Device

IN

OUT

GND

COUT

CIN

Schottky Diode

Internal Body Diode

21

TPS746

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SBVS337A – APRIL 2018 – REVISED DECEMBER 2018

Product Folder Links: TPS746

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Application Information (continued)

If reverse current flow is expected in the application, external protection must be used to protect the device.

Figure 49 shows one approach of protecting the device.

Figure 49. Example Circuit for Reverse Current Protection Using a Schottky Diode

8.1.6 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. Equation 4 calculates power dissipation (PD).

PD = (VIN – VOUT) × IOUT

(4)

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 Equation 5, 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)

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.

8.1.7 Power-Good Function

The power-good circuit monitors the voltage at the feedback pin to indicate the status of the output voltage.

When the output voltage falls below the PG threshold voltage (PGLTH), the PG pin open-drain output engages

and pulls the PG pin close to GND. When the output voltage exceeds PGHTH, the PG pin becomes high

impedance. By connecting a pullup resistor to an external supply, any downstream device can receive power-

good as a logic signal that can be used for sequencing. Make sure that the external pullup supply voltage results

in a valid logic signal for the receiving device. Using a pullup resistor from 10 kΩ to 100 kΩ is recommended.