LTZ1000CH / LTZ1000ACH / LM399AH Heater temperature control stability - Page 1
Forum section: electronics / general

author: zhtoor
author_rank: Frequent Contributor
timestamp: April 10, 2018, 02:18:24 pm
content: -

author: Kleinstein
author_rank: Super Contributor
timestamp: April 10, 2018, 06:48:25 pm
content: One can calculate / simulate the typical LTZ1000 temperature control circuit part.The transistor has a gain of around 200, so at the collector the temperature sensor signal is about 400 mV/K.The heater is good for some 50 K at 5 V and thus a differential gain in the 10 K / V range, though dependent on the voltage as the heater is proportional to the square of the voltage. In the middle frequency range there is a gain of 100 (1 M/10 K). So there is a rather high loop gain of maybe 1000 and more at low frequencies. So regulation can be very good.Another indication for this is that the performance of the LTZ1000 is still quite good even just at the dropout limit. So even with the reduced loop gain (e.g. 1/10 of the normal value) the temperature control seems to be still good enough.For the long term stability there is a change in the set point (divider and Zener itself): A every 1 ppm drift would correspond to about a 0.5 µV voltage change for U_BE and thus about 0.25 mK of temperature drift. Aging of the transistor might cause similar drift, maybe more ?I would expect the LM399 control loop to be less powerful, as it is all internal and can not use large caps to easily get the slow time constants for the slow thermal system. Also the temperature control uses a separate likely less stable reference voltage.

author: zhtoor
author_rank: Frequent Contributor
timestamp: April 10, 2018, 08:08:31 pm
content: the die temperature would also be a function of thermodynamical model of the die-attach and the heat dissipation by the zener / temp. compensation transistor.it might be an interesting exercise to do some "precision thermometry" on the LTZ1000(A)CH for "tuning" the thermal control loop.what would be tempco of the unheated LTZ?if this tempco is FIRST minimized using the series resistor as marked in LTZ datasheet around the ultimate temp setpoint,and THEN close the thermal control loop.does this make sense?best regards.-zia

author: Kleinstein
author_rank: Super Contributor
timestamp: April 10, 2018, 08:47:46 pm
content: AFAIK the unheated TC of the LTZ1000 is around 50 ppm/K (not a lot of scattering). Adding TC adjustment is a two sided approach: It adds a rather critical resistor ratio. So the sensitivity to resistor change gets larger, while the temperature regulation and set-point divider get less critical. Overall I don't think this is a really viable way.The temperature control loop can not be very well optimized: LT suggests the same circuit despite of the quite different thermal properties for the A and non A parts. In addition the heater is square law and thus the loop gain power dependent. So the loop has to be stable at high power and is thus lower than possible gain at lower power. So the DS circuit is a kind of compromise, likely on the very conservative side, especially when the power is low.However it looks like the performance is not limited by the control loop in most cases. So there is very little to gain in using a better optimized and thus faster control loop. Much of the residual thermal effects comes from the wires and contacts. So it is more about optimizing the thermal setup than optimizing the loop gain to get an even more stable chip temperature.For optimizing the loop, there is not need for precision temperature readings, if would more about measuring the puls or step response of the heater. So nothing super accurate, more like relatively fast (e.g. up to ms scale) for a thermal response. There are thermal impedance curves - so if one knows the theory and relies on this date one may not need an own measurement. Most of the data are already there. The other point would be to linearized the heater so there would be less variations with power and thus no need to compromise here. This would be a step that could be done even without remeasuring the thermal response. There is no need to be super accurate here - even a crude approximation can be better than no correction.

author: zhtoor
author_rank: Frequent Contributor
timestamp: April 10, 2018, 11:06:26 pm
content: Quote from: Kleinstein on April 10, 2018, 08:47:46 pmagreed. a step response is all what is needed to optimize the temp loop,but how about correlating Vref and Temperature variations of the Die?best regards.-zia