MIC2133 datasheet(39/50 Pages) MICROCHIP

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DS20006653B-page 39

MIC2133

Power dissipation in the MIC2133 is calculated in the

equation below when EXTVDD is used.

EQUATION 5-42:

The junction temperature of the MIC2133 can be

estimated using the equation below.

EQUATION 5-43:

The maximum recommended operating junction

temperature for the MIC2133 is 125°C. Using the out-

put voltage of the same switching converter when it is

between 4.7V (typical) and 14V, as the voltage at the

EXTVDD pin, significantly reduces the power dissipa-

tion inside the MIC2133. This reduces the junction tem-

perature rise as illustrated further.

For a typical case of:

VIN = 36V, VOUT = 5V, IG(TOTAL) = 20 mA, IQ = 5 mA, and

maximum ambient temperature of TA = 85°C.

When the EXTVDD pin is not used, the MIC2133 junc-

tion temperature is calculated as shown in the equation

below.

EQUATION 5-44:

When the EXTVDD is used and the 5V output of the

MIC2133 buck converter is used as the input to the

EXTVDD pin, the MIC2133 junction temperature is cal-

culated as shown in the equation below. The junction

temperature is significantly reduced from 115.6°C to

89.3°C when the EXTVDD is used.

EQUATION 5-45:

5.10

Thermal Measurements

It is a good idea to measure the IC's case temperature

to make sure it is within its operating limits. Although

this might seem an elementary task, it is easy to get

false results. The most common mistake is to use the

standard thermal couple that comes with a thermal

meter. This thermal couple wire gauge is large, typically

22 gauge, and behaves like a heatsink, which results in

a lower-case temperature measurement.

There are two methods of temperature measurement:

using a smaller thermal couple wire or using an infrared

thermometer. If a thermal couple wire is used, it must

be constructed of 36-gauge wire or higher (smaller wire

size) to minimize the wire heat-sinking effect. In

addition, the thermal couple tip must be covered in

either thermal grease or thermal glue to ensure that the

thermal couple junction makes good contact with the

case of the IC. Wherever possible, an infrared

thermometer is recommended. The measurement spot

size of most infrared thermometers is too large for an

accurate reading on small form factor ICs. However, an

IR thermometer with a 1-mm spot size is a good choice

for measuring the hottest point on the case. An optional

stand can be used to make it easy to hold the beam on

the IC for long periods of time. In addition, a more

advanced, convenient and accurate infrared thermal

camera can be used, although such equipment is much

more expensive.

Where:

VEXTVDD = Voltage at EXTVDD Pin

(4.7V ≤ VEXTVDD ≤ 14V typically)

IG(TOTAL) = Total Average Gate Drive Current for All

Phases

IQ = Quiescent Current of MIC2133

PIC

VEXTVDD

IGTOTAL





Where:

TJ = Junction Temperature of MIC2133

TA = Ambient Temperature

PIC = Power Dissipation of MIC2133

JA = Junction-to-Ambient Thermal Resistance of

MIC2133 (34°C/W typical)

PIC JA TA



PIC

36V20 mA

5 mA





PIC

0.9W

0.9W 34

CW



C



115.6

C

PIC

20 mA

5 mA





PIC

0.125W

0.125W 34

CW



C



89.3

C

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