Figure 8-1 shows one approach for protecting the device.

IN

OUT

GND

CIN

GND

GND

COUT

GND

Internal Body Diode

Schottky Diode

Figure 8-1. Example Circuit for Reverse Current Protection Using a Schottky Diode

8.1.5 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

(2)

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)

(3)

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.

8.1.6 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

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LP2985

SLVS522Q – JULY 2004 – REVISED DECEMBER 2022

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