MIC5249(2004) データシートの表示（PDF） - Micrel
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MIC5249 Datasheet PDF : 7 Pages
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MIC5249
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:
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 i s
layout dependent; Table 1 shows examples of the junction-
to-ambient thermal resistance for the MIC5249.
Package
θJA Recommended
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:
PD = (VIN – VOUT ) IOUT + VIN IGND
Micrel
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 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 calcula-
tion.
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.
M0382-033004
6
March 2004
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