Two questions about a ESD circuit

two questions about ESD

Hi all,

The attachment is a typical ESD circuit schematic for analog pin.The Dp1 and Dn1 is the parasitic diode in the drain of Mp1 and Mn1.There are two questions about such an ESD circuit:

1) what is the role of the resistor(Rn1 and Rp1)?and how to select their values?
2) Many papers mention that the W dimension of Mn1(Mp1) is very large(typically 200~400um) in order to sustain an acceptable ESD level. Why?

thank you very much.

Re: two questions about ESD

the two resistor is in order to decrease the MOS trigger voltage. because when the ESD pulse input, the gate voltage is also increasing through Cgd, then the Vgs is larger than VTH, the mos turn on. the resistor value maybe larger as possible, but they can't be too large to triger the mos when input normal signal.
The mos size determine the largest curent though mos, when exceed this current the mos will be destoyed.

Re: two questions about ESD

rambus_ddr is correct about the resistor. The resistor is used to develop a small gate voltage (~ Vt, threshold voltage) to help provide more uniform triggering of the transistors. If your transistor model accurately models the parasitic capacitance then you can size the resistor to produce between 1 and 2 Vt's on the gate in response to a 10 ns rise time on the drain voltage going from 0 to the breakdown voltage of the drain.

The NMOS & PMOS transistors under ESD operates as a diode if there is another clamp between the supply rails (Rail Clamp) otherwise they operate as breakdown/snap-back clamps to the respective supplies.

The NMOS device is typically made up of muliple gate strips to produce the desired width (i.e. 8x25um, 4x50, etc.). The snap-back action uses the parasitic NPN built into each NMOS device as the main current carrier during ESD. If the gate was grounded the drain would have to reach breakdown to turn-on. With some gate biase there will be some channel current and inturn some hot carrier substrate current. This substrate current is the base current in the NPN allowing it to turn-on earlier.

The large size is needed because ESD current is very large compared to typical operating current. A 3000 volt Human Body Pulse has a peak current near 2 amps.

Dr.Prof

two questions about ESD

hi,DoctorProf

as you said "The NMOS device is typically made up of muliple gate strips to produce the desired width (i.e. 8x25um, 4x50, etc.). 8x25um, 4x50 are both 4 multiple,if one choose 6x34,it is ok?

and if the analog pad is used not to connect op input but connect analog switch, must the switch be layouted with esd rule?
ss

Re: two questions about ESD

Make sure that current distribution is done evenly .

Re: two questions about ESD

resister function is to protect gate of mosfet cuz volatge feedthrough Cgs
esd mosfet's drain size is very large to get large current sot that fast discharge