or LC pi networksusing individual surface mount components. One of themain problems with using discrete components is that thereare parasitics that can negatively affect the performance ofthe filter . Overall, most commonly used EMI filters forportable applications take a form similar to that of Figure 1.

4 Common RC Pi FilterThese filters are typically low pass filters that rejectsignals over 800 MHz. The component values for the filterare typically determined by how much attenuation is neededin the rejection band (800 MHz to GHz for example), thesource and load impedances, and how much the desiredsignal can be attenuated without losing applications where both ESD and EMI filtering arerequired, it is advantageous to choose a single integratedESD/EMI filter that replaces a number of discrete surfacemount components. Where board space is a premium, asingle integrated component is hard to beat.

5 If cost is anissue, a solution composed of a number of discrete surfacemount components cannot compete with a single integratedESD/EMI filter . If it is decided that a single integratedESD/EMI filter is to be used, a number of questions need tobe much ESD protection is required?What are the EMI filtering requirements?What is the acceptable Group Delay?How much insertion loss can be tolerated?What type of package should be used?What else?ESD ProtectionHow much ESD protection is needed? As much aspossible would be the correct answer. Having too little ESDhas the obvious consequences of inviting damage tosensitive components that were meant to be protected.

6 Isthere such a thing as too much ESD protection? For highspeed data lines and high frequency lines the answer is yes . The component that does the work for ESD protectionis the TVS or Transient Voltage Suppressor. One of the waysto measure a TVS s ability to suppress ESD is in the form ofthe IEC 61000 4 2. Level 4 is required for accessiblecontacts on cell phones. For further details refer toAND8074 1. IEC 61000 4 2 Ratings for ESD Protection by Contact Discharge and Air Discharge for VariousVoltage Levels1a Contact Discharge1b Air DischargeLevelTest Voltage (kV)LevelTest Voltage (kV)12122424363848415X*SpecialX*Special* X is an open level.

7 The level has to be specified in the dedicated equipment specification. If higher voltages than those shown are specified,special test equipment may be an integrated circuit a TVS would be in the form of aspecial Zener diode. The breakdown voltage of the Zenerdiode (BVZ) would be determined by the breakdown voltageof the IC (BVIC) being protected. In applications where asingle TVS diode is used for ESD and EMI protection, BVZneeds to be less than BVIC. The limiting factor comes to bethe current handling/power dissipation capability of thediode. Smaller diodes can not handle as much instantaneouscurrent as a larger diode.

8 A TVS s ability to protect againstESD is, for the most part, a function of how muchinstantaneous power can be dropped across the diode. To getmore ESD protection from these Zener diodes, the physicalsize of the diode has to increase. The result of increasing thesize of the diode is that the junction capacitance alsoincreases. The dual nature of the Zener diode is that it notonly takes care of the TVS role, but it also behaves as acapacitor (Figure 2).Figure 2. The Zener diodes alsofunction as capacitors to ground. The capacitance of the diode is a key component of theEMI filter used in the tuning of the filter which will bediscussed later.

9 Another consideration when determiningthe size of the TVS/Zener diode and its capacitance, is itseffect on the Group Delay or Envelope Delay. For ESDprotection it comes down to:1. What level of ESD protection is needed?2. What is the breakdown voltage of the circuit to beprotected?3. What effect will the capacitance of the diode haveon the EMI filter ?The level of ESD protection needed will be determinedgreatly by the environment of the end function. Data linesthat interface with external components such as USB orFireWire peripherals will need greater ESD protection thandata lines that do not have an external interface.

10 Data linesthat have connectors to interface with external componentsquite often can face ESD contact discharges of a fewthousand volts, enough to destroy sensitive electronics. Theinternal data lines also need a degree of ESD that have moving parts; scanners, flip phones,etc., can generate enough internal charge to cause an high speed data lines operate at V. Thesesystems however, typically can handle a little over two timesthis for very brief periods of time. So an acceptablebreakdown voltage, (VBZ), would be around V to V,as in the SM05C.