TC1307 datasheet(18/28 Pages) MICROCHIP

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TC1307

DS21702A-page 18

 2002 Microchip Technology Inc.

5.5

Typical Application

Internal power dissipation, junction temperature rise,

junction temperature and maximum power dissipation

are calculated in the following example. The power dis-

sipation as a result of quiescent current and RESET

output are small enough to be neglected.

Input Voltage:

VIN = 3.1V to 4.1V

LDO Output Voltages and Currents:

VOUT1 = 3.0V

IIOUT1 = 100 mA

VOUT2 = 3.0V

IIOUT2 = 100 mA

VOUT3 = 1.8V

IIOUT3 = 60 mA

VOUT4 = 1.8V

IIOUT4 = 60 mA

Maximum Ambient Temperature:

TA(MAX)= 50°C

Internal Power Dissipation:

Internal Power dissipation is the sum of the power dis-

sipation for each LDO pass device.

PLDO1 = (VIN(MAX)-VOUT1(MIN)) x IOUT1(MAX)

PLDO1 = (4.1V - (0.975 x 3.0V)) x 100 mA

PLDO1 = 117.5 milli-Watts

PLDO2 = (4.1V - (0.975 x 3.0V)) x 100 mA

PLDO2 = 117.5 milli-Watts

PLDO3 = (4.1V - (0.975 x 1.8V)) x 60 mA

PLDO3 = (2.35V x 60 mA)

PLDO3 = 140.7 milli-Watts

PLDO4 = (4.1V - (0.975 x 1.8V)) x 60 mA

PLDO4 = 140.7 milli-Watts

PTOTAL = PLDO1 + PLDO2 + PLDO3 + PLDO4

PTOTAL = 516.4 milli-Watts

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 ther-

mal resistance from junction to air (

θJA) is derived from

an EIA/JEDEC standard for measuring thermal resis-

tance for small surface mount packages. The EIA/

JEDEC specification is JESD51-7 “High Effective Ther-

mal 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 case. The actual thermal

resistance for a particular application can vary depend-

ing on many factors such as copper area and thick-

ness. Refer to AN792 for more information regarding

this subject.

TJRISE = PTOTAL x θJA

TJRISE = 516.4 milli-Watts x 112.4°C/Watt

TJRISE = 58.1°C

Junction Temperature Estimate

To estimate the internal junction temperature (TJ), the

calculated junction temperature rise (TJRISE) is added

to the ambient or offset temperature (TAMBIENT). For

this example the worst case junction temperature is

estimated below.

TJ =TJRISE + TAMBIENT

TJ =108.1°C

Maximum Package Power Dissipation

The maximum power dissipation capability for the

TC1307 can be approximated by finding the maximum

allowable temperature rise from junction to case and

dividing that by the estimated thermal resistance of the

application. For this example, the maximum allowable

junction temperature rise is 125°C - 50°C or 75°C. By

dividing 75°C by the estimated thermal resistance

(112.4°C/Watt), the maximum allowable power dissipa-

tion is calculated to be 667.3 milli-Watts.

5.6

Device Protection

5.6.1

OVER CURRENT LIMIT

In the event of a faulted output load, the maximum cur-

rent the LDO will permit to flow is limited internally. For

each of the four LDO’s internal to the TC1307, the limit

in the event of a short circuit will be 360 mA typical.

This limit can be used to prevent damage to the circuit

board or connectors. The over current protection for

each LDO output is independent. For example, if LDO1

output is shorted to ground, the over current protection

will limit the output current for LDO1. If the junction tem-

perature does not rise above the typical 150°C thermal

shutdown point the other three LDO outputs (LDO2,

LDO3, LDO4) will remain within regulation.

PDMAX

()

TJMAX

()

TAMAX

()

–

()

θJA

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