9.3.6 Thermal Foldback

The TPS92630-Q1 device integrates thermal shutdown protection to prevent the device from overheating. In addition, to prevent LEDs from flickering because of rapid thermal changes, the device includes a programmable thermal current-foldback feature to reduce power dissipation at high junction temperatures.

The TPS92630-Q1 device reduces the LED current as the silicon junction temperature of the TPS92630-Q1 device increases (see Figure 23). By mounting the TPS92630-Q1 device on the same thermal substrate as the LEDs, use of this feature can also limit the dissipation of the LEDs. As the junction temperature of the TPS92630-Q1 device increases, the device reduces the regulated current, reducing the dissipated power in the TPS92630-Q1 device and in the LEDs. The current reduction is from the 100% level at typically 2% of I_(setting) per ºC until the point at which the current drops to 50% of the full value.

Figure 23. Thermal Foldback

Above this temperature, the current continues to decrease at a lower rate until the temperature reaches the overtemperature shutdown threshold temperature, T_(shutdown). Changing the voltage on the TEMP pin adjusts the temperature at which the current reduction begins. With TEMP floating, the definition of thermal monitor activation temperature, T_(th), is the temperature at which the current reduction begins. The specification of T_(th) in the characteristics table is at the 90% current level. T_(th) increases as the voltage at the TEMP pin, V_(TEMP), declines and is defined as approximately:

Figure 24. TEMP Pin Voltage vs Temperature

A resistor connected between TEMP and GND reduces V_(TEMP) and increases T_(th). A resistor connected between TEMP and a reference supply greater than 1 V increases V_(TEMP) and reduces T_(th).

Figure 25. Pullup and Pulldown Resistors vs T_(th)

Figure 25 shows how the nominal value of the thermal-monitor activation temperature varies with the voltage at TEMP and with either a pulldown resistor to GND or with a pullup resistor to 3.3 V or 5 V.

In extreme cases, if the junction temperature exceeds the overtemperature limit, T_(shutdown), the device disables all channels. Temperature monitoring continues, and channel reactivation occurs when the temperature drops below the threshold provided by the specified hysteresis.

Note the possibility of the TPS92630-Q1 device transitioning rapidly between thermal shutdown and normal operation. This can happen if the thermal mass attached to the exposed thermal pad is small and T_(th) is increased to close to the shutdown temperature. The period of oscillation depends on T_(th), the dissipated power, the thermal mass of any heatsink present, and the ambient temperature.