Difference between LTZ1000A and LTZ1000

Hi 

We consider  LTZ1000 replacement to LTZ1000A.

Because LTZ1000A is getable.

Can we use with same characteristics in our considering Θja?

In datasheet, Θja is different and Icc cause temperature’s rising (equivalent to about 10°)

We have question about ltspice.

Please refer below result.

・heater is 0V

・7V pullup 70kΩ to 8pin collector 

・Base connect(pin 6) to pin8. 

・Temperature sweep -30 to 40.

・plot pin8 (Vf voltage) in each other.

Both of Vf are same.

Vf/℃=(614mV-520mV)/70℃=1.342mV/℃

This model isn't reflected on ICC consumption, is it? 

But if the heater is applied, 

LTZ1000A=2.23606V (root5) and LTZ1000=5V because LTZ1000A  Θja=400℃/W and LTZ1000 Θja=80℃.

pin8'(Vf) voltage have difference between LTZ1000 and TLZ1000A.

Their heater resistant is same in LTSPICE. (from heater current)

In same IC(LTZ1000 or LTZ1000A), each temperature characteristics is good.(changed heater voltage and Vf  )

But LTZ1000 vs LTZ1000A characteristics have any difference.（Heater power*Θja don't correspond to Vf ）

Are there any good way about these comparision?

Of course, Vf is exponential characteristics and the correspondance is difficult.

Best regards 

Thank you very much.