# Diode configuration of BJT

I came across this configuration while trying to understand the remote temp sensing ability of the TPS4811.

It seems as if the diode pin voltage is responsible for turning on the BJT. V(BE) being >= 0.7V. The same is also controlling the Collector current & Base current.

How exactly is this working to enable remote temperature sensing ability of this IC?

Or rather how is it able to sense temperature?

The terminal behaviour of a diode can be approximately represented by the equation

$$I_\mathrm{D} \approx I_\mathrm{S}\mathrm{e}^{V_\mathrm{D}/V_T},\tag{1}$$

where $$\I_\mathrm{D}\$$ is the diode current, $$\I_\mathrm{S}\$$ is the reverse saturation current and $$\V_T = k_\mathrm{B}T/e\$$ is the thermal voltage, being $$\k_\mathrm{B}\$$ the Boltzmann constant, $$\e\$$ the elementary charge, and $$\T\$$ the thermodynamic temperature. The above relationship holds for $$\I_\mathrm{D}\gg I_\mathrm{S}\$$.

If we periodically switch the current $$\I_\mathrm{D}\$$ between two values $$\I_1\$$ and $$\I_2\$$, we obtain

$$\frac{I_2}{I_1} \approx \mathrm{e}^\frac{V_2-V_1}{V_T},$$

where $$\V_1\$$ and $$\V_2\$$ are the diode voltages measured with the two currents. From the above equation, solving for $$\T\$$,

$$T \approx \frac{e(V_2-V_1)}{k_\mathrm{B}\ln(I_2/I_1)}.$$

The main idea of this arrangement is that it allows to measure the temperature $$\T\$$ cancelling the dependence on the diode parameter $$\I_\mathrm{S}\$$, which is difficult to characterise and is also temperature-dependent. Note that the diode is not used as an ON/OFF device.

Now, why using a diode-connected BJT instead of a proper diode? The reason is that the characteristic of a diode-connected BJT follows the model (1) much better than a diode. Hence, the temperature measurement is more accurate.

In the TPS4811x, $$\I_1\approx 10\,\mu\mathrm{A}\$$ and $$\I_2\approx 160\,\mu\mathrm{A}\$$.

Other details on this type of measurement can be found in the following document:

Texas Instruments, Optimizing Remote Temperature Sensor Design, SBOA173A Application Report, 2019.