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TK73235M Datasheet(PDF) 10 Page - TOKO, Inc |
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TK73235M Datasheet(HTML) 10 Page - TOKO, Inc |
10 / 16 page Page 10 January 1999 TOKO, Inc. TK732xx deliberately increase the output current (or raise the input voltage) until the thermal protection circuit is activated. Calculate the power dissipation of the device by subtracting the output power from the input power. These measurements should allow for the ambient temperature of the PCB. The value obtained from P D /(150 °C - TA) is the derating factor. The PCB mounting pad should provide maximum thermal conductivity in order to maintain low device temperatures. As a general rule, the lower the temperature, the better the reliability of the device. The thermal resistance when mounted is expressed as follows: T j = 0jA x PD + TA For Toko ICs, the internal limit for junction temperature is 150 °C. If the ambient temperature (T A) is 25 °C, then: 150 °C = 0 jA x PD + 25 °C 0 jA = 125 °C / PD P D is the value when the thermal sensor is activated. A simple way to determine P D is to calculate VIN x IIN when the output side is shorted. Input current gradually falls as temperature rises. You should use the value when thermal equilibrium is reached. The range of usable currents can also be found from the graph below. Procedure: 1) Find P D 2) P D1 is taken to be PD x (Note: It is not necessary to connect a ceramic capacitor in parallel with an aluminum or tantalum output capacitor. (~0.8 - 0.9) SENSOR CIRCUITS Overcurrent Sensor The overcurrent sensor protects the device if the output is shorted to ground. Thermal Sensor The thermal sensor protects the device if the junction temperature exceeds the safe value (T j = 150 °C). This temperature rise can be caused by extreme heat, excessive power dissipation caused by large output voltage drops, or excessive output current. The regulator will shut off when the temperature exceeds the safe value. As the junction temperature decreases, the regulator will begin to operate again. Under sustained fault conditions, the regulator output will oscillate as the device turns off then resets. Damage may occur to the device under extreme fault conditions. Reverse Voltage Protection Reverse voltage protection prevents damage due to the output voltage being higher than the input voltage. This fault condition can occur when the output capacitor remains charged and the input is reduced to zero, or when an external voltage higher than the input voltage is applied to the output side. PACKAGE POWER DISSIPATION (P D) This is the power dissipation level at which the thermal sensor is activated. The IC contains an internal thermal sensor which monitors the junction temperature. When the junction temperature exceeds the monitor threshold of 150 °C, the IC is shut down. The junction temperature rises as the difference between the input power (V IN x IIN) and the output power (V OUT x IOUT) increases. The rate of temperature rise is greatly affected by the mounting pad configuration on the PCB, the board material, and the ambient temperature. When the IC mounting has good thermal conductivity, the junction temperature will be low even if the power dissipation is great. When mounted on the recommended mounting pad, the power dissipation of the SOT-23L-8 is increased to 600 mW. For operation at ambient temperatures over 25 °C, the power dissipation of the SOT-23L-8 device should be derated at 4.8 mW/ °C. To determine the power dissipation for shutdown when mounted, attach the device on the actual PCB and DEFINITION AND EXPLANATION OF TECHNICAL TERMS (CONT.) PD DPD 25 50 75 150 (mW) TA (°C) 3 6 5 4 |
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