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MIC5249

300mA µCap LDO with Programmable Power-On Reset Delay

Micrel 

Micrel, Inc.

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 thermal resistance

of the device and the following basic equation:

PD(max)

=

⎜⎜⎝⎛

TJ(max) −

θ JA

TA

⎟⎟⎠⎞

TJ(max) is the maximum junction temperature of the die,

125°C and TA is the ambient operating temperature. θJA

is layout dependent; Table 1 shows examples of the

junction-to-ambient thermal resistance for the MIC5249.

Package

MSOP-8

θJA Recommended

Minimum Footprint

200°C/W

Table 1. MSOP-8 Thermal Resistance

The actual power dissipation of the regulator circuit can

be determined using the equation:

PD = (VIN – VOUT ) IOUT + VIN IGND

Substituting PD(max) for PD and solving for the operating

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

MIC5249

50°C with a minimum footprint layout, the maximum

input voltage for a set output current can be determined

as follows:

PD(max)

=

⎜⎛ 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 90µA over the load range,

the power dissipation contributed by the ground current

is < 1.0% and can be ignored for this calculation.

375mW = (VIN – 3.0V) 300mA

375mW = VIN × 300mA – 900mW

1275mW = VIN × 300mA

VIN(max) = 4.25V

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.

September 2006

6

M9999-090806

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