TC1301A-ADAVUATR datasheet(18/28 Pages) MICROCHIP

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TC1301A/B

DS21798C-page 18

EQUATION 6-3:

The maximum power dissipation capability for a

package can be calculated given the junction to

ambient thermal resistance and the maximum ambient

temperature for the application. The following equation

can be used to determine the package maximum

internal power dissipation.

EQUATION 6-4:

EQUATION 6-5:

EQUATION 6-6:

6.3

Typical Application

Internal power dissipation, junction temperature rise,

junction temperature, and maximum power dissipation

are calculated in the following example. The power

dissipation as a result of ground current is small

enough to be neglected.

6.3.1

POWER DISSIPATION EXAMPLE

Device Junction Temperature Rise

The internal junction temperature rise is a function of

internal power dissipation and the thermal resistance

from junction to ambient for the application. The

thermal resistance from junction to ambient (R

JA) is

derived from an EIA/JEDEC standard for measuring

thermal resistance for small surface-mount packages.

The EIA/JEDEC specification is JESD51-7, “High

Effective Thermal Conductivity Test Board for Leaded

Surface Mount Packages”. The standard describes the

test method and board specifications for measuring the

thermal resistance from junction to ambient. The actual

thermal resistance for a particular application can vary

depending on many factors such as copper area and

thickness. Refer to AN792, “A Method To Determine

How Much Power a SOT-23 Can Dissipate in Your

Application” (DS00792), for more information regarding

this subject.

JMAX

()

TOTAL

AMAX

Where:

TJ(MAX)

Maximum continuous junction tem-

perature

PTOTAL

Total device power dissipation

Thermal resistance from junction-

to-ambient

TAMAX

Maximum ambient temperature

DMAX

()

JMAX

()

AMAX

()

–

()

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

Where:

PD(MAX)

Maximum device power

dissipation

TJ(MAX)

Maximum continuous junction

temperature

TAMAX

Maximum ambient temperature

Thermal resistance from junction-

to-ambient

JRISE

()

DMAX

()

Where:

TJ(RISE)

Rise in device junction

temperature over the ambient

temperature

PD(MAX)

Maximum device power

dissipation

Thermal resistance from junction-

to-ambient

JRISE

()

Where:

Junction Temperature

TJ(RISE)

Rise in device junction

temperature over the ambient

temperature

Ambient Temperature

Package

Package Type =

3x3 DFN8

Input Voltage

VIN = 2.7V to 4.2V

LDO Output Voltages and Currents

VOUT1 = 2.8V

IOUT1 = 300 mA

VOUT2 = 1.8V

IOUT2 = 150 mA

Maximum Ambient Temperature

TA(MAX) = 50°C

Internal Power Dissipation

Internal power dissipation is the sum of the power

dissipation for each LDO pass device.

PLDO1(MAX) =(VIN(MAX) - VOUT1(MIN)) x

IOUT1(MAX)

PLDO1 = (4.2V - (0.975 x 2.8V)) x 300 mA

PLDO1 = 441.0 milliWatts

PLDO2 = (4.2V - (0.975 X 1.8V)) x 150 mA

PLDO2 = 366.8 milliWatts

PTOTAL =PLDO1 + PLDO2

PTOTAL= 807.8 milliWatts

TJ(RISE) =PTOTAL x RqJA

TJRISE = 807.8 milliWatts x 41.0° C/W

TJRISE =33.1°C

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