LP2997 datasheet(9/12 Pages) NSC

9 / 12 page

Thermal Dissipation (Continued)

top side of the board can also help. With careful layout it is

possible to reduce the

JA further than the nominal values

shown in Figure 1

Optimizing the

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:

D =PAVIN +PVDDQ +PVTT

Where,

AVIN =IAVIN *VAVIN

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,

TT cannot source and sink current simultaneously.

VTT =VVTT xILOAD (Sinking) or

VTT =(VPVIN -VVTT)xILOAD (Sourcing)

The power dissipation of the LP2997 can also be calculated

during the shutdown state. During this condition the output

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.

D =PAVIN +PVDDQ

AVIN =IAVIN xVAVIN

VDDQ =VVDDQ *IVDDQ =VVDDQ2 xRVDDQ

LP2997 데이터시트(PDF) 9 Page - National Semiconductor (TI)