MCP1792 datasheet(17/32 Pages) MICROCHIP

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DS20006229C-page 17

MCP1792/3

EQUATION 5-4:

EQUATION 5-5:

EQUATION 5-6:

5.3

Typical Application Examples

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

EXAMPLE 5-1:

5.3.1.1

Device Junction Temperature Rise

The internal junction temperature rise is a function of

internal power dissipation and of the thermal resistance

from junction-to-ambient for the application. The

thermal resistance from junction-to-ambient (

JA) is

derived from EIA/JEDEC standards for measuring

thermal resistance. The EIA/JEDEC specification is

JESD51. 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 Application Note AN792, “A Method

to Determine How Much Power a SOT23 Can

Dissipate in an Application” (DS00792), for more

information regarding this subject.

EXAMPLE 5-2:

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:

EXAMPLE 5-3:

5.3.1.3

Maximum Package Power

Dissipation at +60°C Ambient

Temperature

EXAMPLE 5-4:

Package

Package Type = 5-Lead SOT223

Input Voltage

VIN =14V ± 5%

LDO Output Voltage and Current

VOUT =5V

PDMAX



TJMAX



TAMAX



–





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

Where:

PD(MAX) = Maximum power dissipation of the

device

TJ(MAX) = Maximum continuous junction

temperature

TA(MAX) = Maximum ambient temperature

JA = Thermal resistance from 

junction-to-ambient

TJRISE



PDMAX







Where:

TJ(RISE) = Rise in the device junction

temperature over the ambient

temperature

PD(MAX) = Maximum power dissipation of the

device

JA = Thermal resistance from

junction-to-ambient

TJRISE



Where:

TJ = Junction temperature

TJ(RISE) = Rise in the device junction

temperature over the ambient

temperature

TA = Ambient temperature

IOUT =50 mA

Maximum Ambient Temperature

TA(MAX) =+60°C

Internal Power Dissipation

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

PLDO = (14.7 – 4.9) x 50 mA

PLDO = 0.49 Watts

TJ(RISE) =PTOTAL x JA

TJ(RISE) = 0.49W x 75°C/W

TJ(RISE) =36.75°C

TJ =TJ(RISE) + TA(MAX)

TJ = 36.75°C + 60.0°C

TJ =96.75°C

5Lead SOT223 (

JA = 75°C/W):

PD(MAX) = (150°C – 60°C)/75°C/W

PD(MAX) =1.2W

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