MIC94080 datasheet(13/16 Pages) MICROCHIP

13 / 16 page

Micrel, Inc.

MIC94080/1/2/3/4/5

January 2011

M9999-013111-D

Application Information

Power Switch SOA

The safe operating area (SOA) curve represents the

boundary of maximum safe operating current and

maximum safe operating junction temperature.

Figure 1. SOA Graph

The curves above show the SOA for various VIN’s

mounted on a typical 1 layer, 1 square inch copper

board.

Power Dissipation Considerations

As with all power switches, the current rating of the

switch is limited mostly 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:

Figure 2. Simple 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 by replacing current

sources with power dissipation (in Watts), resistance

with thermal resistance (in ºC/W) and voltage sources

with temperature (in ºC).

Figure 3. Simple Thermal Circuit

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).

TJ = PDISS x (RθJC + RθCA) + TA

PDISS is calculated as ISWITCH

2 x R

SWmax. RθJC is found in

the operating ratings section of the datasheet and R

θCA

(the PCB thermal resistance) values for various PCB

copper areas is discussed in the document “Designing

with Low Dropout Voltage Regulators” available from the

Micrel website (LDO Application Hints).

Example:

A switch is intended to drive a 1A load and is placed on

a printed circuit board which has a ground plane area of

at least 25mm by 25mm (625mm

2). 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 50ºC.

Summary of variables:

ISW = 1A

VIN = 3V to 4.2V

TA = 50

θJC = 85ºC/W

θCA = 53ºC/W Read from Graph in Figure 4

Figure 4. Excerpt from the LDO Book

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