DROPOUT VOLTAGE

The dropout voltage of a regulator is defined as the minimum

input-to-output differential required to stay within 2% of the

nominal output voltage. For CMOS LDOs, the dropout volt-

age is the product of the load current and the Rds(on) of the

internal MOSFET.

REVERSE CURRENT PATH

The internal MOSFET in LP3872 and LP3875 has an inher-

ent parasitic diode. During normal operation, the input volt-

age is higher than the output voltage and the parasitic diode

is reverse biased. However, if the output is pulled above the

input in an application, then current flows from the output to

the input as the parasitic diode gets forward biased. The

output can be pulled above the input as long as the current

in the parasitic diode is limited to 200mA continuous and 1A

peak.

POWER DISSIPATION/HEATSINKING

LP3872 and LP3875 can deliver a continuous current of

1.5A over the full operating temperature range. A heatsink

may be required depending on the maximum power dissipa-

tion and maximum ambient temperature of the application.

Under all possible conditions, the junction temperature must

be within the range specified under operating conditions.

The total power dissipation of the device is given by:

P

where I

(specified under Electrical Characteristics).

The maximum allowable temperature rise (T

on the maximum ambient temperature (T

cation, and the maximum allowable junction temperature

(T

T

The maximum allowable value for junction to ambient Ther-

mal Resistance,

θ

θ

LP3872 and LP3875 are available in TO-220 and TO-263

packages. The thermal resistance depends on amount of

copper area or heat sink, and on air flow. If the maximum

allowable value of

θ

≥ 60 ˚C/W for

TO-220 package and

≥ 60 ˚C/W for TO-263 package no

heatsink is needed since the package can dissipate enough

heat to satisfy these requirements. If the value for allowable

θ

HEATSINKING TO-220 PACKAGE

The thermal resistance of a TO220 package can be reduced

by attaching it to a heat sink or a copper plane on a PC

board. If a copper plane is to be used, the values of

θ

be same as shown in next section for TO263 package.

The heatsink to be used in the application should have a

heatsink to ambient thermal resistance,

θ

HA

≤θ

θ

θ

In this equation,

θ

to the surface of the heat sink and

θ

tance from the junction to the surface of the case.

θ

about 3˚C/W for a TO220 package. The value for

θ

pends on method of attachment, insulator, etc.

θ

between 1.5˚C/W to 2.5˚C/W. If the exact value is unknown,

2˚C/W can be assumed.

HEATSINKING TO-263 PACKAGE

The TO-263 package uses the copper plane on the PCB as

a heatsink. The tab of these packages are soldered to the

copper plane for heat sinking. Figure 3 shows a curve for the

θ

a typical PCB with 1 ounce copper and no solder mask over

the copper area for heat sinking.

As shown in the figure, increasing the copper area beyond 1

square inch produces very little improvement. The minimum

value for

θ

32˚C/W.

Figure 4 shows the maximum allowable power dissipation

for TO-263 packages for different ambient temperatures,

assuming

θ

ture is 125˚C.

HEATSINKING SOT223-5 PACKAGE

Figure 5 shows a curve for the

θ

different copper area sizes, using a typical PCB with 1 ounce

copper and no solder mask over the copper area for heat

sinking.

20063332

FIGURE 3.

θ

package

20063333

FIGURE 4. Maximum power dissipation vs ambient

temperature for TO-263 package

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