MIC2875 datasheet(15/24 Pages) MICROCHIP

15 / 24 page

 2015 Microchip Technology Inc.

DS20005549A-page 15

MIC2875

7.0

POWER DISSIPATION

As with all power devices, the ultimate current rating of

the output is limited by the thermal properties of the

device package and the PCB on which the device is

mounted. There is a simple, Ohm’s law-type

relationship between thermal resistance, power

dissipation, and temperature which are analogous to

an electrical circuit (see Figure 7-1):

FIGURE 7-1:

Series Electrical Resistance

Circuit.

From this simple circuit we can calculate VX if we know

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

using Equation 7-1:

EQUATION 7-1:

Thermal circuits can be considered using this same

rule and can be drawn similarly by replacing current

sources with power dissipation (in watts), resistance

with thermal resistance (in °C/W) and voltage sources

with temperature (in °C).

FIGURE 7-2:

Series Thermal Resistance

Circuit.

Now replacing the variables in the equation for VX, we

can find the junction temperature (TJ) from the power

dissipation, ambient temperature and the known

thermal resistance of the PCB (θCA) and the package

(θJC).

EQUATION 7-2:

As can be seen in the diagram, total thermal resistance

θJA = θJC + θCA. This can also be written as in

Equation 7-3:

EQUATION 7-3:

Given that all of the power losses (minus the inductor

losses) are effectively in the converter are dissipated

within the MIC2875 package, PDISS can be calculated

thusly:

EQUATION 7-4:

LINEAR MODE

EQUATION 7-5:

BOOST MODE

EQUATION 7-6:

DUTY CYCLE (BOOST)

In the equations above, ƞ is the efficiency taken from

the efficiency curves and DCR represents the inductor

DCR. θJC and θJA are found in the temperature

specifications section of the data sheet.

Where the real board area differs from 1” square, θCA

(the PCB thermal resistance), values for various PCB

copper areas can be taken from Figure 7-3.

SOURCE

 V



DISS



 T



DISS



 T



DISS

OUT



---

–







OUT

–

DCR



DISS

OUT



---

–







OUT

–

-------------





–

DCR



OUT

–

OUT

-------------------------------

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