MIC94092 datasheet(13/16 Pages) MICREL

13 / 16 page

Micrel, Inc.

MIC94090/1/2/3/4/5

March 2009

M9999-033109-A

Application Information

Power Dissipation Considerations

As with all power switches, the ultimate current rating of

the switch 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:

Figure 1. 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).

Figure 2. Thermal Equivalent 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

CA) and the package (RθJA).

()

AMB

DISS

⋅

(1)

It is this equation that is used to determine the graphs on

page 7. PDISS is calculated as (ISWITCH

2 x R

SWmax). RθJC is

found in the operating ratings section of the datasheet

and R

(the PCB thermal resistance) values for

various PCB copper areas can be taken from ‘Designing

with Low Dropout Voltage Regulators’

(1) available from

the Micrel website (LDO Application Hints).

Example:

A switch is intended to drive a 500mA load 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:

ISW = 0.5A

VIN = 3V to 4.2V

Tamb = 50

JC = 60

oC/W from Datasheet (P. 3)

CA = 53

oC/W Read from Graph in Fig. 3

Figure 3. Excerpt from the LDO Book

(1)

PDISS = ISW

2 x R

SWmax

The worst case switch resistance (RSWmax) at the lowest

VIN of 3V is not available in the datasheet, so the next

lower value of VIN is used.

RSWmax @ 2.5v = 315mΩ

If this were a figure for worst case RSWmax for 25

oC, an

additional consideration is to allow for the maximum

junction temperature of 125

oC, the actual worst case

resistance in this case will be 30% higher (See RDS(on)

variance vs. temperature graph).

RSWmax @ 2.5v (@ 125’C) = 315 x 1.3 = 410mΩ

Therefore junction temperature (TJ):

TJ = 0.5

2 x 0.41 x (60+53) + 50

from (Eqn. 1)

TJ = 62

This is well below the maximum 125

oC.

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