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J +

A )

Dmax x R

θJC ) RθCS ) RθSA

(2)

J +

A )

Dmax x R

θJA

(3)

RθJA +

J–TA

Dmax

(4)

TO-220 Power Dissipation

Dmax +

(3.3 – 2.5) V x 3 A + 2.4 W

(5)

RθJAmax + (125 – 55)°C 2.4 W + 29°C W

(6)

TPS75801, TPS75815

TPS75818, TPS75825

TPS75833

SLVS330D – JUNE 2001 – REVISED MARCH 2004

THERMAL INFORMATION (continued)

Equation 2 summarizes the computation:

The RΘJC is specific to each regulator as determined by its package, lead frame, and die size provided in the

regulator's data sheet. The RΘSA is a function of the type and size of heatsink. For example, black body radiator

type heatsinks, like the one attached to the TO-220 package in Figure 21(a), can have RΘCS values ranging from

°C/W for very large heatsinks to 50 °C/W for very small heatsinks. The RΘCS is a function of how the package is

attached to the heatsink. For example, if a thermal compound is used to attach a heatsink to a TO-220 package,

RΘCSof 1°C/W is reasonable.

Even if no external black body radiator type heatsink is attached to the package, the board on which the

regulator is mounted will provide some heatsinking through the pin solder connections. Some packages, like the

TO-263 and TI's TSSOP PowerPAD™ packages, use a copper plane underneath the package or the circuit

board's ground plane for additional heatsinking to improve their thermal performance. Computer aided thermal

modeling can be used to compute very accurate approximations of an integrated circuit's thermal performance in

different operating environments (e.g., different types of circuit boards, different types and sizes of heatsinks,

different air flows, etc.). Using these models, the three thermal resistances can be combined into one thermal

resistance between junction and ambient (RΘJA). This RΘJA is valid only for the specific operating environment

used in the computer model.

Equation 2 simplifies into Equation 3:

Rearranging Equation 3 gives Equation 4:

Using Equation 3 and the computer model generated curves shown in Figure 22 and Figure 25, a designer can

quickly compute the required heatsink thermal resistance/board area for a given ambient temperature, power

dissipation, and operating environment.

The TO-220 package provides an effective means of managing power dissipation in through-hole applications.

The TO-220 package dimensions are provided in the Mechanical Data section at the end of the data sheet. A

heatsink can be used with the TO-220 package to effectively lower the junction-to-ambient thermal resistance.

To illustrate, the TPS75825 in a TO-220 package was chosen. For this example, the average input voltage is 3.3

V, the average output voltage is 2.5 V, the average output current is 3 A, the ambient temperature 55

°C, the air

flow is 150 LFM, and the operating environment is the same as documented below. Neglecting the quiescent

current, the maximum average power is:

Substituting TJmax for TJ into Equation 4 gives Equation 6:

From Figure 22, RΘJA vs Heatsink Thermal Resistance, a heatsink with RΘSA = 22°C/W is required to dissipate

2.4 W. The model operating environment used in the computer model to construct Figure 22 consisted of a

standard JEDEC High-K board (2S2P) with a 1 oz. internal copper plane and ground plane. Since the package

pins were soldered to the board, 450 mm2 of the board was modeled as a heatsink. Figure 23 shows the side

view of the operating environment used in the computer model.

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