Thermal protection of power systems in many very important. Figure 1 shows a low-cost thermal protection circuit. One LTC1998 is a 6-pin for battery monitoring comparator SOT-23 package, in this circuit is used as heat protection. The thermal protection circuit can provide very useful features, benefits such as adjustable transition temperature, programmable hysteresis voltage, and remote temperature sensing.
This circuit uses a negative temperature coefficient (NTC) thermistor RT to examine the circuit board temperature. Under normal circumstances, LTC1998 pin 1 of voltage (Vbatt) higher than 2.5V, so LTC1998 pin 6 voltage (Vbattlo) is logic high.
When the detection point temperature rises, LTC1998 pin 1 of voltage (Vbatt) declined because of RT resistance. If Vbatt drops below 2.5V, then the LTC1998 internal comparator transition occurs, and Vbattlo (pin 6) to become logic low. Pin 6 can be connected to the operation of the relevant power / soft-start (enable / shutdown) control side. Therefore, if the temperature rises to a pre-set trip point, you can turn off the power to prevent overheating.
Figure 1: Threshold with adjustable temperature, programmable hysteresis voltage, and remote temperature sensing properties of the thermal protection circuit.
Transition temperature threshold can be set using the following equation:
Vbatt = Vz × RT (trip) / [R1 + RT (trip)] = 2.5V (1)
NTC thermistor selected and after the temperature jump threshold to reach the temperature trip point when the RT (trip) value for the resistance – temperature curve or thermistor manufacturer’s data sheet to determine the formula. If known Vz (the voltage on the zener diode), then the resistance will be able to use the R1 equation (1) calculated.
Programmable hysteresis voltage is added to prevent the pin 6 (Battlo) at the transition point oscillation. When a temperature jump and the power shut off after the circuit board temperature began to decline. NTC thermistor resistance increases, LTC1998 pin 1 (Vbatt) also will increase the voltage. When Vbatt rose more than 2.5V after a certain predetermined value, LTC1998 internal comparator transition from happening again. At this point, pin 6 (battlo, RUN / SS) into a logic high, and the power to resume normal work.
Hysteresis voltage can be pin 4 (VH.A) on the voltage setting:
VHYST = VH.A / 2 = Vz × R3 / (R2 + R3) / 2 (2)
Choose a good restart the power supply temperature, hysteresis voltage can be based on the resistance – temperature curve or thermistor data sheet to determine the formula. Then by equation (2), R2 and R3 selected value.
Chip temperature sensor with different modes, the thermal protection circuit using NTC thermistor to detect temperature, it is possible to the remote temperature sensor. A tiny thermistor circuit can be placed in the position away from the LTC1998, and any points of interest board to monitor the temperature.
Proposed circuit the input voltage Vz at about 2.7V voltage setting to 5.5V. For example, if the system has a 3.3V voltage supply, then Vz can be directly connected to the 3.3V of power supply, then would not need Rz and Zener diodes.
If the system does not provide the appropriate bias supply, you need to use the Rz and the Zener diode protection circuit connected to the power input. Select the input voltage range Rz and zener diodes.
The circuit in Figure 1 produced by the Murata 0603 specifications 100K NTC thermistor (NCP18WF104J03RB). Rz and Zener diode device at about 3V to 40V input voltage range to work well.
When VIN is 24V when, Vz is about 3.3V, hysteresis voltage of 150mV. Jump pin 1 voltage 2.5V, which is through the LTC1998’s internal reference voltage setting. Jump threshold temperature is about 90oC, and a 10oC lag (in this temperature range the temperature coefficient thermistor is about -0.3KO/oC). That is, at a temperature higher than 90oC controlled power will be turned off when, and when the temperature is below 80oC return to work.