9.2.1.2.1 Thermal Protection With Comparator

The engineer can use the TMP64-Q1, a voltage reference, and a comparator to program the thermal protection.

As shown in Figure 9-5, the output of the comparator remains low until the voltage of the thermistor divider, with

comparator signals an overtemperature warning signal. The engineer can also program the hysteresis to prevent

the output from continuously toggling around the temperature threshold when the output returns low. Either a

comparator with built-in hysteresis or feedback resistors may be used.

Figure 9-5. Temperature Switch Using TMP64-Q1 Voltage Divider and a Comparator

9.2.1.2.2 Thermal Foldback

One application that uses the output voltage of the TMP64-Q1 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 TMP64-Q1 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

current is held at a specified level until a high temperature is reached, known as the knee point, where the

current must be rapidly reduced. To better control the temperature/voltage sensitivity of the TMP64-Q1, a rail-to-

rail operational amplifier is used. In the example shown in Figure 9-6, the temperature “knee” where the foldback

begins is 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 (like 110 °C, for example). A buffer is used in-between the voltage

TMP64-Q1

SNIS220B – MARCH 2020 – REVISED NOVEMBER 2020

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