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POwer clamp circuit in chip design

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Vivek Chandran

Newbie level 2
Aug 26, 2015
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Please explain me how the RC clamp circuit protects a chip from ESD?

What is the purpose of resistor and capacitor in power clamp circuit?

Why we need inverter chain in RC Clamp circuit?
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The RC clamp is usually connected to a big ESD transistor (the so called power clamp).

Whenever you get an ESD event, says between a supply and a ground, a lot of current will be forced into that supply, trying to find its way to the ground.

This huge current will trigger the ESD transistor, it will turn it on and this big transistor will absorb all the current, and send it to the ground, while keeping a VDS quite low, in the acceptable range of ESD voltages on the chip.

Regarding the inverter chain, could you maybe upload a scheme of your power clamp so we can discuss it?


I have never seen a capacitor in an ESD protection circuit, it would slow down the circuit too much.
A Cmos logic IC uses a series input resistor to limit the current and a diode to the positive supply and another diode to ground. Then the diodes limit (clamp) the input voltage to a diode drop above and below ground.

The "power clamp" is one piece of the ESD protection network.
In a "star" arrangement it would only protect the VDD-VSS
path. In a "ring" arrangement, common in "normal" digital ICs,
the ESD shunts steer current into the VDD rail and out through
VSS, with the rail clamp completing that current loop. ESD
design is all about the current loops, and whether along any
loop locally you can generate a destructive voltage across
a sensitive feature (overcurrent, that's another matter).

Draw out the entire pin-pin path for, say, an input#M to
output#N stroke, and see how in a VSS-diode-pin-diode-VDD
pin clamp design, your loop takes you either through the
supplied core (bad) or a supply-supply shunt (power clamp).

Active clamps may use a dV/dt trigger as part of the trigger
scheme (also wants a DC level trigger, to hold the clamp on
after the leading edge, else the HBM source's time constant
can hold back significant energy that might still damage the
part if dynamic trigger releases too soon. A C-R differentiator
plus a breakdown-R (or R-R divider) path, taper chain and
active shunt is not unusual. This is the probable function of
the capacitor mentioned.

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