TC1017-1.8VCT datasheet(13/22 Pages) MICROCHIP

13 / 22 page

 2003 Microchip Technology Inc.

DS21813B-page 13

TC1017

FIGURE 5-2:

Maximum Current vs.

Ambient Temperature (SC-70 package).

5.3

Power Dissipation: SOT-23

The TC1017 is also available in a SOT-23 package for

improved thermal performance. The thermal resistance

for the SOT-23 package is approximately 255°C/W

when the copper area used in the printed circuit board

layout is similar to the JEDEC J51-7 low thermal

conductivity standard or semi-G42-88 standard. For

applications with larger or thicker copper area, the

thermal resistance can be lowered. See AN792, “A

Method to Determine How Much Power a SOT-23 Can

Dissipate in an Application”, DS00792, for a method to

determine the thermal resistance for a particular

application.

The TC1017 power dissipation capability is dependant

upon several variables: input voltage, output voltage,

load current, ambient temperature and maximum

junction temperature. The absolute maximum steady-

state junction temperature is rated at +125°C. The

power dissipation within the device is equal to:

EQUATION:

The V

IN x IGND term is typically very small when

compared to the (V

IN-VOUT) x ILOAD term, simplifying the

power dissipation within the LDO to be:

EQUATION:

To determine the maximum power dissipation

capability, the following equation is used:

EQUATION:

Given the following example:

Find:

Internal power dissipation:

Maximum allowable ambient temperature:

Maximum allowable power dissipation at

desired ambient:

In this example, the TC1017 dissipates approximately

158.5 mWatts and the junction temperature is raised

40.5°C over the ambient. The absolute maximum

power dissipation is 157 mW when given a maximum

ambient temperature of +85°C.

Input voltage, output voltage or load current limits can

also be determined by substituting known values in the

power dissipation equations.

Figure 5-3 and Figure 5-4 depict typical maximum

power dissipation versus ambient temperature and

typical maximum current versus ambient temperature

with a one volt input voltage to output voltage

differential, respectively.

100

120

140

160

-40

-15

110

Ambient Temperature (°C)

VIN - VOUT = 1V

VIN VOUT

–

() ILOAD VIN IGND

VIN VOUT

–

() ILOAD

VIN =

3.0V to 4.1V

VOUT =

2.85V ±2.5%

ILOAD =

120 mA (output current)

TA =

+85°C (max. desired ambient)

PDMAX

TJ_MAX TA_MAX

–

()

θJA

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

Where:

TJ_MAX = the maximum junction

temperature allowed

TA_MAX = the maximum ambient

temperature

θJA = the thermal resistance from

junction to air

PDMAX

VIN_MAX VOUT_MIN

–

() ILOAD

4.1V 2.85

0.975

()

–

() 120mA

158.5mW

TA_MAX

TJ_MAX P

–

DMAX

θJA

125

°C 158.5mW 255°C/W

–

()

84.5

°C

125

°C40.5°C

–

()

TJ_MAX TA

–

θJA

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

157mW

125

°C85°C

–

255

°C/W

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

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