This is a Big On-Chip Cap

[ebuddy]

I am looking at a chip spec. It uses a single wire to do both power supply and digital serial input/output. This pin, plus a ground pin, are all the pins this chip has.

In normal use, the power pin is connected to VDD thru a pull-up resistor. The digital signal can be transmitted on this same wire by pulling it low momentarily. When the power line is pulled low, the chip internal supply is provided with the on-chip parasitic capacitor holding the charges.

The interesting thing is, the spec claims the parasitic capacitor is 1000pf. How did they do that? The die size is roughly 2mm x 2mm. Even if the chip is filled with by-pass caps, and uses all the metals to do the cap, it is still hard to have such big cap. Any possible tricks?

 

[erikl]

The interesting thing is, the spec claims the parasitic capacitor is 1000pf. How did they do that? The die size is roughly 2mm x 2mm. Even if the chip is filled with by-pass caps, and uses all the metals to do the cap, it is still hard to have such big cap. Any possible tricks?

Could be an off-chip cap in the same package - below or above the silicon chip.
If not, I'd bet this is a typo: perhaps it's 1000fF = 1pF - still a lot, but a reasonable value. Did you try and measure it?

 

[dick_freebird]

I see about 1.5pF from a 50x50 MIM cap in my favorite
technology. That's about a 20V cap. I'd bet on a newer
technology using MOS caps you could get your 1000pF
in 50x5000um or so, meaning you could (say) pave the
underside of power busses and pads with MOS caps and
get it all for free in the I/O ring alone. At 8000um of
periphery and only 4 pads, there's a -lot- of periphery
to be used, that isn't (by pads alone).

I'm sure the cap doesn't have to stand off any more
voltage than the core, so no need for high voltage
low C-density structures.

 

[keith1200rs]

It depends on the voltage but you can get up to 9fF/sq um on 0.18um process so 1000pF is only around 0.35mm x 0.3mm. For higher Vdd the capacitance would reduce to maybe 5fF/sq um which is still reasonable. I have used 50pF floating poly-poly capacitors in 0.8um which have only been 0.2mm x 0.3mm.

Keith