MCP1825 datasheet(21/38 Pages) MICROCHIP

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DS22056B-page 21

MCP1825/MCP1825S

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. Equation 5-4 can be

used to determine the package maximum internal

power dissipation.

EQUATION 5-4:

EQUATION 5-5:

EQUATION 5-6:

5.3

Typical Application

Internal power dissipation, junction temperature rise,

junction temperature and maximum power dissipation

is 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 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 AN792, “A Method to Determine

How Much Power a SOT23 Can Dissipate in an

Application” (DS00792), for more information regarding

this subject.

DMAX

()

JMAX

()

AMAX

()

–

()

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

PD(MAX) = Maximum device power dissipation

TJ(MAX) = maximum continuous junction

temperature

TA(MAX) = maximum ambient temperature

JA = 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

JA = Thermal resistance from junction-to-

ambient

JRISE

()

TJ = Junction temperature

TJ(RISE) = Rise in device junction temperature

over the ambient temperature

TA = Ambient temperature

Package

Package Type = TO-220-5

Input Voltage

VIN =3.3V ± 5%

LDO Output Voltage and Current

VOUT =2.5V

IOUT =500 mA

Maximum Ambient Temperature

TA(MAX) =60°C

Internal Power Dissipation

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

PLDO = ((3.3V x 1.05) – (2.5V x 0.975))

x 500 mA

PLDO = 0.514 Watts

TJ(RISE) =PTOTAL x RθJA

TJRISE = 0.514 W x 29.3° C/W

TJRISE = 15.06°C

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