Subject: IBIS EGG9 - Stored Charge

From: [email-masked] ( Bob Ross)
Date: Sun, 10 Dec 95 18:45 PST

To IBIS members:

Attached is an EGG concerning an old topic - Stored Charge effects in Diodes.
Because of stored charge, a clamping diode will not release instanteously
from the clamped mode as predicted by IBIS models.  There will be a delay
while the diode is turning off.  This creates a glitch which itself can
propogate up and down a line.  This glitch is significant enough that IBIS
should support a way to model its effect.

The major contributor to the effect is the TT parameter which contributes
a non-linear transit time capacitance when a diode is conducting.  The
proposal here is to use a similar parameter by adding [TTgnd] and [TTpower]
keywords to the [Model] keyword description.

Bob Ross,
Interconnectix, Inc.

EGG9 - Proposal for handing Stored Charge Effects:

|==============================================================================
|    Keywords:  [TTgnd], [TTpower]
|    Required:  No.
| Description:  The data for these keywords enters the transist time parameters
|               to estimate the transit time capacitances for the [Gnd Clamp]
|               and [Power Clamp] tables.
| Usage Rules:  For each of these keywords, the three columns hold the transit
|               values corresponding to the typical, minimum and maximum
|               [Gnd Clamp] or [Power Clamp] tables, respectively.  The
|               entries for TT(typ), TT(min), and TT(max) must be placed on
|               a single line and must be separated by at least one white
|               space or tab or tab character.  All three columns are 
|               required under these keywords.  However, data is required
|               only in the typical column.  If minimum and/or maximum values
|               are not available, the reserved word "NA" must be used 
|               indicating the TT(typ) value by default.
| Other Notes:  The transit time capacitance is added to C_comp.  Its value
|               value Ct is approximated by Ct = TT * (q/kT) * Id where TT
|               is transit time, q is electron charge, k is Boltzmann's 
|               constant, T temperature in degrees Kelvin, and Id is the 
|               dynamic diode current when the diode is conducting.  Id can
|               can be found from the [Gnd Clamp] or [Power Clamp] operating
|               points.  If the [Temperature] keyword is not defined, use
|               the default temperature for all Ct calculations
|  |------------------------------------------------------------------------------
| variable      TT(typ)         TT(min)         TT(max)
[TTgnd]         10n             12n             9n
[TTpower]       12n             NA              NA
|
|==============================================================================

Refer to Spice diode reference information concerning the complete equations.
The emission coefficient (N) is assumed to be 1.  For measured values or
cases where N is not 1, use the effective TT values.  So the TT values
may not be the exact values used in a Spice model.

The Spice diode and transistor models differs slightly from the IBIS Clamp
models.  The definition and position of the capacitances is different.
Furthermore, the IBIS table combines the diode and resistor, but can support
more complex non-linear characteristics.
                          
                   Intrinsic
             RS      Diode                        +-----+
                      |\ | ----> Id               |     | ----> Id
       o---/\/\/\--+--| >|--+----o          +-----|Clamp|-----+
                   |  |/ |  |               |     |     |     |
                   |        |          o----|     +-----+     |----o
                   |   | |  |               |       | |       |
                   +---| |--+               +-------| |-------+
                       | |                          | |

                     Cj + Ct                     C_comp + Ct

Fixed values of C_comp serve to estimate voltage dependent non-linear
junction capacitance plus other metalization effects

The Ct value is a function of absolute temperature.  As an approximation,
the default typical temperature is sufficient if [Temperature] is not
specified.  It is recommended to include the [Temperature] keyword when
TT is specified to remove ambiguity regarding minimum and maximum transit
time calculations for CMOS versus Bipolar technologies.

An underlying assumption is that the equations will be applied only to
the Clamping data, not the combined data that includes [Pulldown] and
[Pullup] data.  So when this detail is needed for Output models, the
clamping data needs to be derived, if not provided.

Because of these differences and possible missing details, the simplified
equation and approximation approach is justified to capture the dominant
behavioral effects.  So the TT values may be the effective values 
consistent with the IBIS model and data.

There are several formatting choices to describe transit time:

(1) A [Model] sub-parameter similar to Vinl and Vinh, e.g.,

TTgnd   = 12n
TTpower = 10n

This was not chosen because minimum and maximum process Spice models may have
different values.

(2) A [Model] sub-parameter similar to C_comp, e.g.,

TTgnd      10n       12n        9n
TTpower    12n       NA         NA

This was not chosen because the rules for minimum and maximum columns for
C_comp are by magnitude, whereas the rules for TT are by process extremes.

(3) A [Gnd Clamp] and [Power Clamp] sub-parameter similar to C_comp, e.g

[Gnd Clamp]
TT        10n       12n        9n

This was not chosen for the same reason as (2) and because of [Gnd Clamp]
and [Power Clamp] table complexity.

(4) A [Model] keyword modifier similar to [Temperature] and other keywords
which are a function of typical, minimum and maximum process and measurement
conditions. e.g.,

[TTgnd]         10n             12n             9n
[TTpower]       12n             NA              NA

Since these keywords are related to the [Gnd Clamp] and [Power Clamp] 
tables and to the conditions under which they were derived, this format was
chosen.

---

From: [email-masked] (Jon Powell)
Date: Mon, 11 Dec 95 08:19:50 PST

Do we already have a place for the Rs? The experiments I did last year when we
were talking about this indicate that Rs is essential (for at least) scientific
study of TT. It is unclear if Rs is ever very much bigger that 0 for real circuits.

jon

If people are looking for a good mathematical expression of this they can look in
any text book on SPICE under diode behavior. Working (and simple) equations are
presented.

                   Intrinsic
             RS      Diode                        +-----+

                     Cj + Ct                     C_comp + Ct