1. BIRD98 Effective current drawing dependence from Vds
Antonio Girardi
Giacomo Bernardi
Roberto Izzi
STMicroelectronics
Flash Memory Group
R&D CAD

2. Gate Modulation Proposal
The ST “Gate Modulation” solution is based on the introduction of two coefficients, one for the Pullup and one for the Pulldown stage, which modulate properly the IBIS standard current (I_IBIS-STD) when a bouncing noise occurs on the power and ground nodes.
I(Vgs, Vds) = Kssn(Vgs,Vds)*I(Vgs=VDD,Vds)
Effective SPICE current
IBIS standard current
I_effective = Kssn(Vgs,Vds)*I_IBIS-STD
March 9, 2007

3. Effective Currents Drawing
Pulldown I(Vgs, Vds=VDD) table extraction
Pullup I(Vgs, Vds=VDD) table extraction
Vsweep=[-VDD, VDD]; Vgate is kept stable -- the collected effective current is related to the source voltage (Vs) changes
The effective current (I_effective) must be drawn within the saturation zone
The above circuits show a possible approach based on holding Vds=VDD
Is the Vds=VDD choice the best one for drawing the I_effective current?
March 9, 2007

4. Test-case Analyzed
Pulldown stage of a production device (130nm) supplied by VDD=1.8V. The Id-Vds characteristics are the following
VGS=2.2V
VGS=2.1V
VGS=2V
VGS=1.9V
VGS=1.8V
VGS=1.7V
VGS=1.6V
VGS=1.5V
VGS=1.4V
VGS=1.3V
VGS=1.2V
VGS=1.1V
VGS=1V
VGS=0.9V
March 9, 2007

5. Effective Current Extraction: Three cases of Vds=constant have been compared
The effective current (I_effective) has been drawn by considering three different values of Vds, within the saturation zone.
Case 1: Vds=VDD=1.8V (current choice proposed)
Case 2: Vds=1.55V
Case 3: Vds=1.3V
Case 3 Vds=1.3V
Case 2 Vds=1.55V
Case 1 Vds=1.8V
March 9, 2007

6. Three Cases’ Effective Currents Comparison
I_effect_vds_18 I_effect_vds_155 I_effect_vds_13
Current (A)
Vsweep voltage (V)
These three cases don’t show a remarkable difference among the currents
March 9, 2007

7. Error Evaluation
The maximum error is less than 3% and it is ~0% in the region in which the first bouncing occurs (Vsweep > 0)
Obviously, the error is related to the channel modulation effect: the lower is such effect, the lower is the error due to the choice of the Vds constant value within the saturation zone
Nevertheless, in the majority of cases, the MOS of push-pull buffers are chosen and designed in order to present a slight channel modulation effect
In addition, considering that the higher is Vgs, the shorter is the saturation region, the best choice may again be Vds=VDD
March 9, 2007

8. Three Cases Output Signal Comparison
Three cases’ output signals comparison by simulating a simultaneous switching output (SSO) by using the VHDL_AMS IBIS implementation. Even the output signals don’t reveal critical discrepancies.
Vout_case1
Vout_case2
Vout_case3
March 9, 2007

9. Effective Current Drawing by Changing Vds
It has also been explored the case in which the Vds voltage is linearly changed within the saturation zone during the effective current drawing
VGS=1.8V
VGS=1.7V
VGS=1.6V
VGS=1.5V
Vds changing
VGS=1.4V
VGS=1.3V
VGS=1.2V
VGS=1.1V
VGS=1V
VGS=0.9V
VGS=0.8V
March 9, 2007

10. Effective Current Dependence from a linear Vds
Comparing the effective current drawn with the one related to Vds=VDD=1.8V, it is observed a good matching between them. The same considerations of the slide 6 are confirmed too.
I_effect_vds_18 I_effect_vds_linear
March 9, 2007

11. Gate Modulation Coefficient Curves
Taking into account the curves of the gate modulation coefficient (Kssn), related to the production device under analysis, it can be noted that in a very large region the Kssn coefficient may be considered independent from the Vds changes. This behavior is an additional confirmation of the previous considerations and results.
VGS=2.2V
VGS=2.1V
VGS=2V
VGS=1.9V
VGS=1.8V
VGS=1.7V
VGS=1.6V
VGS=1.5V
VGS=1.4V
VGS=1.3V
VGS=1.2V
VGS=1.1V
VGS=1V
VGS=0.9V
March 9, 2007

12. Conclusions
The choice of Vds values, constant or no, within the saturation zone, don’t impact on the effective current values if the channel modulation effect is slight
The above statement is also confirmed by observing the Kssn curves
As a result, the proposed choice of holding Vds=VDD during the effective current drawing may remain a good one
Nevertheless, in case of no negligible channel modulation effect, the choice of drawing the effective current by changing linearly the Vds voltage may better interpolate the error due to such effect.
A final decision should be make.
March 9, 2007

13. Effective Current Drawing
The same results can be observed considering another device, even in 130nm technology.
I_effect_vds_18 I_effect_vds_155 I_effect_vds_13
Back-up Slide
March 9, 2007

14. Output Signals Comparison
A good matching is again observed.
Vout_case1 Vout_case2 Vout_case3
Vout_case1 Vout_case2 Vout_case3
Back-up Slide
March 9, 2007