LP2995 Datasheet, PDF (9/24 Pages) National Semiconductor (TI) – DDR Termination Regulator

LP2995
www.ti.com
SNVS190M – FEBRUARY 2002 – REVISED MARCH 2013
performance are critical, although their cost is typically higher than any other capacitor.
Capacitor recommendations for different application circuits can be seen in the accompanying application notes
with supporting evaluation boards.
Thermal Dissipation
Since the LP2995 is a linear regulator any current flow from V_TT will result in internal power dissipation
generating heat. To prevent damaging the part from exceeding the maximum allowable junction temperature,
care should be taken to derate the part dependent on the maximum expected ambient temperature and power
dissipation. The maximum allowable internal temperature rise (T_Rmax) can be calculated given the maximum
ambient temperature (T_Amax) of the application and the maximum allowable junction temperature (T_Jmax).
T_Rmax = T_Jmax − T_Amax
From this equation, the maximum power dissipation (P_Dmax) of the part can be calculated:
P_Dmax = T_Rmax / θ_JA
The θ_JA of the LP2995 will be dependent on several variables: the package used; the thickness of copper; the
number of vias and the airflow. For instance, the θ_JA of the SOIC-8 is 163°C/W with the package mounted to a
standard 8x4 2-layer board with 1oz. copper, no airflow, and 0.5W dissipation at room temperature. This value
can be reduced to 151.2°C/W by changing to a 3x4 board with 2 oz. copper that is the JEDEC standard.
Figure 14 shows how the θ_JA varies with airflow for the two boards mentioned.
180
170
160
150
140
130
120
110
100
90
80
0
SOP Board
JEDEC Board
200 400 600 800 1000
AIRFLOW (Linear Feet per Minute)
Figure 14. θ_JA vs Airflow (SOIC-8)
Layout is also extremely critical to maximize the output current with the WQFN package. By simply placing vias
under the DAP the θ_JA can be lowered significantly. Figure 15 shows the WQFN thermal data when placed on a
4-layer JEDEC board with copper thickness of 0.5/1/1/0.5 oz. The number of vias, with a pitch of 1.27 mm, has
been increased to the maximum of 4 where a θ_JA of 50.41°C/W can be obtained. Via wall thickness for this
calculation is 0.036 mm for 1oz. Copper.
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