UA78L05CLPR datasheet(21/48 Pages) TI

21 / 48 page

Note

Power dissipation can be minimized, and therefore greater efficiency can be achieved, by correct

selection of the system voltage rails. For the lowest power dissipation, use the minimum input voltage

required for correct output regulation.

For devices with a thermal pad, the primary heat conduction path for the device package is through the thermal

pad to the PCB. Solder the thermal pad to a copper pad area under the device. This pad area must contain an

array of plated vias that conduct heat to additional copper planes for increased heat dissipation.

The maximum power dissipation determines the maximum allowable ambient temperature (TA) for the device.

According to the following equation, power dissipation and junction temperature are most often related by the

junction-to-ambient thermal resistance (RθJA) of the combined PCB and device package and the temperature of

the ambient air (TA).

TJ = TA + (RθJA × PD)

(2)

Thermal resistance (RθJA) is highly dependent on the heat-spreading capability built into the particular PCB

design, and therefore varies according to the total copper area, copper weight, and location of the planes.

The junction-to-ambient thermal resistance listed in the Thermal Information table is determined by the JEDEC

standard PCB and copper-spreading area, and is used as a relative measure of package thermal performance.

As mentioned in the An empirical analysis of the impact of board layout on LDO thermal performance application

note, RθJA can be improved by 35% to 55% compared to the Thermal Information table value with the PCB

board layout optimization.

8.2.2.3 Estimating Junction Temperature

The JEDEC standard now recommends the use of psi (Ψ) thermal metrics to estimate the junction temperatures

of the linear regulator when in circuit on a typical PCB board application. These metrics are not thermal

resistance parameters and instead offer a practical and relative way to estimate junction temperature. These

psi metrics are determined to be significantly independent of the copper area available for heat-spreading.

The Thermal Information table lists the primary thermal metrics, which are the junction-to-top characterization

parameter (ψJT) and junction-to-board characterization parameter (ψJB). These parameters provide two methods

for calculating the junction temperature (TJ), as described in the following equations. Use the junction-to-top

characterization parameter (ψJT) with the temperature at the center-top of device package (TT) to calculate

the junction temperature. Use the junction-to-board characterization parameter (ψJB) with the PCB surface

temperature 1 mm from the device package (TB) to calculate the junction temperature.

TJ = TT + ψJT × PD

(3)

where:

• PD is the dissipated power

• TT is the temperature at the center-top of the device package

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UA78L

SLVS010X – JANUARY 1976 – REVISED JUNE 2023

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