SP78LC00 datasheet(5/10 Pages) SIPEX

5 / 10 page

Rev. 5/19/03

SP78LC00

100mA CMOS LDO Regulator

THEORY OF OPERATION: Continued

tion in the package, use the junction-to-ambi-

ent thermal resistance of the device and the

following basic equation:

PD =

(TJ(max) - TA)

θJA

TJ(max) is the maximum junction temperature

of the die and is 125

°C. TA is the ambient

operating.

θJA is the junction-to-ambient ther-

mal resistance for the regulator and is layout

dependent.

The actual power dissipation of the regulator

circuit can be determined using one simple

equation:

PD = (VIN - VOUT)*IOUT + VIN*IGND

≅ (VIN - VOUT) * IOUT

Substituting PD(max) for PD and solving for the

operating conditions that are critical to the

application will give the maximum operating

conditions for the regulator circuit. For ex-

ample, if we are operating the SP78LC00-

3.0V at room temperature, with a minimum

footprint layout, we can determine the maxi-

mum input voltage for a set output current.

PD(max) =

(125

°C -25°C) = 454mW

(220

°C/W)

To prevent the device from entering thermal

shutdown, maximum power dissipation can

not be exceeded. Using the output voltage of

3.0V and an output current of 100 mA, the

maximum input voltage can be determined.

Ground pin current can be taken from the

electrical spec’s- table (I

GND=110 µ A

OUT=100mA). The maximum input voltage is

determined as follows:

454mW = (VIN – 3.0V)*100mA + VIN*0.11mA

Solving for VIN, we get:

VIN =

(454mW + 300mW)

100.11mA

After calculations, we find that the maximum

input voltage of a 3.0V application at 100mA

of output current in an SOT-23-5 package

is7.53V.

Dual-Supply Operation

When used in dual supply systems where the

regulator load is returned to a negative sup-

ply, the output voltage must be diode clamped

to ground.

Dropout vs. Io ( fixed 3.0V)

100

150

200

250

300

350

400

050

100

150

200

Io (mA)

T=125°

T=27°

T=-40°

Dropout vs. Temp (fixed 3.0V)

100

150

200

250

-50

-20

100

130

Temp (deg)

IOUT=100mA

IOUT=50mA

TYPICAL CHARACTERISTICS

27°C, VIN = 5.5V, IO = 0.1mA, CIN = COUT = 1µF unless otherwise specified.

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