One application that uses the output voltage of the TMP61 in an active control circuit is thermal foldback. This is performed to reduce, or fold back, the current driving a string of LEDs, for example. At high temperatures, the LEDs begin to heat up due to environmental conditions and self heating. Thus, at a certain temperature threshold based on the LED's safe operating area, the driving current must be reduced to cool down the LEDs and prevent thermal runaway. The device voltage output increases with temperature when the output is in the lower position of the voltage divider and can provide a response used to fold back the current. Typically, the device holds the current at a specified level until a high temperature is reached, known as the knee point, at which the current must be rapidly reduced in order to continue operation. To better control the temperature/voltage sensitivity, the device uses a rail-to-rail operational amplifier. Figure 8-11 shows the temperature knee point where the foldback begins. The set by the reference voltage (2.5 V) at the positive input, and the feedback resistors set the response of the foldback curve. The foldback knee point may be chosen based on the output of the voltage divider and the corresponding temperature from Equation 5 (110°C, for example). The device uses a buffer between the voltage divider with R_TMP61 and the input to the op amp to prevent loading and variations in V_TEMP.

Figure 8-11 Thermal Foldback Using TMP61 Voltage Divider and a Rail-to-Rail Op Amp

The op amp remains high as long as the voltage output is below V_REF. When the temperature goes above 110°C, the output falls to the 0-V rail of the op amp. The rate at which the foldback occurs depends on the feedback network, R_FB and R₁, which varies the gain of the op amp, G, as shown in Equation 6. The foldback behavior controls the voltage and temperature sensitivity of the circuit. The device feeds this voltage output into a LED driver circuit that adjusts output current accordingly. V_OUT is the final output voltage used for thermal foldback and is calculated in Equation 7. Figure 8-12 describes the output voltage curve in this example which sets the knee point at 110°C.

Equation 5.

Equation 6.

Equation 7.

Figure 8-12 Thermal Foldback Voltage Output Curve