Electronics Forum / CAD / August 2008

Hello Helmut

I did some more looking and I found a model in C.Basso's second book
"Switch-Mode Power Supplies Spice Simulations and Practical Designs".
I haven't had too much of an opportunity to test it, but it does
provide the correct reference.

This is the schematic that SUBCKT TL431 refers to:

http://i36.tinypic.com/1j0aq.png

This is the .txt file

.SUBCKT TL431 a k ref
C1 7 a 1n
Q1 k 2 1 QN3904
R1 1 7 240
R2 7 a 6.6k
Q2 k 7 a QN3904 50
D1 ref k D4mod
D2 a k D2mod
V1 5 a DC=2.49
D3 a 2 D3mod
C4 k 2 43p
G2 a 2 5 ref -250u
*
.MODEL QN3904 NPN AF=1.0 BF=300 BR=7.5 CJC=3.5PF CJE=4.5PF
+ IKF=.025 IS=1.4E-14 ISE=3E-13 KF=9E-16 NE=1.5 RC=2.4
+ TF=4E-10 TR=21E-9 VAF=100 XTB=1.5
.MODEL D3mod D N=0.01
.MODEL D2mod D BV=36 CJO=4PF IS=7E-09 M=.45 N=1.3 RS=40
+ TT=6E-09 VJ=.6V
.MODEL D4mod D BV=100V CJO=4PF IS=7E-09 M=.45 N=2 RS=.8
+ TT=6E-09 VJ=.6V
*
.ENDS

He shows good agreement with most of the graphs in the TL431 data
sheet.

I just changed the reference voltage (V15) to 1.24V. There doesn't
seem to be any significant difference between the LV version and the
standard version other then the REF voltage, bias current and max
cathode voltage.

He uses this TL431 model all through his book for small signal
simulations as well as DC transient for SMPS's.

Thanks for the response.