Dropout Voltage

The dropout voltage of a regulator is defined as the minimum

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

output voltage. The LP3961/LP3964 use an internal MOS-

FET with an Rds(on) of 240m

Ω (typically). For CMOS LDOs,

the dropout voltage is the product of the load current and the

Rds(on) of the internal MOSFET.

Reverse Current Path

The internal MOSFET in LP3961and LP3964 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 out-

put can be pulled above the input as long as the current in

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

peak.

Maximum Output Current Capability

LP3961 and LP3964 can deliver a continuous current of

800mA 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

J-

max

):

T

The maximum allowable value for junction to ambient Ther-

mal Resistance,

θ

θ

LP3961 and LP3964 are available in TO-220, TO-263, and

SOT-223 packages. The thermal resistance depends on

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

maximum allowable value of

θ

˚C/W for TO-220 package,

≥60 ˚C/W for TO-263 package,

and

≥ 140 ˚C/W for SOT-223 package, no heatsink is

needed since the package can dissipate enough heat to sat-

isfy these requirements. If the value for allowable

θ

below these limits, a heat sink is required.

Heatsinking TO-220 Packages

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

In this equation,

θ

tion to the surface of the heat sink and

θ

sistance 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 and SOT-223 Packages

The TO-263 and SOT223 packages use the copper plane on

the PCB as a heatsink. The tab of these packages are sol-

dered to the copper plane for heat sinking.

Figure 3 shows a

curve for the

θ

sizes, using 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, as-

suming

θ

ture is 125˚C.

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.

DS101129-32

FIGURE 3.

θ

package

DS101129-33

FIGURE 4. Maximum power dissipation vs ambient

temperature for TO-263 package

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