MIC2873YCS datasheet(18/24 Pages) MICREL

18 / 24 page

Micrel, Inc.

MIC2873

Power Dissipation Consideration

As with all power devices, the ultimate current rating of the

output is limited by the thermal properties of the device

package and the PCB on which the device is mounted.

There is a simple, Ohm’s law type relationship between

thermal resistance, power dissipation and temperature

which are analogous to an electrical circuit:

Figure 5. Series Electrical Resistance Circuit

From this simple circuit we can calculate VX if we know

ISOURCE, VZ and the resistor values, RXY and RYZ using

Equation 1:

SOURCE

)

Eq. 1

Thermal circuits can be considered using this same rule

and can be drawn similarly by replacing current sources

with power dissipation (in watts), resistance with thermal

resistance (in °C/W) and voltage sources with temperature

(in °C).

Figure 6. Series Thermal Resistance Circuit

Now replacing the variables in the equation for VX, we can

find

the

junction

temperature

(TJ) from the power

dissipation, ambient temperature and the known thermal

resistance of the PCB (

CA) and the package (θJC).

DISS

)

(

Eq. 2

As can be seen in the diagram, total thermal resistance

JA = θJC + θCA. Hence this can also be written as in

Equation 3:

DISS

)

(

Eq. 3

Since effectively all of the power losses (minus the

inductor losses) in the converter are dissipated within the

MIC2873 package, PDISS can be calculated thus:

Linear Mode:

DCR

]

OUT

DISS

−













Eq. 4

Boost Mode:

DCR

]

OUT

DISS

−

























Eq. 5

Duty Cycle in Boost Mode:

OUT

−

Eq. 6

where:

η = Efficiency taken from efficiency curves and DCR =

inductor DCR.

and

are found in the operating

ratings section of the data sheet.

July 17, 2014

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