TC2117 datasheet(8/18 Pages) MICROCHIP

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TC2117

DS21665C-page 8

TABLE 4-2:

THERMAL RESISTANCE

GUIDELINES FOR TC2117 IN

3-PIN DDPAK PACKAGE

Equation 4-1 can be used in conjunction with

Equation 4-2 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 786 mW; below the allowable limit of 1.186W. In a

similar manner, Equation 4-1 and Equation 4-2 can be

used to calculate maximum current and/or input volt-

age limits.

Copper

Area

(Topside)*

Copper

Area

(Backside)

Board Area

Thermal

Resistance

(

θJA)

2500 sq mm

25°C/W

1000 sq mm

2500 sq mm

27°C/W

125 sq mm

2500 sq mm

35°C/W

*Tab of device attached to topside copper.

Given:

VINMAX =5.0V ± 5%

VOUTMIN = 3.3V ± 0.5%

ILOADMAX = 400 mA

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

= 786 mW

PDMAX

=(TJMAX – TAMAX)

θJA

(125 – 55)

1.186W

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