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TPS79633KTTT Fiches technique(PDF) 11 Page - Texas Instruments |
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TPS79633KTTT Fiches technique(HTML) 11 Page - Texas Instruments |
11 / 18 page www.ti.com THERMAL INFORMATION T J + T A ) P Dmax x R θJC ) RθCS ) RθSA P D max + VI(avg) * VO(avg) I O(avg) ) V I(avg) x I (Q) A B C TJ A RθJC TC B RθCS TA C RθSA (a) (b) DDPAK Package SOT223 Package CIRCUIT BOARD COPPER AREA B A C TPS79601, TPS79618, TPS79625 TPS79628, TPS79630, TPS79633 SLVS351D – SEPTEMBER 2002 – REVISED OCTOBER 2004 temperature due to the regulator's power dissipation. The amount of heat that an LDO linear regulator The temperature rise is computed by multiplying the generates is directly proportional to the amount of maximum expected power dissipation by the sum of power it dissipates during operation. All integrated the thermal resistances between the junction and the circuits have a maximum allowable junction tempera- case (RΘJC), the case to heatsink (RΘCS), and the ture (TJmax) above which normal operation is not heatsink to ambient (RΘSA). Thermal resistances are assured. A system designer must design the measures of how effectively an object dissipates operating environment so that the operating junction heat. Typically, the larger the device, the more temperature (TJ) does not exceed the maximum surface area available for power dissipation and the junction temperature (TJmax). The two main environ- lower the object's thermal resistance. mental variables that a designer can use to improve thermal performance are air flow and external Figure 24 illustrates these thermal resistances for (a) heatsinks. The purpose of this information is to aid a SOT223 package mounted in a JEDEC low-K the designer in determining the proper operating board, and (b) a DDPAK package mounted on a environment for a linear regulator that is operating at JEDEC high-K board. a specific power level. Equation 5 summarizes the computation: In general, the maximum expected power (PD(max)) consumed by a linear regulator is computed as Equation 4: (5) The RΘJC is specific to each regulator as determined by its package, lead frame, and die size provided in (4) the regulator's data sheet. The RΘSA is a function of where: the type and size of heatsink. For example, black body radiator type heatsinks can have RΘCS values • VI(avg) is the average input voltage. ranging from 5 °C/W for very large heatsinks to • VO(avg) is the average output voltage. 50 °C/W for very small heatsinks. The RΘ CS is a • IO(avg) is the average output current. function of how the package is attached to the • I(Q) is the quiescent current. heatsink. For example, if a thermal compound is used to attach a heatsink to a SOT223 package, RΘCS of For most TI LDO regulators, the quiescent current is 1 °C/W is reasonable. insignificant compared to the average output current; therefore, the term VI(avg) x I(Q) can be neglected. The operating junction temperature is computed by adding the ambient temperature (TA) and the increase in Figure 24. Thermal Resistances 11 |
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