LP2994 datasheet(9/15 Pages) NSC

9 / 15 page

Component Selection (Continued)

dielectric types have poor capacitance characteristics as a

function of voltage and temperature. Because of the typically

low value of capacitance it is recommended to use ceramic

capacitors in parallel with another capacitor such as an

aluminum electrolytic. A dielectric of X5R or better is recom-

mended for all ceramic capacitors.

Hybrid - Several hybrid capacitors such as OS-CON and SP

are available from several manufacturers. These offer a

large capacitance while maintaining a low ESR. These are

the best solution when size and performance are critical,

although their cost is typically higher than other capacitors.

Thermal Dissipation

Since the LP2994 is a linear regulator any current flow from

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 ambi-

ent temperature and power dissipation. The maximum allow-

able internal temperature rise (T

Rmax) can be calculated

given the maximum ambient temperature (T

Amax)ofthe

application and the maximum allowable junction temperature

Jmax).

Rmax =TJmax −TAmax

From this equation, the maximum power dissipation (P

D)of

the part can be calculated:

Dmax =TRmax / θJA

The

JA of the LP2994 will be dependent on several vari-

ables: the package used; the thickness of copper; the num-

ber of vias and the airflow. For instance, the

JA of the SO-8

is 163˚C/W with the package mounted to a standard 8x4

2-layer board with 1oz. copper, no airflow, and 0.5W dissi-

pation 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 3 shows how the

JA varies

with airflow for the two boards mentioned.

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

possible to reduce the

JA further than the nominal values

shown in

Figure 3.

Optimizing the

JA and placing the LP2994 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 V

DDQ. 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 xVAVIN

VDDQ =VVDDQ xIVDDQ =VVDDQ

2 xR

VDDQ

To calculate the maximum power dissipation at V

TT, both

sinking and sourcing current conditions at V

TT need to be

examined. Although only one equation will add into the total,

TT cannot source and sink current simultaneously.

VTT =VVTT xILOAD (Sinking)

VTT =(VPVIN -VVTT)xILOAD (Sourcing)

The power dissipation of the LP2994 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 V

DDQ pin.

D =PAVIN +PVDDQ

Where,

AVIN =IAVIN xVAVIN

VDDQ =VVDDQ xIVDDQ =VVDDQ

2 xR

VDDQ

20045928

FIGURE 3.

JA vs Airflow

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