TC2117 datasheet(5/12 Pages) MICROCHIP

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DS21665B-page 5

TC2117

TABLE 3-2:

THERMAL RESISTANCE

GUIDELINES FOR TC2117 IN

3-PIN DDPAK PACKAGE

Equation 3-1

can

used

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

(

JA)

2500 sq mm

25°C/W

1000 sq mm

2500 sq mm

27°C/W

125sqmm

2500sq mm

35°C/W

*Tab of device attached to topside copper.

Given:

VINMAX =5.0V ± 5%

VOUTMIN = 3.3V ± 0.5%

ILOADMAX = 400mA

TJMAX = 125°C

TAMAX = 55°C

θJA = 59°C/W (SOT-223)

PD ≈ (VINMAX –VOUTMIN)ILOADMAX

= [(5.0 x 1.05) – (3.3 x .995)] 400 x 10-3

= 786mW

PDMAX

=(TJMAX –TAMAX)

θJA

(125 – 55)

1.186mW

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