P

D

max

+ VI(avg) * VO(avg)

I

O(avg) )

V

I(avg)

x I

(Q)

A

B

C

TJ

A

RθJC

TC

B

RθCS

TA

C

RθSA

(a)

(b)

DDPAK Package

SOT223 Package

CIRCUIT BOARD COPPER AREA

B

A

C

T

J +

T

A )

P

Dmax x R

θJC ) RθCS ) RθSA

TPS72501

TPS72515, TPS72516

TPS72518, TPS72525

www.ti.com

SLVS341E – MAY 2002 – REVISED JUNE 2010

THERMAL INFORMATION

The amount of heat that an LDO linear regulator generates is directly proportional to the amount of power it

dissipates during operation. All integrated circuits have a maximum allowable junction temperature (TJmax)

above which normal operation is not assured. A system designer must design the operating environment so that

the operating junction temperature (TJ) does not exceed the maximum junction temperature (TJmax). The two

main environmental variables that a designer can use to improve thermal performance are air flow and external

heatsinks. The purpose of this information is to aid the designer in determining the proper operating environment

for a linear regulator that is operating at a specific power level.

In general, the maximum expected power (PD(max)) consumed by a linear regulator is computed as:

(3)

Where:

•

VI(avg) is the average input voltage.

•

VO(avg) is the average output voltage.

•

IO(avg) is the average output current.

•

I(Q) is the quiescent current.

For most TI LDO regulators, the quiescent current is insignificant compared to the average output current;

therefore, the term VI(avg) x I(Q) can be neglected. The operating junction temperature is computed by adding the

ambient temperature (TA) and the increase in temperature due to the regulator's power dissipation. The

temperature rise is computed by multiplying the maximum expected power dissipation by the sum of the thermal

resistances between the junction and the case (RqJC), the case to heatsink (RqCS), and the heatsink to ambient

(RqSA). Thermal resistances are measures of how effectively an object dissipates heat. Typically, the larger the

device, the more surface area available for power dissipation and the lower the object's thermal resistance.

Figure 20 illustrates these thermal resistances for (a) a SOT223 package mounted in a JEDEC low-K board, and

(b) a DDPAK package mounted on a JEDEC high-K board.

Figure 20. Thermal Resistances

Equation 4 summarizes the computation:

(4)

The RqJC is specific to each regulator as determined by its package, lead frame, and die size provided in the

regulator's data sheet. The RqSA is a function of the type and size of heatsink. For example, black body radiator

type heatsinks can have RqCS values ranging from 5°C/W for very large heatsinks to 50°C/W for very small

heatsinks. The RqCS is a function of how the package is attached to the heatsink. For example, if a thermal

compound is used to attach a heatsink to a SOT223 package, RqCSof 1°C/W is reasonable.

Copyright © 2002–2010, Texas Instruments Incorporated

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