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1、ESD Protection Design Seminar Jim Sutherland Senior Applications Engineer,Outline,What is ESD? What damage can it cause? Why is the problem growing? What are the issues for the designer? How can we measure it? How can we protect equipment from ESD?,What Is ESD?,ESD = Electro Static Discharge Generat
2、ion Triboelectric (friction causes accumulation of charge) Induction (field induces charge) Discharge Dielectric (air) breakdown Electric field increases when charged bodies approach each other Current flow into circuitry,ESD Damage of ICs,Permanent Oxide breakdown, shorts, opens, latch-up Temporary
3、 Latch-up, ground bounce Latent Degradation from an ESD event,ESD problem is growing,Circuits/Systems Old - Robust ICs & Low speed signals New - Sensitive ICs & High speed signals Environment Old - Manufacturing / Corporate New - Home / Outdoors / Person,ESD Issues for the Designer,Must meet ESD spe
4、cifications Select ESD tolerant components Minimize signal degradation (from R,L & C) Board space / weight / proper design Component cost Assembly cost Lifetime cost (stability) Test the system,International ESD Standards,Human Body Model (HBM) - for devices EIA/JESD22-A114-A ANSI/EOS/ESD-S5.1-1993
5、MIL-STD-883 (method 3015) IEC 1000-4-2:1995 - for systems Machine Model (MM) - less common EIA/JESD22-A115-A ANSI/EOS/ESD-S5.1-1993 Charge Device Model (CDM) - less common JESD22-c101,Human Body Model (HBM),Discharge from 100pF capacitor through 1.5 kOhm resistor 6 ESD pulses 3 positive, 3 negative
6、1 sec separation Pin-to-pin testing N(N-1)/2 combinations Used for component characterization Widely used,HBM Current Waveform,Rise Time: 2 nS Tr 10 nS,IEC 1000-4-2:1995 Standard,Discharge from 150 pF capacitor through 330 ohm resistor 6 ESD pulses 3 positive, 3 negative Used for system characteriza
7、tion “Contact” v. “Air” discharge Different levels Different applications,IEC 1000-4-2 Current Waveform,Very fast rise time: Tr 1nS,60ns,IEC 1000-4-2 Test Levels,Contact discharge is the preferred test method - air discharges are not repeatable Air discharges used where contact discharge cannot be a
8、pplied No implied equivalence in test severity between the two test methods,IEC 1000-4-2 Bench Test Specification,ESD Protection Techniques,Clamp diodes in IC Not sufficient protection Shielding Low effectiveness Bypass capacitor or series resistor/inductor Can degrade signal; many components; large
9、 board area Spark gap Low cost; low stability; large board area Discrete Zener diodes High capacitance, many components; large board area Discrete PN diodes Low capacitance; many components; large board area Integrated PN diodes,Integrated Diode Networks,Superior downstream ESD protection High speed
10、 response ESD current steered to GND or VCC Minimum Signal Degradation (Low C) Minimal board space, weight Low assembly/manufacturing costs Minimal Design-In Time Long-term reliability,Choosing an ESD Diode Network,How many lines are needed? How much capacitance? (e.g. 5 pF) What is the HBM rating?
11、(e.g. 15 kV) What is the downstream clamp voltage? (e.g. 13 V 15 kV HBM pulse) What is the contact discharge rating? (e.g. 8 kV) What is the air discharge rating? (e.g. 15 kV) What package? (e.g. 24-pin QSOP),ESD Diode Network Placement,The Need to Keep ESD Diodes,Downstream of Line Inductances,Also
12、 put protection diodes at most likely ESD entry point - the connector,Designing for Minimal Power Rail Inductance,Add Bypass Capacitor,Place Ceramic bypass capacitor (0.1 0.2 uF) as close as possible to ESD diode network power rail to shunt ESD current to both power rails Maybe add Zener in parallel
13、 with capacitor to minimize parasitic inductance of bypass capacitor,Protected,Device,Gnd,Vcc,C,Using a Series Resistor to Minimize Downstream Current,Can be considered for latch-up sensitive applications Guaranteed clamping voltage limits current downstream (I = V / R) Only for inputs with high Z O
14、nly for output drivers with low Z watch out for filtering of signal,Power-down Issues,Diode protected systems that are powered down can drain current from an active high input through the diode to VCC This can drain batteries and/or damage devices on the same line To avoid this, isolate VCC from the
15、 bypass capacitor with a blocking diode One diode solution,Component and System Specifications,There is no simple formula to translate system specifications into component specifications IEC 1000-4-2 Specification is more severe than HBM Line capacitance and inductance shape the ESD pulse, reducing
16、its peak value Poor device placement can degrade performance If there are multiple devices on a line, decide which to protect The relationship between downstream clamp voltage and downstream protection is not exact,Validating the Design,Define the practical limits of functional failure (e.g. Data integrity, recovery time) Test only at those places subject to touch during normal
