Maxim Leads Protection This application note describes threatens

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Application Note 639: 2000
Maxim Leads Protection
This application note describes threatens electronic systems, type damage inflicted, generated, test methods waveforms used, human body machine models testing, compliance levels, contact discharge. Protection methods described Maxim's approach protection detailed. Guidelines given selecting with high resistance ESD. RS-485 ports analog switches multiplexers specifically considered. Electrostatic discharge (ESD) often found root cause equipment failure factory floor field. Such failures difficult track down when they masquerade other types failures, they often manufacturing, instance, yield loss final test might traced component subassembly then (with further investigation) associated with manufacturing test process that subjects part ESD. Early field failures post-installation problems industrial equipment often caused during installation. most insidious damage that which degrades performance instrument least initially) does affect operation obvious way. Such events cause erratic nonlinear operation immediately produce "hard" failure months years. This article explains protect your products from ESD. outlines standard test methods required electronics industry, offers some common techniques protect against ESD, highlights some Maxim's patented ESD-protected devices. With careful design, these devices improve product quality while saving money your company's reputation.
Damage Potential
photomicrograph (Figure shows damage sustained competitor's RS-232 interface after exposure strike 15kV common test level). result gross failure that easily visible, because overstress actually vaporized metalization chip. other cases, investigation invisible failures gate-oxide layers buried layers requires careful removal metalization other layers. strikes also find paths into core instrument. Somewhat like lightning strike, will course through circuit until energy dissipated, often with unexpected effects.
Figure This photomicrograph shows gross damage unprotected RS-232 receiver.
Where Does High Voltage Come From?
Mechanically separating materials creates electrostatic charge. surfaces these otherwise neutral materials electrically double-layered varying degrees, meaning that outer layer might have www.maxim-ic.com/an639 Page
majority electrons balanced positive charge bulk material. Other materials exhibit opposite surface charge. When materials with opposite surface charges come contact, transfer electrons leaves material with negative charge other with positive charge. Called triboelectric generation, this effect basis static generation transfer. triboelectric series shown Table positions materials like glass nylon positive silicon teflon negative end. materials' conductivity also affects their ability build surface charge. many materials, conductivity surface resisitivity strongly dependent humidity. humidity promotes conductivity, which maintains localized charges preventing them from moving. Table Triboelectric Series (most pos.) Hands Lead Asbestos Silk Rabbit Aluminum Glass Paper Mica Cotton Human hair Steel Nylon Wood Wool Amber Sealing Hard rubber Nickel, copper Brass, silver Gold, platinum Sulfur Acetate, rayon Polyester Celluloid Orlon Saran Polyurethane Polyethylene KEL-F (CTE) Silicon Teflon (most neg.)
real world, static high voltage produced usually interaction people their surroundings. Imagine person sitting formica table plastic chair, wearing wool slacks socks, leather shoes, cotton shirt, silk tie. This soup triboelectric materials defies quantitative analysis, imagine responsible some serious charge buildup. Several accepted models describe charged human body different situations. most generic model (Figure assumes 100pF capacitance charged 15,000V 1500 series resistance.
Figure When discharged, this circuit (the Human Body Model) produces very fast rise time with current peaks 15kV/1.5k over 10A!
Test Methods Standards
methods used commonly testing susceptibility integrated circuits have been adapted end-equipment testing, outlined next section. oldest method, MIL-STD-883 Method 3015.7, developed understanding precautions necessary packaging handling ICs. This method tests each package against other groups pins classifies device according lowest voltage which failure occurs. applied signal this test current waveform derived from circuit called Human Body Model (Figure which simulates capacitance source impedance typical human body. Circuit layout critical, because actual waveform delivered also depends parasitic inductance capacitance associated with test connections board. resulting current waveform represents that occurs when person touches object such other method, which differs from above only values developed Electronic Industries Association Japan (EIAJ). Called IC-121 based circuit called Machine www.maxim-ic.com/an639 Page
Model (Figure applies current waveform similar that produced when makes contact with handling machinery. mimicking events caused charges that accumulate moving parts, waveform simulates static discharges seen during machine assembly.
HUMAN BODY MACHINE MODEL MODEL COMPON 1000-4-2 (MIL-STD 883, (IC-121, EIAJ) METHOD 3015.7) 100M 1500 (pF) 100±10% Figure Substituting different component values shown yields discharge circuits known Human Body Model, Machine Model, 1000-4-2 Model (human holding metallic object). methods complementary, shouldn't choose over other. Because affect during manufacturing, during board assembly, after product into service, test based Human Body Model Machine Model together provides adequate assurance regarding IC's tolerance rigors manufacturing product life. These tests ICs. Other specific tests govern exposed interfaces equipment. example, pins exposed outside world through connectors encounter even when mounted board within enclosure. exposure less likely other pins, which connected circuitry board. this class test method such Method 3015.7 (which tests combinations) does provide adequate representation susceptibility input/output (I/O) pins. Machine Model Method 3015.7, which offer ratings according lowest voltage failure pin, justice higher levels internal protection required pins (and provided some manufacturers). device might have pins that withstand ±15kV non-I/O pins that fail ±2kV. With methods described above, resulting rating could less than ±2kV. Fortunately, however, better test methods available rating pins.
Tests Ports
port allows communication with other pieces equipment. ports comprise logical groups pins that give access equipment external system containing These pins subject static discharge other abuse, operators connect disconnect cables from system. pins external interface ideal test method susceptibility would accomplish following: Test pins only ways that simulate exposure events actual equipment. Apply test waveforms that model electrostatic discharges produced human body. Different circuit models specify different values amplitude, rise/fall time, transferred power.
