Subject: Re: Confused in Massachussets, 2n7000 data sheet
Date: Tue, 07 Jan 2003 20:11:06 +0100
Organization: Wanadoo, l'internet avec France Telecom
NNTP-Posting-Date: 7 Jan 2003 19:11:07 GMT
X-Mailer: Mozilla 4.79 [en] (X11; U; Linux 2.4.20 i686)
Robert Strand wrote:
> Hi Win,
> > Page 4 has a graph, Normalized Drain Source Resistance vs Temperature.
> > The x-axis is temperature and y-axis shows r_DSn = r_DS(Ta)/r_DS(25C),
> > the ratio of Rds(T) to Rds at 25C, which is fine. We see four plots,
> > labeled delta-V 0.5V, 1V, 2V and 3V, where delta-V = V_GS - V_GS(To),
> > and also V_GS(To) at I_D = 1mA. Whew, what's that about? Are they
> > showing the variation of R_ds(on) with temperature for four different
> > gate voltages, above some (linear operation) gate-voltage threshold?
> > I.e., if it takes Vgs = 2.0V to get 1mA linear conduction, then are
> > these Rds(on) plots for Vgs = 2.5, 3.0, 4.0 and 5.0 volts?
> Win, your interpretation is correct. The both Vgs and R_ds(on) are
> normalized. V_GS(To) is the voltage when Id=1mA, what is unclear is whether
> this is at TA or 25C. I have a feeling it's a 25C
I believe you meant TA, correct? Since V_(GS)(TA) has a negative tempco,
V_(GS)(TA)+deltaV_0 decreases with TA and can compensate (over large delta TA)
the positive tempco of I_ds at a fixed V_GS, for some delta V_0 such that
V(GS)(25°C)+delta V_0 is below the 3.7-4.3 v threshold (which one can see,
e.g. on the OnSemi's datasheet). If V_GS(TO) was for 25C then
V_GS(TO)+delta_V would be a fixed voltage, and we won't get zero tempco for
delta_V below 1.5V.
So "TO" is probably a typo for "TA". I wonder however about the utility of
this graph compared to the
graph of the family of the transfer characteristics at different temperatures.
> and this might explain
> the negative R_ds(on) TC for low deltaV, since V_GS(To) has a negative
> tempco, and also the linearization of R_ds(on) with T at higher deltaV.
> R_ds(on)(25C) is the resistance at 25C and Vgs = deltaV + V_GS(To),
> R_ds(on)(TA) is the resistance at TA and delta Vgs = deltaV + V_GS(To).
> > Page 5 has
> > an Rds(on) vs. Vgs plot; here Rds plots to about 13, 8.5, 6 and 5 ohms
> > respectively. The page 4 plot shows a strong negative Rds tempco for
> > ...etc.
> Agreed on those points as well.