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# ADN8834

Question asked by hpkamen on Mar 5, 2017
Latest reply on Apr 18, 2017 by ytu

Dear Sir/Madam,

I am using the ADN8834  for constant temperature control, design of circuit diagram is shown in Figure 1. ADN8834 internal structure is shown in Figure 2.

figure 1 design circuit

Figure 2. ADN8834 internal circuit diagram

According to these two graphs, descriptions of problems I encountered are shown below:

I first test the relaiton between  OUT2 and LDR and SW,  the steps are shown below:
1. do not connect devices such as Thermistors and TEC, do not connect PID network, excluding the front end impact on OUT2 output voltage.
2. short the two pins IN2N and OUT2, input 1.25V voltage on IN2P pin, setting the COMPENSATION circuit to emitter follower , resulting in VOUT2=1.25V.

3. VREF=2.5V,VDD=5V。

4. According to the resistance values shown in Figure 3 sets VLIM and ILIM voltage through the multimeter to measure the voltage in the table VVLIM, and VILIM close.

Figure 3. VLIM and ILIM voltage settings
5. According to the following two equations, VB=2.5V, when VOUT2=1.25V, and VLDR=2.5V,VSFB=2.5V, but based on actual measurements, VLDR=0.159V,VSFB=0.61V, does not match the calculated value.

6. Measuring VTEC pin voltage VVTEC=1.12V. However, according to the formula , the value should be  VTEC=0.52V, and the actual VLDR and VSFB measured voltage are different

II.　I try to find the difference between my circuit and ADI offical circuit.
The only difference is  a 49.9 ohm resisotor on SFB pin, I wonder what is the role of the resistance

Figure 4. ADI official ADN8834 evaluation board circuit diagram

Above all, the questions are summarized as follows:

1. LDR and SFB chip output voltage does not match the calculated value

2. .what is the role of the resistance

Because I am eager to use this component.
Thanks.