AND8230 - Application Hints for Transient Voltage ...

1 Semiconductor Components Industries, LLC, 2016 May, 2016 Rev. 21 Publication Order Number: AND8230 /DAND8230/DApplication Hints forTransient VoltageSuppression Diode CircuitsIntroductionTransient Voltage Suppression (TVS) diodes providea simple solution to increase the EMI and ESD immunitylevel of a circuit and only a few guidelines must be followedto provide effective surge protection. This document willanalyze several important Application features of avalancheTVS and diode arrays. In addition, examples will beprovided to demonstrate the advantages and disadvantagesof uni and bidirectional TVS devices. The following circuitdesign considerations will be analyzed: Internal IC versus External TVS Protection Circuits TVS Diode Turn-on Voltage Diode Array Application Hints Common Mode Offset Voltages Back Drive Protection Differential Input and Output Application GuidelinesInternal IC versus External TVS Protection CircuitsTransient Voltage Suppression (TVS) diodes can be usedto supplement the surge immunity level of an IC.

2 Most ICscontain internal protection circuits that function well atpreventing ESD failures that occur in assembly; however,they are often inadequate for protecting against surge eventsthat occur in normal product usage. The surge ability ofa silicon TVS diode is directly related to its size and externaldevices are typically a factor of at least ten times larger thanthe internal IC TVS devices. External TVS diodes providea higher level of surge protection because it is typically notpractical for an IC to incorporate large protection devices. Inaddition, the internal protection circuit of most ICs isdesigned to handle only a few ESD events while an externalTVS device provides immunity for an indefinite amount of the IC s internal surge protection circuitcan be helpful in selecting an external TVS device withan appropriate power rating and turn-on , the data sheets of most ICs provide only anESD rating and do not disclose the internal protectioncircuit.

3 Internal IC protection circuits can be created usinghigh Voltage transistors, Zener diodes, diode arrays,thyristors and overvoltage detection switches. Figure 1provides two popular IC circuits used to provide internalsurge protection. Guidelines to ensure that a surge event willnot exceed the power rating of the internal protection circuitwill be shown in the following 1. Zener Diodes are a Popular Choice for the Internal Protection Circuit of a Transceiver IC thatRequires Power Surge and ESD Protection. Diode Arrays are a Frequent Choice for the Internal ESDP rotection Circuit of a Logic ICTransceiver ICLogic ICTransmitterReceiverInternal ICProtectionCircuitsVDDGNDVDDVSSD_HighD_ NOTEAND8230 Diode Array Turn-On VoltageThe main function of an external TVS device is to limit thecurrent through an IC by virtue of decreasing the magnitudeof the surge Voltage .

4 An ideal external TVS device willturn-on before the IC s internal circuit and absorb the entireenergy of the surge pulse. In practice both the external andinternal protection circuits will usually turn-on duringa surge event. The IC s reliability will not be impacted if theinternal protection circuit current is limited to a low addition, the location of the TVS devices is a key factorthat determines whether the majority of the surge energy isabsorbed by the external protection circuit. Reference [4]provides PCB layout guidelines that help to ensure that thesurge protection will be provided by the external TVSdevices rather than the IC s internal protection ICs incorporate a diode array for ESD protection;thus, the internal and external protection circuits often havea similar topology, as shown in Figure 2.

5 A value of V V can be used to estimate the turn-on Voltage of externalswitching and Schottky diodes, respectively. The turn-onvoltage of the internal IC protection circuit is typically equalto V for a bipolar process; however, the value for a CMOS device is a function of several process variables, as shownbelow by the Voltage equation of a MOSFET +VT)IDS12moCox WL (eq. 1)Where:VT = Threshold VoltageIDS = Drain-to-Source Currentmo = Electron MobilityCox = Gate Oxide Capacitance Per Unit AreaW/L = Width and Length DimensionsFigure 2. An External Diode Array often has a SimilarTopology to the IC s Internal ESD Protection Circuit;however, the Turn-on Voltages of the Two Arrays canbe DifferentVDDGNDI/OESDC ircuitICInputVDDVDDO ften CMOS ICs are designed with MOSFET diodes thathave a VT greater than V to increase their immunityagainst ground noise; thus, the external array will usuallyhave a lower turn-on Voltage .

6 The VT of a low Voltage ICmay be low enough that the turn-on of the MOSFETs can belower than the value of a standard diode. One solution tothis problem is to use a Schottky diode array, as shown inFigure 3. Schottky diodes typically have a turn-on voltageof approximately 3. The Low Turn-on Voltage of a SchottkyDiode can be Used to Provide Surge Protectionfor Low Voltage ICsVDDGNDI/OESDC ircuitICInputVDDVDDThe circuit shown in Figure 4 is another alternative tosolve the potential problem that can occur if the internal andexternal arrays have a similar turn-on Voltage . The resistorthat is located between the two arrays ensures that themajority of the surge energy will be dissipated by theexternal circuit.

7 The IC s internal diodes also turn-on, butthe current through these devices will be relatively lowcompared to the external diode 4. R Forces I1 >> I2, which Ensures that theMajority of the Surge Energy is Diverted by theExternal Diode Array rather than by the IC s InternalESD CircuitVDDGNDI/OESDC ircuitICInputVDDVDDRI1I1 >> I2I2 AND8230 Array Application HintsDecoupling the Power SupplyDiodes arrays steer the surge current into the power supplyrails where the energy of the Transient Voltage pulse isdissipated, as shown in Figure 5. A positive surge pulse willbe clamped to a Voltage that is equal to a forward diodevoltage drop above the supply Voltage (VDD).

8 Typically theVSS pin is grounded; thus, a negative pulse will be clampedto a Voltage of one diode drop below 5. Adding a Decoupling Capacitor and Avalanche TVS to a Diode Array Enhances the Ability to Clamp the Surge Voltage to a Diode Drop above or below the Power Rails+V V0 VP1P2D2D1Z1 mFD1 Clamps Positive VoltagesVC = VDD + VFD2 Clamps Negative VoltagesVC = VFZ1 Provides Surge Protection for VDDVBR_Z1 > VDDD ecoupling capacitors and avalanche diodes are twosimple solutions to improve the load regulation of a powersupply during a surge effect. Placing an RF ceramiccapacitor of approximately to mF across the powerpins reduces the magnitude of the surge pulse.

9 Additionalsurge protection can be provided by using a diode array witha built-in avalanche diode that has a breakdown voltageslightly higher than VDD. Integrating the avalanche diode inthe TVS IC minimizes the inductances associated with thedevice connections, which reduces the magnitude of thesurge pulse due to the V = L (DI/Dt) diode array clamping equations assume that thepower supply rails VDD and VSS are a constant voltagesource. This is a good assumption for low frequency loadchanges, but may not be valid during the high frequency loaddemand of a surge pulse. For example, the IEC 61000 4 2 ESD pulse has a rise time of less than ns and a peakcurrent of 30 A.

10 The high peak energy of the ESD pulse canincrease the power supply s output impedance. The changein impedance produces a peak clamping voltagesignificantly larger in magnitude than what Figure 5 sequations predict. Reference [4] provides additionalrecommendations that can be used to maximize theclamping performance of the diode Array Surge RatingsCareful interpretation of a diode array s data sheetspecifications is required because the surge rating isa function of the test configuration. Some diode arrays havea power rating that is measured with power applied to theVDD pin, while others float the power pin during the surgetest.