LP2951 datasheet(23/47 Pages) TI

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7.1.3 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 1mm from the device package (TB) to calculate the junction temperature.

TJ = TT + ψJT × PD

(2)

where:

• PD is the dissipated power

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

TJ = TB + ψJB × PD

(3)

where:

• TB is the PCB surface temperature measured 1mm from the device package and centered on the package

edge

For detailed information on the thermal metrics and how to use the metrics, see the Semiconductor and IC

Package Thermal Metrics application note.

7.1.4 Power Dissipation (PD)

Circuit reliability requires consideration of the device power dissipation, location of the circuit on the printed

circuit board (PCB), and correct sizing of the thermal plane. The PCB area around the regulator must have few

or no other heat-generating devices that cause added thermal stress.

To first-order approximation, power dissipation in the regulator depends on the input-to-output voltage difference

and load conditions. The following equation calculates power dissipation (PD).

PD = (VIN – VOUT) × IOUT

(4)

Note

Power dissipation is minimized, and therefore greater efficiency is 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. Make sure this pad area

contains 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)

(5)

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LP2951-Q1

SLVSAW6H – JUNE 2011 – REVISED NOVEMBER 2024

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