LP2954IMSLASHNOPB datasheet(15/28 Pages) TI1

15 / 28 page

LP2954, LP2954A

www.ti.com

SNVS096E – JUNE 1999 – REVISED JULY 2016

Product Folder Links: LP2954 LP2954A

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8.2.2.4 Power Dissipation

Knowing the device power dissipation and proper sizing of the thermal plane connected to the tab or pad is

critical to ensuring reliable operation. Device power dissipation depends on input voltage, output voltage, and

load conditions and can be calculated with Equation 3.

PD(MAX) = (VIN(MAX) – VOUT) × IOUT

(3)

Power dissipation can be minimized, and greater efficiency can be achieved, by using the lowest available

voltage drop option that would still be greater than the dropout voltage (VDO). However, keep in mind that higher

voltage drops result in better dynamic (that is, PSRR and transient) performance.

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), according to

Equation 4 or Equation 5:

TJ(MAX) = TA(MAX) + (RθJA × PD(MAX))

(4)

PD(MAX) = (TJ(MAX) – TA(MAX)) / RθJA

(5)

Unfortunately, this RθJA is highly dependent on the heat-spreading capability of the particular PCB design, and

therefore varies according to the total copper area, copper weight, and location of the planes. The RθJA recorded

in Thermal Information is determined by the specific EIA/JEDEC JESD51-7 standard for PCB and copper-

spreading area, and is to be used only as a relative measure of package thermal performance. For a well-

designed thermal layout, RθJA is actually the sum of the package junction-to-case (bottom) thermal resistance

(RθJCbot) plus the thermal resistance contribution by the PCB copper area acting as a heat sink.

8.2.2.5 Estimating Junction Temperature

The EIA/JEDEC standard recommends the use of psi (Ψ) thermal characteristics to estimate the junction

temperatures of surface mount devices on a typical PCB board application. These characteristics are not true

thermal resistance values, but rather package specific thermal characteristics that offer practical and relative

means of estimating junction temperatures. These psi metrics are determined to be significantly independent of

copper-spreading area. The key thermal characteristics (ΨJT and ΨJB) are given in Thermal Information and are

used in accordance with Equation 6 or Equation 7.

TJ(MAX) = TTOP + (ΨJT × PD(MAX))

where

•

PD(MAX) is explained in Equation 5

•

TTOP is the temperature measured at the center-top of the device package.

(6)

TJ(MAX) = TBOARD + (ΨJB × PD(MAX))

where

•

PD(MAX) is explained in Equation 5.

•

TBOARD is the PCB surface temperature measured 1-mm from the device package and centered on the

package edge.

(7)

For more information about the thermal characteristics ΨJT and ΨJB, Semiconductor and IC Package Thermal

Metrics; for more information about measuring TTOP and TBOARD, see Using New Thermal Metrics; and for more

information about the EIA/JEDEC JESD51 PCB used for validating RθJA, see Thermal Characteristics of Linear

and Logic Packages Using JEDEC PCB Designs. These application notes are available at www.ti.com.

8.2.2.6 Heatsinking the TO-220 Package

A heat sink may be required with the LP2954IT depending on the maximum power dissipation and maximum

ambient temperature of the application. Under all possible operating conditions, the junction temperature must be

within the range specified under Recommended Operating Conditions.

To determine if a heat sink is required, the maximum power dissipated by the regulator, P(MAX), must be

calculated. It is important to remember that if the regulator is powered from a transformer connected to the AC

line, the maximum specified AC input voltage must be used (because this produces the maximum DC input

voltage to the regulator). Figure 20 shows the voltages and currents that are present in the circuit. The formula

for calculating the power dissipated in the regulator is also shown in Figure 20.

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