MIC23303 datasheet(15/21 Pages) MICREL

15 / 21 page

Micrel, Inc.

MIC23303

September 6, 2013

090613-2.0

From this simple circuit we can calculate VX if we know

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

equation:

(

)

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 °C/W) and voltage sources with temperature

(in °C).

Figure 6. Thermal Circuit Description

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 (RθJC).

(

)

AMB

DISS

As can be seen in the diagram, total thermal resistance

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

(

)

AMB

DISS

Since effectively all of the power losses (minus the

inductor losses) in the converter are dissipated within the

MIC23303 package, PDISS can be calculated thus:

DCR

)

(

OUT

DISS

−













−

Where:

η = Efficiency taken from efficiency curves and DCR =

Inductor DCR.

JC and RθJA are found in the Operating Ratings section

of the datasheet. Where the reel board area differs from

1in square, R

CA (the PCB thermal resistance) values for

various PCB copper areas can be taken from Figure 7

below. This graph is taken from Designing with Low

Dropout Voltage Regulators, which is available from the

Micrel website (LDO Application Hints).

Example:

A MIC23303 is intended to drive a 2A 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°C.

Summary of variables:

IOUT = 2A

VOUT = 1.8V

VIN = 3V to 4.2V

TAMB = 50°C

JA = 61°C/W from datasheet

η @ 2A = 85% (worst case @ 5V from Figure 3)

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