8.2.2.7 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 used in accordance with Equation 4

or Equation 5 and are given in the Thermal Information table.

TJ(MAX) = TTOP + (ΨJT × PD(MAX))

(4)

where:

• PD(MAX) is explained in Equation 3

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

TJ(MAX) = TBOARD + (ΨJB × PD(MAX))

(5)

where:

• PD(MAX) is explained in the Power Dissipation section

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

package edge

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

Thermal Metrics application note; for more information about measuring TTOP and TBOARD, see the Using

New Thermal Metrics application note; and for more information about the EIA/JEDEC JESD51 PCB used for

validating RθJA, see the Thermal Characteristics of Linear and Logic Packages Using JEDEC PCB Designs

application note. These application notes are available at www.ti.com.

8.2.3 Application Curves

Time (µs)

0

50

100

150

200

250

300

350

400

450

500

0.0

0.5

1.0

1.5

2.0

2.5

D029

VEN (V)

VOUT (V)

VPG (V)

VIN = 2.3 V, IOUT = 500 mA, COUT = 1 µF

Figure 8-2. LP5912-1.8 VOUT vs VEN (On)

Time (µs)

0

5

10

15

20

25

0.0

0.5

1.0

1.5

2.0

2.5

D030

VEN (V)

VOUT (V)

VPG (V)

VIN = 2.3 V, IOUT = 500 mA (3.6 Ω), COUT = 1 µF

Figure 8-3. LP5912-1.8 VOUT vs VEN (Off)

LP5912-EP

SNVSC57 – SEPTEMBER 2022

www.ti.com

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