MIC2874YCS datasheet(18/22 Pages) MICREL

18 / 22 page

Micrel, Inc.

MIC2874

DISS

)

(

Eq. 2

As can be seen in the diagram, total thermal resistance

JA = θJC + θCA. Hence this can also be written as in

Equation 3:

DISS

)

(

Eq. 3

Since effectively all of the power losses (minus the

inductor losses) in the converter are dissipated within the

MIC2874 package, PDISS can be calculated thus:

Linear Mode:

DCR

]

OUT

DISS

−













Eq. 4

Boost Mode:

DCR

]

OUT

DISS

−

























Eq. 5

Duty Cycle in Boost Mode:

OUT

−

Eq. 6

where:

η = Efficiency taken from efficiency curves and DCR =

inductor DCR.

and

are found in the operating

ratings section of the data sheet.

Where the real board area differs from 1 inch square,

(the PCB thermal resistance) values for various PCB

copper areas can be taken from Figure 7. Figure 7 is taken

from Designing with Low Dropout Voltage Regulators

available from the Micrel website (“LDO Application

Hints”).

Figure 7. Graph to Determine PC Board Area for a Given

PCB Thermal Resistance

Figure 7 shows the total area of a round or square pad,

centered on the device. The solid trace represents the

area of a square, single sided, in horizontal orientation,

solder masked, copper PC board trace heat sink,

measured in square millimeters. No airflow is assumed.

The dashed line shows PC boards trace heat sink covered

in black oil-based paint and with 1.3m/sec (250 feet per

minute) airflow. This approaches a “best case” pad heat

sink. Conservative design dictates using the solid trace

data, which indicates that a maximum pad size of 5000

2 is needed. This is a pad 71mm by 71mm (2.8 inches

per side).

August 26, 2014

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