## [SI-LIST] Re: Driver strength and trace capacitance

• From: "Mangipudi, Prasad" <Prasad_Mangipudi@xxxxxxxxxxx>
• To: <Ravinder.Ajmani@xxxxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
• Date: Tue, 30 Aug 2005 17:13:13 -0700

```Ravinder,

Since the drive strength is high, the rise and fall times may be small
and this increases the knee frequency (0.5/tR or .5/tF whichever is
more, where tR is the rise time and tF is fall time).  The trace will be
considered distributed if it is longer than L/6 where L is the length of
the rising edge =3D tR/D where D is the delay in ps/in.  If the rise =
time
is approximately 1 ns, then traces longer than 1" are considered as
distributed.  The above formulas (1.1, and 1.3 ) are given in Howard
Johnson's book "High Speed Digital Design" and you can do your math.

Regards,
Prasad

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Ravinder.Ajmani@xxxxxxxxxxxxxx
Sent: Tuesday, August 30, 2005 4:11 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Driver strength and trace capacitance

I was simulating a 2" long net driven by an ASIC and noticed significant

ringing and non-monotonicity.  This is a bidirectional net and the
driver=20
at the other end is a 50 ohm driver, which does not require any=20
termination.  From the IBIS model I deduced the driver impedance to be=20
about 3 ohms, which I thought was quite low.  I was told that the ASIC=20
vendor was asked to design the driver to drive a 100 MHz signal in to a
35=20
pF load.  The trace capacitance is about 10 pF (including vias), and the

receiver capacitance is less than 5 pF (package plus die).  To me this=20
appears to be a case of over-design.  However, I have one question about

the trace capacitance.  Can it be considered a lumped load or a=20
distributed load.  The Lambda/20 length at 100 MHz is about 3" so I am=20
inclined to treat the total trace capacitance as a lumped load.
Regards, Ravinder
Server PCB Development
Hitachi Global Storage Technologies

Email: Ravinder.Ajmani@xxxxxxxxxxxxxx

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```