MIC33030 datasheet(13/18 Pages) MICREL

13 / 18 page

Micrel Inc.

MIC33030

February 2011

M9999-020311-C

Power Dissipation Considerations

As with all power devices, the ultimate current rating of

the output is limited by the thermal properties of the

package and the PCB it is mounted on. There is a

simple, ohms law type relationship between thermal

resistance, power dissipation and temperature which are

analogous to an electrical circuit:

From this simple circuit we can calculate Vx if we know

Isource, Vz and the resistor values, Rxy and Ryz using

the equation:

() Vz

Ryz

Rxy

Isource

⋅

Thermal circuits can be considered using these same

rules and can be drawn similarly replacing current

sources with Power dissipation (in Watts), Resistance

with Thermal Resistance (in

ºC/W) and Voltage sources

with temperature (in

ºC):

Now replacing the variables in the equation for Vx, we

can find the junction temperature (TJ) from power

dissipation, ambient temperature and the known thermal

resistance of the PCB (R

θCA) and the package (RθJC):

()

DISS

⋅

AMB

As can be seen in the diagram, total thermal resistance

θJA = RθJC + RθCA. Hence this can also be written:

()

AMB

DISS

⋅

Since effectively all of the power loss in the converter is

dissipated within the MIC33030 package, PDISS can be

calculated thus:

)

(

OUT

DISS

−

⋅

Where

η = Efficiency taken from efficiency curves

θJC and RθJA are found in the operating ratings section

of the datasheet.

Example:

A MIC33030 is intended to drive a 300mA load at 1.8V

and is placed on a printed circuit board which has a

ground plane area of at least 25mm square. The Voltage

source is a Li-ion battery with a lower operating

threshold of 3V and the ambient temperature of the

assembly can be up to 50

ºC.

Summary of variables:

IOUT = 0.3A

VOUT = 1.8V

VIN = 3V to 4.2V

TAMB = 50

ºC

θJA = 76

ºC/W from Datasheet

η @ 300mA = 75% (worst case with VIN=4.2V from the

Typical Characteristics Efficiency vs. Load graphs)

)

(

PDISS

−

⋅

= 0.18W

The worst case switch and inductor resistance will

increase at higher temperatures, so a margin of 20% can

be added to account for this:

PDISS = 0.18 x 1.2 = .216W

Therefore:

TJ = 0.216W. (76

ºC/W) + 50ºC

TJ = 66

ºC

This is well below the maximum 125

ºC.

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