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TPS79518DCQ Datasheet(PDF) 11 Page - Texas Instruments |
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TPS79518DCQ Datasheet(HTML) 11 Page - Texas Instruments |
11 / 15 page www.ti.com THERMAL INFORMATION TJ A RθJC TC B RθCS TA C RθSA (a) SOT223 Package CIRCUIT BOARD COPPER AREA B A C T J + TA ) PD max RθJC ) RθCS ) RθSA (5) P D max + VIN(avg) * VOUT(avg) I OUT(avg) ) VI(avg) I (Q) TPS79501, TPS79516 TPS79518, TPS79525 TPS79530, TPS79533 SLVS350C – OCTOBER 2002 – REVISED JANUARY 2005 The amount of heat that an LDO linear regulator generates is directly proportional to the amount of power it dissipates during operation. All integrated circuits have a maximum allowable junction tempera- ture (TJ(max)) above which normal operation is not assured. A system designer must design the operating environment so that the operating junction temperature (TJ) does not exceed the maximum junction temperature (TJ(max)). The two main environ- mental variables that a designer can use to improve thermal performance are air flow and external heatsinks. The purpose of this information is to aid the designer in determining the proper operating environment for a linear regulator that is operating at a specific power level. Figure 25. Thermal Resistances In general, the maximum expected power (PD(max)) consumed by a linear regulator is computed as Equation 5 summarizes the computation: Equation 4: The RΘJC is specific to each regulator as determined (4) by its package, lead frame, and die size provided in where: the regulator's data sheet. The RΘSA is a function of the type and size of heatsink. For example, black • VIN(avg) is the average input voltage body radiator type heatsinks can have RΘCS values • VOUT(avg) is the average output voltage ranging from 5 °C/W for very large heatsinks to • IOUT(avg) is the average output current 50 °C/W for very small heatsinks. The RΘ CS is a • I(Q) is the quiescent current function of how the package is attached to the heatsink. For example, if a thermal compound is used For most TI LDO regulators, the quiescent current is to attach a heatsink to a SOT223 package, RΘCS of insignificant compared to the average output current; 1 °C/W is reasonable. therefore, the term VIN(avg) x I(Q) can be neglected. The operating junction temperature is computed by Even if no external black body radiator type heatsink adding the ambient temperature (TA) and the in- is attached to the package, the board on which the crease in temperature due to the regulator's power regulator is mounted provides some heatsinking dissipation. The temperature rise is computed by through the pin solder connections. Some packages, multiplying the maximum expected power dissipation like the DDPAK and SOT223 packages, use a copper by the sum of the thermal resistances between the plane underneath the package or the circuit board's junction and the case (RΘJC), the case to heatsink ground plane for additional heatsinking to improve (RΘCS), and the heatsink to ambient (RΘSA). Thermal their thermal performance. Computer aided thermal resistances are measures of how effectively an object modeling can be used to compute very accurate dissipates heat. Typically, the larger the device, the approximations of an integrated circuit's thermal per- more surface area available for power dissipation and formance in different operating environments (e.g., the lower the object's thermal resistance. different types of circuit boards, different types and sizes of heatsinks, and different air flows, etc.). Using Figure 25 illustrates these thermal resistances for (a) these models, the three thermal resistances can be a SOT223 package mounted in a JEDEC low-K combined into one thermal resistance between junc- board. tion and ambient (RΘJA). This RΘJA is valid only for the specific operating environment used in the computer model. 11 |
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