LP3876ES-ADJ datasheet(11/14 Pages) NSC

11 / 14 page

Application Hints (Continued)

the current drawn by the internal reference increases the

total supply current (ground pin current). Using an optimized

trade-off of ground pin current and die size, LP387X

achieves low noise performance and low quiescent current

operation.

The total output noise specification for LP387X is presented

in the Electrical Characteristics table. The Output noise den-

sity at different frequencies is represented by a curve under

typical performance characteristics.

SHORT-CIRCUIT PROTECTION

The LP3876-ADJ is short circuit protected and in the event of

a peak over-current condition, the short-circuit control loop

will rapidly drive the output PMOS pass element off. Once

the power pass element shuts down, the control loop will

rapidly cycle the output on and off until the average power

dissipation causes the thermal shutdown circuit to respond

to servo the on/off cycling to a lower frequency. Please refer

to the section on thermal information for power dissipation

calculations.

SHUTDOWN OPERATION

A CMOS Logic level signal at the shutdown ( SD) pin will

turn-off the regulator. Pin SD must be actively terminated

through a 10k

Ω pull-up resistor for a proper operation. If this

pin is driven from a source that actively pulls high and low

(such as a CMOS rail to rail comparator), the pull-up resistor

is not required. This pin must be tied to Vin if not used.

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 LP3876-ADJ has an inherent para-

sitic diode. During normal operation, the input voltage 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

The LP3876-ADJ can deliver a continuous current of 3A over

the full operating temperature range. A heatsink may be

required depending on the maximum power dissipation 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:

D =(VIN−VOUT)IOUT+(VIN)IGND

where I

GND is the operating ground current of the device

(specified under Electrical Characteristics).

The maximum allowable temperature rise (T

Rmax) depends

on the maximum ambient temperature (T

Amax) of the appli-

cation, and the maximum allowable junction temperature

Jmax):

Rmax =TJmax−TAmax

The maximum allowable value for junction to ambient Ther-

mal Resistance,

JA, can be calculated using the formula:

JA =TRmax /PD

The LP3876-ADJ is available in TO-220 and TO-263 pack-

ages. The thermal resistance depends on amount of copper

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

value of

JA calculated above is

≥ 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

JA falls

below these limits, a heat sink is required.

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

JA will

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

JA −

CH −

JC.

In this equation,

CH is the thermal resistance from the case

to the surface of the heat sink and

JC is the thermal resis-

tance from the junction to the surface of the case.

JC is

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

CH de-

pends on method of attachment, insulator, etc.

CH varies

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 1 shows a curve for the

JA of TO-263 package for different copper area 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

JA for the TO-263 package mounted to a PCB is

32˚C/W.

Figure 2 shows the maximum allowable power dissipation

for TO-263 packages for different ambient temperatures,

assuming

JA is 35˚C/W and the maximum junction tempera-

ture is 125˚C.

20074432

FIGURE 1.

JA vs Copper (1 Ounce) Area for TO-263

package

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