TC1302A datasheet(15/26 Pages) MICROCHIP

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DS21333B-page 15

TC1302A/B

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

θJA) 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 SOT23 Can Dissipate in an

Application”,

(DS00792),

for

information

regarding this subject.

Junction Temperature Estimate

To estimate the internal junction temperature, the

calculated temperature rise is added to the ambient or

offset temperature. For this example, the worst-case

junction temperature is estimated below.

Maximum Package Power Dissipation at 50°C

Ambient Temperature

Package

Package Type = 3x3DFN8

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

DMAX

()

JMAX

() TAMAX

()

–

()

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

PD(MAX) = maximum device power dissipation.

TJ(MAX) = maximum continuous junction

temperature.

TA(MAX) = maximum ambient temperature.

θJA

= Thermal resistance from junction to

ambient.

JRISE

()

DMAX

()

TJ(RISE) = Rise in device junction temperature over

the ambient temperature.

PD(MAX) = Maximum device power dissipation.

θJA

= Thermal resistance from junction-to-

ambient.

JRISE

()

= Junction temperature.

TJ(RISE) = Rise in device junction temperature over

the ambient temperature.

= Ambient Temperature.

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

TJ =TJRISE + TA(MAX)

TJ = 83.1°C

3x3DFN8 (41°C/Watt R

θJA)

PD(MAX) = (125°C - 50°C)/41° C/W

PD(MAX) = 1.83 Watts

MSOP8 (208°C/Watt R

θJA)

PD(MAX) = (125°C - 50°C)/208° C/W

PD(MAX) = 0.360 Watts

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