© 2002 Microchip Technology Inc.

DS21665B-page 5

TC2117

TABLE 3-2:

THERMAL RESISTANCE

GUIDELINES FOR TC2117 IN

3-PIN DDPAK PACKAGE

Equation 3-1

can

be

used

in

conjunction

with

Equation 3-2 to ensure regulator thermal operation is

within limits. For example:

Find:

1. Actual power dissipation

2. Maximum allowable dissipation

Actual power dissipation:

Maximum allowable power dissipation:

In this example, the TC2117 dissipates a maximum of

only 786mW; below the allowable limit of 1.186mW. In

a similar manner, Equation 3-1 and Equation 3-2 can

be used to calculate maximum current and/or input

voltage limits.

Copper

Area

(Topside)*

Copper

Area

(Backside)

Board Area

Thermal

Resistance

(

θ

2500 sq mm

2500 sq mm

2500 sq mm

25°C/W

1000 sq mm

2500 sq mm

2500 sq mm

27°C/W

125sqmm

2500sq mm

2500sq mm

35°C/W

*Tab of device attached to topside copper.

Given:

= 786mW

=

(125 – 55)

59

=

1.186mW