TPS786XX Datasheet, PDF (10/21 Pages) Texas Instruments – ULTRALOW-NOISE, HIGH-PSRR, FAST, RF, 1.5-A LOW-DROPOUT LINEAR REGULATORS

TPS786xx
SLVS389G – SEPTEMBER 2002 – REVISED JULY 2006
www.ti.com
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 (T_Jmax) above which normal operation
is not assured. A system designer must design the
operating environment so that the operating junction
temperature (T_J) does not exceed the maximum
junction temperature (T_Jmax). 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.
increase in temperature due to the regulator 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 (R_θJC), the case to heatsink
(R_θCS), and the heatsink to ambient (R_θSA). 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 26 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.
In general, the maximum expected power (P_Dmax)
consumed by a linear regulator is computed as
shown in Equation 4:
Equation 5 summarizes the computation:
T_J + T_A ) P_Dmax ^ǒR_qJC ) R_qCS ) R_qSA^Ǔ ₍₅₎
ǒ Ǔ _{PD max +}
V * V _IN(avg)
OUT(avg)
I ) V _OUT(avg)
IN(avg)
I_Q
(4)
where:
• V_IN(avg) is the average input voltage.
• V_OUT(avg) is the average output voltage.
• I_OUT(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 V_IN(avg)× I_Q can be neglected. The
operating junction temperature is computed by
adding the ambient temperature (T_A) and the
The R_θJC is specific to each regulator as determined
by its package, lead frame, and die size provided in
the regulator data sheet. The R_θSA is a function of
the type and size of heatsink. For example, black
body radiator type heatsinks can have R_θCS values
ranging from 5°C/W for very large heatsinks to
50°C/W for very small heatsinks. The R_θCS 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,
R_θCS of 1°C/W is reasonable.
CIRCUIT BOARD COPPER AREA
C
A T_J
R_θJC
B
B T_C
R_θCS
A
C
R_θSA
SOT223 Package
(a)
T_A
Figure 26. Thermal Resistances
A
B
C
DDPAK Package
(b)
10
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