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Test with without power applied. Define failures include latchup momentary loss operation) well catastrophic parametric failure. Latchup considered failure mechanism, because, left undetected, lead reliability problems system malfunctions.
methods, both compliant with requirements listed, have seen increasing equipment manufacturers testing susceptibility ports (Figure first modification Method 3015.7, MIL-STD-883. makes same circuit model waveform original method applies pulses only pins device. intent simulate fault currents seen installed board operating target system. second method 1000-4-2, which become world standard. specifies higher capacitance lower resistance than that specified Method 3015.7. 1000-4-2 applied universally testing end-equipment interfaces (Table Note that Maxim's analog switches RS-232/RS-485 interface comply with these standards, without need external components. Table 1000-4-2 Classification Four Voltage Ranges 1000-4-2 Test Voltage, Test Voltage, Compliance Contact Discharge Discharge (kV) Level (kV)
ESD-Protection Methods
Protecting interface from damage designer's responsibility. industry offers choice several methods, each with certain strengths weaknesses. Lots misunderstanding black magic surround remedies; following discussion intended dispel some that mystery. Capacitor Protection This method common high-volume consumer automotive equipment. protects input pins with simple shunt capacitor from input ground. idea that capacitor sufficient value will absorb discharge without exceeding rating attached pin. illustrate, consider exposed outside world, with rating 2kV. 1000-4-2 Model specifies 150pF capacitance charged 15kV. 1500pF capacitance added exposed pin, will charge maximum equal 1/10 test voltage (1.5kV). Because this level below IC's ESD-protection rating, assumes that well. This method used widely, involves simplistic view physics involved. gives some protection exercise extreme care circuit layout, provided that circuit operation affected either necessary capacitance parasitic inductance. emphasize sensitivity layout, consider that trace board about inductance. When current pulse with rise time (the 1000-4-2 waveform, Figure applied 7nH, produces voltage spike 210V each centimeter ground path.
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Figure Parameters this waveform (rise time, peak current, amplitude 30ns, amplitude 60ns) specified 1000-4-2. Resistor Protection Resistor protection added series interface pins. This resistance limits peak currents helps dissipate some power transient. with capacitor protection, precautions should taken ensure that circuit operation affected adversely this increased impedance. Another caveat that resistors themselves ESD-sensitive. Metal-film resistors fabricated with methods similar semiconductor metalization often have similar limitations. parasitic capacitance these series resistors also issue. narrow spike, through even picofarads, wreak havoc input. Resistor-Capacitor Protection This approach hybrid protection techniques discussed above. Having components interface consumes real estate while increasing costs decreasing reliability. networks often used suppression concert with ferrite beads feedthroughs. MOVs TransZorbProtection Adding metal-oxide varistors (MOVs) silicon avalanche suppressors (TransZorbs) pins very effective. They tend expensive, large, unwanted capacitance interface. with capacitor protection, these devices require low-inductance (short) paths ground. Layout Guidelines Enhancing Protection Follow standard analog-layout techniques, placing bypass charge-pump capacitors close possible (interface especially). Include ground plane board. Place protection close port possible.
Protection Internal Maxim
Maxim invested substantial effort developing with internal protection. Starting with RS232 RS-485 interface ICs, these protected devices include several analog switches MAX681_ family switch debouncers. withstand application 1000-4-2 events directly
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their pins. Maxim believes this best control system. robust, readily available, requires external real estate, costs less than most alternatives. Maxim offers variety ESD-protected RS-232 interface ICs, representing every useful combination drivers receivers. Included ultra-low-power RS-232 devices incorporating Maxim's AutoShutdownfeature. Several innovations have been introduced area ESD-protected RS232 interface devices this year. example, single RS-232 receivers transmitters with full protection tiny packages (the MAX318_ series) real problem solvers small portable equipment. Also scheduled introduction unique ESD-protected interface data cables. Data cables cellular phones usually include RS-232 interface cable rather than phone. Thus, protection this application required just RS-232 side interface logic side well. MAX3237E only available that provides solution this problem. offers complete fivetransmitter, three-receiver interface (like modem). Maxim plans introduce additional RS-232 interface with this double protection other applications such cradles other phone configurations.
RS-485 Interfaces
Maxim also leader ESD-protected RS-485 interfaces. After pioneering protection such devices, Maxim offers products this area, with many more way. Significant over last year introduction MAX3095/MAX3096, which extend Maxim's robust structures low-power operation venerable 26LS32 quad-receiver pinout. Also released last year full line 3.3V, ESD-protected, RS-485 interface ICs. Maxim innovations MAX348_E family, example, include protection, fractional unit loads, slew-rate limiting, power.
Analog Switches
Maxim first company recognize value protection analog switches multiplexers that serve interface external systems host applications which protection necessary. First series ESD-protected switches multiplexers. This series includes several low-voltage with ±15kV protection: 8-to-1 CMOS analog multiplexer with 4051 pinout (MAX4558), 4-to-1 CMOS analog multiplexer with 4052 pinout (MAX4559), triple SPDT switch with 4053 pinout (MAX4560), SPST, CMOS analog switch with 4066 pinout (MAX4551). second series devices, SOT23 packages with ±15kV protection, includes single SPST-NO switch (MAX4568), SPST-NC switch (MAX4569), SPDT switch (MAX4561). References Maxim Engineering Journal #25, "ESD Protection Ports." Electrostatic Discharge Control, Owen McAteer, McGraw Hill. 1989. ISBN 0-07-044838-8.
TransZorb trademark General Semiconductor Industries, Inc. AutoShutdown trademark Maxim Integrated Products.
EJ40-2, August 2000
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