MIC33153 datasheet(13/17 Pages) MICREL

13 / 17 page

Micrel Inc.

MIC33153

September 2010

M9999-092910-A

As can be seen in the diagram, total thermal resistance

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

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, Ohm’s law type of relationship between thermal

resistance, power dissipation and temperature which is

analogous to an electrical circuit:

(

)

AMB

DISS

Rθ

Since effectively all of the power loss in the converter is

dissipated within the MIC33153 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.

From this simple circuit, one can calculate VX if one

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

using the equation:

Example:

A MIC33153 is intended to drive a 1A 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.

()

SOURCE

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):

Summary of variables:

IOUT = 1A

VOUT = 1.8V

VIN = 3V to 4.2V

TAMB = 50

ºC

θJA = 55

ºC/W from Datasheet

η @ 1A = 80% (worst case with VIN=4.2V from the

Typical Characteristics Efficiency vs. Load graphs)

0.80

(

1.8

PDISS

−

⋅

= 0.45W

The worst case switch and inductor resistance will

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

be added to account for this:

Now replacing the variables in the equation for VX, one

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):

PDISS = 0.45 x 1.2 = .54W

()

AMB

DISS

Rθ

Therefore:

TJ = 0.54W x (55

ºC/W) + 50ºC

TJ = 79.7

ºC

This is well below the maximum 125

ºC.

MIC33153 Datasheet(PDF) 13 Page - Micrel Semiconductor

MIC33153 Datasheet(HTML) 13 Page - Micrel Semiconductor

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Datasheet Download

Link URL