MIC33153 datasheet(16/28 Pages) MICROCHIP

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MIC33153

DS20006223B-page 16



2019 - 2022 Microchip Technology Inc.

FIGURE 5-3:

Electrical Circuit Analogous

to the Thermal Relief.

From this simple circuit Vx can be calculated if ISOURCE,

Vz and the resistor values, Rxy and Ryz are known,

using the Equation 5-5:

EQUATION 5-5:

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 5-4:

Thermal Relief 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 (RθCA) and the package

(RθJC):

EQUATION 5-6:

As can be seen in Figure 5-4, total thermal resistance

RθJA = RθJC + RθCA. This can also be calculated using

Equation 5-6:

EQUATION 5-7:

Because effectively all of the power loss in the

converter is dissipated within the MIC33153 package,

PDISS can be calculated by using Equation 5-8:

EQUATION 5-8:

RθJC and RθJA are found in the Section “Operating

Ratings ‡” of the data sheet.

EXAMPLE:

A MIC33153 is intended to drive a 1A load at 1.8V and

is placed on a printed circuit board which has a ground

plane area of at least 25 mm 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 = 1A

• VOUT = 1.8V

• VIN = 3V to 4.2V

• TAMB = 50°C

• RθJA = 55°C/W

η @ 1A = 80% (worst case with VIN = 4.2V) See

Section 2.0, Typical Performance Curves.

Isource

Rxy

Ryz

VX ISOURCE RXY RYZ







Pdiss

R JC

Tamb

R CA

TJ PDISS RJC RCA





TAMB



TJ PDISS RJA





TAMB



PDISS POUT 1--- 1

–











Where:

η =

Efficiency taken from Efficiency Curves

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