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TK71550ASCL Datasheet(PDF) 9 Page - TOKO, Inc |
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TK71550ASCL Datasheet(HTML) 9 Page - TOKO, Inc |
9 / 15 page March 2001 TOKO, Inc. Page 9 TK715xxAS DEFINITION AND EXPLANATION OF TECHNICAL TERMS (CONT.) determine the power dissipation for shutdown when mounted, attach the device on the actual PCB and delib- erately increase the output current (or raise the input voltage) until the thermal protection circuit is activated. Calculate the power dissipation of the device by subtract- ing the output power from the input power. These mea- surements 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 (~ 0.8 - 0.9) 3) Plot P D1 against 25 °C 4) Connect P D1 to the point corresponding to the 150 °C with a straight line. 5) In design, take a vertical line from the maximum operating temperature (e.g., 75 °C) to the derating curve. 6) Read off the value of P D against the point at which the vertical line intersects the derating curve. This is taken as the maximum power dissipation, D PD. The maximum operating current is: I OUT = (DPD / (VIN(MAX) - VOUT) SOT23-3 POWER DISSIPATION CURVE PD DPD 25 50 75 150 (mW) TA (°C) 3 6 5 4 0 50 100 150 TA (°C) 0 300 500 100 200 400 MOUNTED AS SHOWN FREE AIR |
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