MCP1790 datasheet(19/34 Pages) MICROCHIP

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DS22075A-page 19

MCP1790/MCP1791

5.3

Power Dissipation Example

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.

5.3.1

POWER DISSIPATION EXAMPLE

5.3.1.1

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 SOT23 Can Dissipate in an

Application”,

(DS00792),

for

information

regarding this subject.

5.3.1.2

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.

5.3.1.3

Maximum Package Power

Dissipation at +40°C Ambient

Temperature

5.4

Pulsed Load Applications

For some applications, there are pulsed load current

events that may exceed the specified 70 mA maximum

specification of the MCP1790/MCP1791. The internal

current foldback feature of the MCP1790/MCP1791 will

prevent high peak load demands from causing

non-recoverable

damage.

The

Current

Foldback

feature of the device will limit the output voltage and

output current during pulsed applications. As the cur-

rent rises above the foldback current threshold, the out-

put voltage will decrease.

Package:

Package Type = SOT-223-5

Input Voltage:

VIN = 8V to 24V

LDO Output Voltages and Currents:

VOUT =5.0V

IOUT =50 mA

Maximum Ambient Temperature:

TA(MAX) =+40°C

Internal Power Dissipation:

Internal Power dissipation is the product of the LDO

output current times the voltage across the LDO

(VIN to VOUT).

PLDO(MAX) =(VIN(MAX) - VOUT(MIN)) x IOUT(MAX)

PLDO = (24V - (0.98 x 5.0V)) x 50 mA

PLDO = 955 milli-Watts

TJ(RISE) =PTOTAL x RqJA

TJRISE = 955 milli-Watts x 62°C/Watt

TJRISE = 59.2°C

TJ =TJRISE + TA(MAX)

TJ = 99.2°C

SOT-223-5 (62°C/Watt = R

JA)

PD(MAX) = (125°C - 40°C) / 62°C/W

PD(MAX) = 1.371 Watts

DDPAK-5 (32°C/Watt = R

JA)

PD(MAX) = (125°C - 40°C) / 32°C/W

PD(MAX) = 2.656 Watts

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