ESD protection with TVS diode array. What is this diode for?

Colleagues,

TVS diodes are used for ESD protection of signals, such as USB. One of the typical solutions consists of 2 diodes per line and one common Zener diode. Some app notes show additional diode anti-series with the Zener (highlighted with purple in the pictures). Not every application note shows it, though.

Unfortunately, I could find the explanation for it in the app notes themselves. What is this diode for :?:

Can this diode be detrimental? Suppose, a positive ESD pulse hits the GND, while the device is powered down. This extra diode becomes reverse-biased. The Zener doesn't conduct.

Any suggestion, insight or reference is really appreciated!

Cheers,
- Nick

fig.1 (Source: **broken link removed**)

fig.2 (Source: **broken link removed**)

kender said:

Colleagues,

TVS diodes are used for ESD protection of signals, such as USB. One of the typical solutions consists of 2 diodes per line and one common Zener diode. Some app notes show additional diode anti-series with the Zener (highlighted with purple in the pictures). Not every application note shows it, though.

Unfortunately, I could find the explanation for it in the app notes themselves. What is this diode for :?:

Can this diode be detrimental? Suppose, a positive ESD pulse hits the GND, while the device is powered down. This extra diode becomes reverse-biased. The Zener doesn't conduct.

Any suggestion, insight or reference is really appreciated!

Cheers,
- Nick

fig.1

(Source: **broken link removed**)

fig.2

(Source: **broken link removed**)

Click to expand...

It's for backdrive protection, more in the same TI note, page 9 ..
:wink:
IanP

IanP said:

It's for backdrive protection, more in the same TI note, page 9 ..
:wink:
IanP

Click to expand...

Thanks, I understand the backdrive issue.

But, doesn't this diode leave the circuit vulnerable to negative ESD spikes (negative spike into VBus, or positive spike into GND) :?: Of course, more Zener can be added to protect against that too...