LP2998 Datasheet, PDF (10/18 Pages) Intersil Corporation – DDR-II and DDR-I Termination Regulator

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FIGURE 2. θ_JA vs Airflow (SO-8)
Additional improvements can be made by the judicious use of
vias to connect the part and dissipate heat to an internal
ground plane. Using larger traces and more copper on the top
side of the board can also help. With careful layout it is pos-
sible to reduce the θ_JA further than the nominal values shown
in Figure 2
Optimizing the θ_JA and placing the LP2998 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 sourc-
ing, 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 following equa-
tions:
Where,
P_D = P_AVIN + P_VDDQ + P_VTT
P_AVIN = I_AVIN * V_AVIN
P_VDDQ = V_VDDQ * I_VDDQ = V_VDDQ2 x R_VDDQ
To calculate the maximum power dissipation at V_TT both con-
ditions 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 = V_VTT x I_LOAD (Sinking) or
P_VTT = ( V_PVIN - V_VTT) x I_LOAD (Sourcing)
The power dissipation of the LP2998 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 (leakage is
negligible). The only losses during shutdown will be the re-
duced quiescent current at AVIN and the constant impedance
that is seen at the VDDQ pin.
P_D = P_AVIN + P_VDDQ
P_AVIN = I_AVIN x V_AVIN
P_VDDQ = V_VDDQ * I_VDDQ = V_VDDQ2 x R_VDDQ
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