[SI-LIST] Re: subscribe: drive ability

Get the SPICE or IBIS model and simulate it.  Output impedance of the driver 
is what really matters.  If you take the SPICE model and determine the 
output impedance you will find it has little to do with the drive current 
rating, especially comparing supplier to supplier and technology to 
technology.  The drive current rating is a DC parameter that finds when Vohl 
and Volh are reached.  I would not use it unless you are designing a DC 
circuit.  You can also get a rough idea of what the output impedance is by 
looking at the I-V curve in the IBIS model file.

Dennis


----- Original Message ----- 
From: "Zhao Yu" <tianfeiyu@xxxxxxxx>
To: <si-list@xxxxxxxxxxxxx>
Sent: Tuesday, January 25, 2005 9:51 AM
Subject: [SI-LIST] subscribe: drive ability


> dear sir,
>      I'm confused with the drive= ability calculation . When designed
>  the circuit, we have to con= sider the drive strength of the drive
> pin. It is said that the maximum = output current of drive pin should
> be more than the sum of all input pi= n driven.   But i don't know
> which current i should use to ca= lculate. Maybe we can divide all the
> time into two phase: the transient= phase and the stable phase. In the
> transient phase, the switches are t= o be closed and all of the
> parasitical capacitors , include output pin = parasitical capacitor,
> transmission line ,input pin parasitical capacit= or and so on, are
> charged. During this time, the value of current is la= rge, but the
> persistence time is short.  In the stable phase, the = current in the
> circuit is only the leakage current of CMOS gate and its= value is
> small ,but it lasts a long time.
>
>   &nb= sp; When i calculate the drive  ability, which current should be
> u= sed,the transient current or the stable current.
>
>   &= nbsp;           &nbs= p;    Best wishes .
>
>
> --=20
>
> ___________________________________________________________
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