MIC5249_12 Datasheet, PDF (6/7 Pages) Micrel Semiconductor – 300mA μCap LDO with Programmable Power-On Reset Delay

Micrel, Inc.
Thermal Considerations
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:
P_D(max)
=
⎜⎜⎝⎛
T_J(max) −
θ _JA
T_A
⎟⎟⎠⎞
T_J(max) is the maximum junction temperature of the die,
125°C and T_A is the ambient operating temperature. θ_JA
is layout dependent; Table 1 shows examples of the
junction-to-ambient thermal resistance for the MIC5249.
Package θ_JA Recommended Minimum Footprint
MSOP-8
160°C/W
Table 1. MSOP-8 Thermal Resistance
The actual power dissipation of the regulator circuit can
be determined using the equation:
P_D = (V_IN – V_OUT ) I_OUT + V_IN I_GND
MIC5249
Substituting P_D(max) for P_D 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 MIC5249-3.0YMM at
50°C with a minimum footprint layout, the maximum
input voltage for a set output current can be determined
as follows:
P_D(max)
=
⎜⎛^{125°C − 50°C} ⎟⎞
⎝ ^160°C/W ⎠
The junction-to-ambient thermal resistance for the
minimum footprint is 160°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:
468mW = (V_IN – 3.0V) 300mA
468mW = V_IN × 300mA – 900mW
1368mW = V_IN × 300mA
V_IN(max) = 4.56V
Therefore, a 3.0V application at 300mA of output current
can accept a maximum input voltage of 4.56V 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.
March 2012
6
M9999-030812