Active ESD protection using BUS port protection array

　　Many portable applications require ESD protection in excess of 8kV while keeping capacitance below 5pF. Z diodes alone can meet this requirement, but when combined with low capacitance PN diodes such as switch or junction diodes, they can meet both requirements for this application: low capacitance, and high ESD and surge immunity.

　　This combination of a Z diode and a small PN diode provides a unidirectional protection device. The clamped current can flow in only one direction—the forward direction of the PN diode—because the reverse path is blocked (Figure 1).

　　Adding another PN diode opens the reverse channel, making the protection device bidirectional. When the clamping voltage levels in the forward and reverse directions are different, the device exhibits bidirectional and asymmetric clamping behavior (BiAs) (Figure 1).

　　Initially, when all three diodes are fully discharged (diode capacitance is empty), the first signal pulse with 0.5V amplitude will drive the upper PN diode (D1) in the forward direction, thus filling or loading the larger empty capacitance of the Z diode (ZD). Depending on the duration of the pulse and the time until the next pulse is interrupted, the capacitance of the Z diode has already been charged to a higher level, so the next pulse charges less capacitance. After a few pulses, the Z diode is fully charged, so subsequent pulses only detect the lower capacitance of the two PN diodes.

　　For some portable applications, capacitance variation is not a problem, but for other applications, the capacitance must be the same for each pulse. For these applications, the Z diode can be connected to a supply voltage, such as VBUS of a USB port (Figure 1), where the supply voltage charges the Z diode and the two PN diodes remain in reverse mode, keeping the capacitance to a minimum.

　　The diode array (VBUS054B-HS3) shown in Figure 2 protects dual high-speed USB ports from transient voltage signals. The array clamps negative transients just below ground level and positive transients just above the 5V operating voltage range.

　　The Z diode clamps the power line (VBUS at pin 5) to ground (pin 2), while the high-speed data lines (D1+, D2+, D1-, and D2-) are connected to pins 6, 3, 4, and 1. As long as the signal voltage on the data line is between ground and VBUS levels, the low-capacitance PN diodes provide very high isolation from VBUS, ground, and other data lines. However, as soon as any transient signal exceeds this operating voltage range, one of the PN diodes returns to forward mode and clamps the transient to ground or avalanche breakdown voltage levels. As a result, the VBUS054B-HS3 provides high ESD immunity of ±15kV while providing a typical capacitance of less than 1pF that meets the requirements of portable electronic equipment applications.

　　The VBUS054B-HS3 is a single chip solution, so the difference between the line capacitances is very small. This is very important for the two data lines, D- and D+, because this “data line pair” transmits the same data pulses at the same time, but with opposite polarity.