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TC1017

150 mA, Tiny CMOS LDO With Shutdown

Microchip Technology 

TC1017

5.0 THERMAL CONSIDERATIONS

5.1 Thermal Shutdown

Integrated thermal protection circuitry shuts the

regulator off when die temperature exceeds

approximately 160°C. The regulator remains off until

the die temperature drops to approximately 150°C.

5.2 Power Dissipation: SC-70

The TC1017 is available in the SC-70 package. The

thermal resistance for the SC-70 package is

approximately 450°C/W when the copper area used in

the printed circuit board layout is similar to the

JEDEC J51-7 high 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:

PD = (VIN – VOUT) × ILOAD + VIN × IGND

The VIN x IGND term is typically very small when

compared to the (VIN-VOUT) x ILOAD term, simplifying

the power dissipation within the LDO to be:

EQUATION:

PD = (VIN – VOUT) × ILOAD

To determine the maximum power dissipation

capability, the following equation is used:

EQUATION:

Where:

PDMAX = -(--T---J--_---M----A--RX----θ-–---J-T-A---A---_--M----A---X---)-

TJ_MAX = the maximum junction

temperature allowed

TA_MAX = the maximum ambient

temperature

RθJA = the thermal resistance from

junction to air

Given the following example:

Find:

VIN = 3.0V to 4.1V

VOUT = 2.85V ±2.5%

ILOAD = 120 mA (output current)

TA = 55°C (max. desired ambient)

1. Internal power dissipation:

PDMAX = (VIN_MAX – VOUT_MIN ) × ILOAD

= (4.1V – 2.85 × (0.975)) × 120mA

= 158.5mW

2. Maximum allowable ambient temperature:

TA_MAX

=

TJ_MAX

–

P

DM

AX

×

R

θJ

A

= (125°C – 158.5mW × 450°C/W)

= (125°C – 71°C)

= 54°C

3. Maximum allowable power dissipation at

desired ambient:

PD

=

T----J--_---M----A--X-----–----T----A-

RθJA

= -1--2---5----°--C------–-----5---5---°---C--

450 ° C /W

= 155mW

In this example, the TC1017 dissipates approximately

158.5 mW and the junction temperature is raised 71°C

over the ambient. The absolute maximum power

dissipation is 155 mW when given a maximum ambient

temperature of 55°C.

Input voltage, output voltage or load current limits can

also be determined by substituting known values in the

power dissipation equations.

Figure 5-1 and Figure 5-2 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.

400

350

300

250

200

150

100

50

0

-40 -15 10 35 60 85 110

Ambient Temperature (°C)

FIGURE 5-1:

Power Dissipation vs.

Ambient Temperature (SC-70 package).

DS21813B-page 12

 2003 Microchip Technology Inc.

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