The ideality factor (η) is a measured characteristic of a remote temperature sensor diode as compared to an ideal diode. Compensating for ideality factor differences is simple if the diode manufacturer specifies the n-factor in the respective data sheet. If the ideality factor of the transistor is not specified, the manufacturer can be able to provide the n-factor value by a special request.

Typical ideality factor specification differences cause a gain variation of the transfer function. The TMP4719 is calibrated for the ideality factor of 1.004, so use η-Factor Lookup Table to compensate for a target ideality factor that differs from 1.004.

For example, if MMBT3906FZ-7B bipolar transistor, which has an ideality factor of approximately 1.008, is selected for this design example, 15h with a ideality factor (η) of 1.0079 is the closest value that can be programmed to compensate for the differing typical ideality factors. Referring to I²C and Register Map, the controller then sends a write command to the address of the target (4Dh) followed by the register pointer (27h) and finally the data (15h) to η-Factor_Ch1 Register.

The design calls for the high temperature limit of 88°C at device power up, which is programmed using I²C. Referring to I²C and Register Map, the controller sends a write command to the address of the target (4Dh) followed by the register pointer (07h) and finally the data (51h) to THigh_Limit_Remote_Ch1_High Register.

Figure 9-2 Application Example: Program Using I²C to Set Temperature High Limit

Remote junction temperature sensors are typically implemented in a noisy environment. Noise is most often created by fast digital signals, and noise can corrupt measurements. The TMP4719 device has a built-in, 65kHz low-pass filter on the inputs of D+ and D– to minimize the effects of noise. However, a bypass capacitor placed differentially across the inputs of the remote temperature sensor is recommended to make the application more robust against unwanted coupled signals. For this capacitor, select a value between 100pF and 1.5nF. Some applications attain better overall accuracy with additional series resistance; however, this increased accuracy is application-specific. When series resistance is added, the total value must not be greater than 1kΩ. If filtering is required, suggested component values are 470pF and 50Ω on each input; exact values are application-specific.