The TMP411-Q1 and TMP411D-Q1 devices are designed to be used with discrete transistors or substrate transistors built into processor chips and ASICs. NPN or PNP transistors can be used, as long as the base-emitter junction is the remote temperature sensor. A transistor or diode connection can be used, as shown in Figure 7-1. The D+ pin waveform is shown in Figure 9-3 while a transistor is connected between the D+ and D– pins. Due to the three different source currents used in Section 7.2, the D+ waveform has three levels of voltage during temperature conversion.

Figure 9-3 D+ Waveform

Errors in remote temperature sensor readings are the result of the ideality factor and current excitation from the TMP411-Q1 and TMP411D-Q1 versus the manufacturer-specified operating current for a given transistor. Some manufacturers specify a high-level and low-level current for the temperature-sensing substrate transistors. The TMP411-Q1 and TMP411D-Q1 have an I_LOW value of 6μA, and an I_HIGH value of 120μA. The TMP411-Q1 and TMP411D-Q1 allow for different n-factor values, as shown in Table 8-6.

The ideality factor (n) is a measured characteristic of a remote temperature sensor diode compared to an ideal diode. The ideality factor reduces to a value of 1.008. For transistors with an ideality factor that does not match the TMP411-Q1 and TMP411D-Q1, Equation 4 calculates the temperature error. Note that the actual temperature (°C) must be converted to Kelvin (K) for the equation to yield the correct results.

Equation 4. or ${\mathrm{T}}_{\mathrm{E}\mathrm{R}\mathrm{R}}=\left(\frac{{\mathrm{n}}_{\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{u}\mathrm{a}\mathrm{l}}\mathrm{ }-{\mathrm{ }\mathrm{n}}_{\mathrm{e}\mathrm{x}\mathrm{p}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{e}\mathrm{d}}}{{\mathrm{n}}_{\mathrm{e}\mathrm{x}\mathrm{p}\mathrm{e}\mathrm{c}\mathrm{t}\mathrm{e}\mathrm{d}}}\right)\times \left(273.15+{\mathrm{T}}_{\mathrm{a}\mathrm{c}\mathrm{t}\mathrm{u}\mathrm{a}\mathrm{l}}\left(\mathrm{֯}\mathrm{C}\right)\right)$

where:

• n or n_actual = the ideality factor of the remote temperature sensor
• T(°C) or T_actual(°C) = actual temperature
• T_ERR = T_reported - T_actual= device reading error due to n or n_actual ≠ 1.008
• Degree delta is the same for °C and K
• n_expected = 1.008

For n = 1.004 and T(°C) = 100°C, use Equation 5:

Equation 5.

If a discrete transistor is used as the remote temperature sensor, please select the transistor according to the following criteria, which results in the best accuracy.

1. Base-emitter voltage > 0.25V at 6µA, at the highest sensed temperature.
2. Base-emitter voltage < 0.95V at 120µA, at the lowest sensed temperature.
3. Base resistance < 100Ω
4. Tight control of V_BE characteristics is indicated by small variations in h_FE (that is, 50 to 150).

Based on these criteria, use two small-signal transistors, such as the 2N3904 (NPN) or 2N3906 (PNP).