9.2.1.2.1 Thermal Protection With Comparator

The engineer can use the TMP64, 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

RBIAS and RTMP64, rises above the threshold voltage set by R1 and R2. When the output goes high, the

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.

RBias

VBias

RTMP64

R1

R2

VTrip

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

9.2.1.2.2 Thermal Foldback

One application that uses the output voltage of the TMP64 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 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, 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

divider with RTMP64 and the input to the op amp to prevent loading and variations in VTEMP.

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TMP64

SNIS212C – DECEMBER 2019 – REVISED SEPTEMBER 2020

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