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Why some circuits have only one diode connected to VSS?

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John Xu

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esd diode to vss

We know there have two diodes which are conneced to VDD and VSS respectively to clamp the positive and negative pulse for the protected pin. But I noticed some pins in some cicruit has only one diode connectd to VSS. Anyone help to explain to me? In what conditions, only one clamp diode is enough?

Thanks
 

Re: the ESD diode

I think the two configures are for two kinds of ESD model.
 

    J

    John Xu

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the ESD diode

One clamp diode can use for the open drain structure pin. And the ESD current path of this pin to VDD is first from pin to vss, then from vss to vdd.
 

    J

    John Xu

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Re: the ESD diode

John Xu said:
We know there have two diodes which are conneced to VDD and VSS respectively to clamp the positive and negative pulse for the protected pin. But I noticed some pins in some cicruit has only one diode connectd to VSS. Anyone help to explain to me? In what conditions, only one clamp diode is enough?

Thanks
Click to expand...

Generally, this is not a good ESD design.
But I think there exits another current path. The path can locate on another pin which is metal-connected to this pin. You should check the whole esd network instead of only one pin.
 

    J

    John Xu

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the ESD diode

One diode to Vss can discharge both positive and negetive pulse.
 

Re: the ESD diode

The ESD diode to VSS is meant for Open-drain I/O only, found in older VLSI circuits in order to save space. Now it is very rare to use single ESD diode. With today's advanced nodes from 180 down to 65, all I/Os are using ESD diodes to VDD and VSS so as to provide adequate protection to CMOS circuits which are very sensitive static charges. Sometimes series resistors are added to the I/O for current-limiting, especially in Open-Collector Bipolar, but was an old method and hardly used today.
 

    J

    John Xu

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