Using BUS-Port Protection Arrays in Portable Electronics for Active ESD Protection

Page Contents:   [ Featured Products | Datasheets | Application Notes | Buy Now ]

Z-Diodes* are widely used for voltage regulation across a circuit. Unlike conventional solid-state diodes, Z-Diodes are designed to have a greatly reduced line impedance, permitting current to flow in the forward direction and also in the reverse direction if the voltage exceeds the breakdown voltage.

A critical parameter of a Z-Diode made for overvoltage protection is the ESD immunity and surge current the diode can short to ground without being destroyed, a parameter that can be adjusted by the size of the silicon chip. As a rule, the bigger the diode’s active area, the higher the amount of current the diode can draw to ground. However, the active area is also a factor in the diode’s capacitance, with the capacitance increasing along with the size of the active area.

Power is not the only parameter that needs to be taken into consideration, however, when selecting the best protection device for a particular application. For example, diode A has a maximum clamping voltage of 30V and a peak pulse current of 5A (30V x 5A = 150W). Diode B has a maximum clamping voltage of 15V and a peak pulse current of 7A (15V x 7A = 35W). Diode A has the higher clamping power, however, the higher peak pulse current combined with the lower clamping voltage makes Diode B the better protection device.

Many portable electronics applications require ESD protection in excess of 8kV, while at the same time maintaining a capacitance lower than 5pF. Due to the relationship between active area size, ESD immunity and capacitance in Z-Diodes, they are incapable of meeting this requirement. However, with a low capacitance PN-diode, such as a switching or junction diode, used in conjunction with a Z-Diode, both requirements of the application can be met: low capacitance with high ESD and surge immunity.

This combination of a Z-Diode with a small PNdiode provides a unidirectional protection device. A clamping current can only flow in one direction, which is the forward direction of the PN-diode, as the reverse path is blocked (see Figure 1).

 

 

Adding another PN-diode opens the back path so that the protection device becomes a bi-directional protection device. As the clamping voltage levels in forward and reverse directions are different, such a device has a bi-directional and asymmetrical clamping behavior (BiAs) (see Figure 2).

 

 

At the very first moment, when all three diodes are completely discharged (meaning the diode capacitances are empty), the first signal pulse with a 0.5V amplitude will drive the upper PN-diode (D1) in a forward direction, filling or loading the empty, big capacitance of the ZDiode (ZD). Depending on the duration of the pulse and the pause until the next one, the Z-Diode’s capacitance is already charged up to a higher level so that the next pulse has less capacitance to charge up. After a few pulses, the Z-Diode is completely charged, so that the following pulses just “see” the low capacitance of the two PN-diodes.

 

 

For some portable electronics applications, a varying capacitance is not a problem, but for others the capacitance must be the same for every pulse. For these applications, the ZDiode can be connected to the supply voltage, such as the VBUS at the USB port (see Figure 3). In this setup, the supply voltage charges the Z-Diode up and both PN-diodes are always in reverse mode, which keeps their capacitance at a minimum.

 

 

The diode array shown in Figure 4 (VBUS054BHS3), which is intended for bus port protection, can protect a double, high speed USB port against transient voltage signals. The array clamps negative transients close below the ground level while positive transients are clamped close above the 5V working range.

A Z-Diode clamps the supply line (VBUS at Pin 5) to ground (Pin 2). The high speed data lines (D1+, D2+, D1- and D2-) are connected to Pin 1, 3, 4 and 6. As long as the signal voltage on the data lines is between the ground- and the VBUS-level, the low-capacitance PN-diodes offer a very high isolation to VBUS, ground, and to the other data lines. However, as soon as any transient signal exceeds this working range, one of the PN-diodes reverts to forward mode and clamps the transient to ground or the avalanche breakthrough voltage level. As a result, the VBUS054B-HS3 can offer a high ESD immunity of ±15kV while offering a typical capacitance of <1pF for portable electronics applications.

The VBUS054B-HS3 is a single chip solution, so the differences among line capacitances is very low, which is important for the two data lines D- and D+. This “data line couple” transmits the same data pulses at the same time, but with opposite polarity. The higher the symmetry of both signals on the two data lines, the higher the data rate can be.

In any PCB layout, it is important to take into consideration that ESD protection devices can only control or clamp the voltage level directly at its own terminals (pins). Any impedance between these terminals and the ports that have to be protected can reduce the protection performance due to additional voltage drop across these impedances.

A T-routing (as shown in Figure 5) creates a serial impedance (L3 and L4) to the protection diode that prevents the optimal clamping of transients. A V-routing (as shown in Figure 6), however, eliminates such serial impedances (L3 and L4).

 

Figure 5: T-routing creates a serial impedance serial impedances

Figure 6: V-routing eliminates a serial impedance serial impedances

 

 

*Z-Diode is a common name for Zener and avalanche diodes, both of which show the typical “Z” shape for their voltage-current characteristic. While Zener diodes have a “soft” Zener-break-through, or tunneling effect, avalanche diodes have a “sharp” avalanche break-through. The junction from the Zener to the avalanche range is at about 6V.

 

 

 Featured Products

Part Number		Description	Data Sheet	App. Notes
VBUS054B-HS3		4-line BUS-port ESD Protection

refers to New Product Introduction