MIC5249

Micrel

M0382-033004

6

March 2004

Thermal Consideration

The MIC5249 is designed to provide 300mA of continuous

current in a very small package. Maximum power dissipation

can be calculated based on the output current and the voltage

drop across the device. To determine the maximum power

dissipation of the package, use the junction-to-ambient ther-

mal resistance of the device and the following basic equation:

P max

T max

T

D

JA

JA

()

()

=

−









θ

T

125

°C and T

layout dependent; Table 1 shows examples of the junction-

to-ambient thermal resistance for the MIC5249.

Package

θθθθθ

Minimum Footprint

MSOP-8 (MM)

200

°C/W

Table 1. MSOP-8 Thermal Resistance

The actual power dissipation of the regulator circuit can be

determined using the equation:

P

Substituting P

conditions that are critical to the application will give the

maximum operating conditions for the regulator circuit. For

example, when operating the MIC5249-3.0BMM at 50

°C with

a minimum footprint layout, the maximum input voltage for a

set output current can be determined as follows:

Pmax

125 C

50 C

200 C/W

°−

°

°









The junction-to-ambient thermal resistance for the minimum

footprint is 200

°C/W, from Table 1. The maximum power

dissipation must not be exceeded for proper operation. Using

the output voltage of 3.0V, and an output current of 300mA,

the maximum input voltage can be determined. Because this

device is CMOS and the ground current is typically 90

µA over

the load range, the power dissipation contributed by the

ground current is < 1.0% and can be ignored for this calcula-

tion.

375mW = (V

375mW = V

1275mW = V

V

Therefore, a 3.0V application at 300mA of output current can

accept a maximum input voltage of 4.25V in the MSOP-8

package. For a full discussion of heat sinking and thermal

effects on the voltage regulators, refer to the “Regulator

Thermals” section of

Micrel’s Designing with Low-Dropout

Voltage Regulators Handbook.