Electronic Design

Diode Challenges Polymer Devices In High-Speed ESD Protection

The emergence of high-speed data protocols such as USB 2.0 has created the need for ESD protection devices with low capacitance—1 pF or less. Traditional methods of ESD protection like diodes and metal-oxide varistors are unsuitable in these high-speed applications because their high capacitance degrades signal integrity. A newer alternative, polymer-based ESD protection devices, have sufficiently low capacitance. But they suffer from high trigger voltages that could damage the ICs being protected.

California Micro Devices has developed a semiconductor alternative that provides low-capacitance ESD protection without the trigger voltage. The PicoGuard CM1210 family provides up to ±6-KV contact protection per the IEC61000-4-2 specification while exhibiting only 1 pF of channel input capacitance. These specs suit USB 2.0, IEEE 1394, and Gigabit Ethernet applications as well as DVI ports and antenna ports in cellular phones and wireless LAN designs.

The CM1210 is available in two-, four-, six-, and eight-channel versions. Each channel consists of a MOS-type diode pair. The CM1210 clamps ESD signals at one diode drop above VCC on positive pulses and at one diode drop below ground on negative pulses. In contrast, polymer-style ESD protection components will clamp an ESD signal at some value in the tens of volts range, but only after the pulse reaches a trigger value that may be as high as 1000 V.

Although many ICs are designed to survive 2-kV events as defined by the human body model, that standard for ESD protection is meant primarily to protect a chip in a manufacturing environment where stringent measures are taken to prevent electrostatic buildup. In particular, the human body model tests the IC with no power applied. As a result, it does not address the possibility of a potentially fatal ESD-induced latchup when the device is under power. Because of the polymer-based device's high trigger voltage, latchup could occur even with the protection device in place.

Another limitation of polymer ESD devices is that their behavior changes with repeated ESD events. Over time, their clamp voltage varies, while their leakage current typically goes up. Although the former effect is beneficial, the latter is not. According to Anthony Chan, director of design at California Micro Devices, "At some point the device is going to fail, probably as a short circuit." In contrast, the PicoGuard can withstand more than 1000 attacks and will not exhibit a change in diode characteristics, including leakage current. Other features include virtually zero insertion loss up to 3 GHz and subnanosecond response times

The CM1210 devices are available in SOT23, SOT143, SC70, and MSOP packages. In quantities of 10,000, pricing ranges from $0.26 for a one-channel SOT-23-housed device to $0.51 for an eight-channel MSOP unit. Products are currently sampling with production targeted for late in the second quarter.

California Micro Devices, www.calmicro.com; Joe Salvadore, (408) 934-3114; [email protected]

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