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LP2997 Datasheet(PDF) 9 Page - National Semiconductor (TI) |
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LP2997 Datasheet(HTML) 9 Page - National Semiconductor (TI) |
9 / 12 page Thermal Dissipation (Continued) top side of the board can also help. With careful layout it is possible to reduce the θ JA further than the nominal values shown in Figure 1 Optimizing the θ JA and placing the LP2997 in a section of a board exposed to lower ambient temperature allows the part to operate with higher power dissipation. The internal power dissipation can be calculated by summing the three main sources of loss: output current at V TT, either sinking or sourcing, and quiescent current at AVIN and VDDQ. During the active state (when shutdown is not held low) the total internal power dissipation can be calculated from the follow- ing equations: P D =PAVIN +PVDDQ +PVTT Where, P AVIN =IAVIN *VAVIN P VDDQ =VVDDQ *IVDDQ =VVDDQ2 xRVDDQ To calculate the maximum power dissipation at V TT both conditions at V TT need to be examined, sinking and sourcing current. Although only one equation will add into the total, V TT cannot source and sink current simultaneously. P VTT =VVTT xILOAD (Sinking) or P VTT =(VPVIN -VVTT)xILOAD (Sourcing) The power dissipation of the LP2997 can also be calculated during the shutdown state. During this condition the output V TT will tri-state, therefore that term in the power equation will disappear as it cannot sink or source any current (leak- age is negligible). The only losses during shutdown will be the reduced quiescent current at AVIN and the constant impedance that is seen at the VDDQ pin. P D =PAVIN +PVDDQ P AVIN =IAVIN xVAVIN P VDDQ =VVDDQ *IVDDQ =VVDDQ2 xRVDDQ www.national.com 9 |
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