ADN8831 Vout2 vs Vtempset

Question

Hi, 
 I have aquirred the evaluation board and started to make some measurements. 
 But first I have some doubts how to understand the datasheet and in that regard how the ADN8831 regulates. 
 Looking at this equation 
 
 V_out2 is equal to V_tempset when the wanted temperature is reached. I am not sure what exactly V_out2 is controlling, but later in the datasheet is another graph shown 
 
 So if my V_tempset is 1.25V then the voltage over the TEC is 0. But if I set my V_tempset to 0.5V then V_out2 is also 0.5V when the temperature is reached, but now the voltage over the TEC is not zero and it will still be heating or cooling. 
 Can someone explain if I have misunderstood something or if I have to always make my wanted temperature to a V_tempset=1.25V? 
 Thank you

kiet · Accepted Answer

Hi Tyassin, 
 Figure 21 is the character of the device ADN8831, when Vout2 = 1.25V, no current flows thru TEC (VTEC = 0V). 
 If using the NTC Thermistor with the Temperature-to-Voltage Coefficient = - 25mV/C (ADN8831 datasheet, bottom of page 15), and: 
 If setting V_Tempset = 0.5V, then your operating Temperature is 55C, another word, if your operating Temperature is 55C, then you need to set V_Tempset = 0.5V. Since V_Tempset < 1.25V (VREF/2 at 25C), so current flows from LFB to SFB, and ADN8831 keeps TEC cool off (Cooling Mode) at the ambient T=55C. Note that, if setting V_Tempset = 0.5V, and put the TEC at room Temp (instead of T=55C), then ADN8831 will bring the TEC down to -5C (delta_C = -30C from 25C), and if you put a small metal, such as a penny coin, then you will see some ice on it, another word, you could make a tiny refrigerator. Note that, Vout2 = V_Tempset after temperature is reached, and VTEC will maintain its level to keep your setting Temperature. 
 From the following table, if setting V_Tempset = 1.75V for 5C, then current flows from SFB to LFB (Heating Mode), the TEC will get hot, not cool as V_Tempset < 1.25V above, and you could warm your little food if putting TEC outside the chamber at room Temp 25C, but you need a higher TEC current (a few Amps).

I hope you get some idea about how the TEC controller functioning. 
 Best Regards, 
 Kiet Tran)

tyassin · Answer

Hi Kiet, 
 Thank you for the very nice long answer. 
 Still not sure I fully understand. But I would like to put it to the test. 
 In the following I consider the resistors(Rx, R, Rfb) to work at 25C. 
 1. Is the "Operating temperature" the ambient temperature? 
 2. The TEC is in cooling mode if I set V_Tempset=0.5V at 25C and also if the ambient temperature is 55C and I set V_Tempset=0.5V? So the difference is that in the first case the TEC will end up at -5C and in the latter case I will maintain the TEC at 55C? 
 
 In the below picture, how can V_IN1N change with temperature? Shoud it not be fixed by the Opamp? 
 
 Thank you

kiet · Answer

Hi Tyassin, 
 Yes, you're correct for both #1 & #2 question. 
 For V_IN1N, it's changed with temperature, since the NTC Thermistor reduces its value at higher temperature, another word, the higher temperature the lower NTC value, so the lower V_IN1N voltage. 
 Best Regards, 
 Kiet

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ADN8831 Vout2 vs Vtempset