Can someone explain the FET problems when moving to QDR memory?

[MadRat]

1. Perhaps someone can explain a FET?
2. Why would QDR require FETs?
3. What would have to be done to avoid a FET solution?

Thanks in advance.

 

[Superdoopercooper]

1) A FET is a Field Effect Transistor. Basically, it is sub-micron transistor on serious steroids.

2) A FET is probably needed due to the current requirments needed to drive the QDR bus at such high speeds. A small RAM module just can't effectively drive enough capacitive load at that speed to make it affective. So, the FET is likely used as a current buffer... it requires approx. ZERO input current to switch... but it has a strong drive capability. Think of it as a power amp.

3) The ram could be laid out so that it had more power (larger output stage transistors). But there are problems... first... there are 168-pins to buffer... maybe half of that per chip. So, if you build that into each RAM chip... now you have more die area where a defect can occur... so your RAM yields will suffer. Second... you still might not be able to effectively make enough drive capability on the RAM die... why? Well.. they are made on 0.18 micron... or 0.25u technologies... maybe a little larger, but not much. FETs are made in 2-5u technology... and are more robust, can handle larger voltages and currents before they breakdown. The 0.18 sized stuff has a max voltage of probably 2.8 or 3.3V, and the Gate Oxide layer is so thin that it doesnt take much to punch through and kill the die, or much current to kill a drain/source.

The "constant current" equation, which is a useful quick and dirty calculation for figuring average current needs... says i (current) = C (capacitance) * V (voltage) * f (frequency).

So, if we're operating at 300MHz, have a voltage of 3.3, and are driving 20pF... then that means we need "constant current" of 300e6 * 3.3 * 20e-12 = 19.8 mA if I do my math corectly. Well.. think.. .you have that average current... times voltage, times X number of pins on the device.... say 50. Well.. if you total all that up... you're looking at some hefty power requirements and SERIOUS heat generation. I bet even when buffered by a FET, the ram modules will still need some passive heatsinks on them.

Hope this makes sense. I'll probably read it tomorrow afternoon when I'm not so tired... and see if I was an idiot.

 

[MadRat]

So basically the manufacturers either make each pin-to-RAM pathway able to handle more load, or they add more pins?

 

[Superdoopercooper]

Hmm... maybe I jumped the gun on my first post. I'm was kinda posting out of "in general why would you need a FET for this particular application", rather than actually knowing somehting about the actual techology to which you are referring.

I know nothing about QDR, and where the extra FETs are needed. I'm assuming their needed on the access ports (I/O) of the QDR ram modules?!

 

[MadRat]

hehe

You probably know more than you give yourself credit.