TPS7A0233DBVR datasheet(25/43 Pages) TI

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TPS7A02

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SBVS277B – JULY 2019 – REVISED MARCH 2020

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Application Information (continued)

The maximum power dissipation determines the maximum allowable junction temperature (TJ) for the device.

According to Equation 3, 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). Equation 4 rearranges Equation 3 for output current.

TJ = TA + (RθJA × PD)

(3)

IOUT = (TJ – TA) / [RθJA × (VIN – VOUT)]

(4)

Unfortunately, this 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 RθJA recorded in the Thermal Information table is determined by the JEDEC standard, PCB, and

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

designed thermal layout, RθJA is actually the sum of the X2SON package junction-to-case (bottom) thermal

resistance (RθJC(bot)) plus the thermal resistance contribution by the PCB copper.

8.1.5.1 Estimating Junction Temperature

The JEDEC standard now recommends the use of psi (Ψ) thermal metrics to estimate the junction temperatures

of the LDO when in-circuit on a typical PCB board application. These metrics are not strictly speaking thermal

resistances, but rather offer practical and relative means of estimating junction temperatures. These psi metrics

are determined to be significantly independent of the copper-spreading area. The key thermal metrics (ΨJT and

ΨJB) are used in accordance with Equation 5 and are given in the Thermal Information table.

ΨJT : TJ = TT + ΨJT × PD and ΨJB : TJ = TB + ΨJB × PD

where:

•

PD is the power dissipated as explained in Equation 2

•

TT is the temperature at the center-top of the device package, and

•

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

edge

(5)

8.1.5.2 Recommended Area for Continuous Operation

The operational area of an LDO is limited by the dropout voltage, output current, junction temperature, and input

voltage. The recommended area for continuous operation for a linear regulator is given in Figure 60 and can be

separated into the following parts:

•

Dropout voltage limits the minimum differential voltage between the input and the output (VIN – VOUT) at a

given output current level. See the Dropout Operation section for more details.

•

The rated output currents limits the maximum recommended output current level. Exceeding this rating

causes the device to fall out of specification.

•

The rated junction temperature limits the maximum junction temperature of the device. Exceeding this rating

causes the device to fall out of specification and reduces long-term reliability.

–

The shape of the slope is given by Equation 4. The slope is nonlinear because the maximum rated

junction temperature of the LDO is controlled by the power dissipation across the LDO; thus when VIN –

VOUT increases the output current must decrease.

•

The rated input voltage range governs both the minimum and maximum of VIN – VOUT.

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