LP3891 datasheet(9/19 Pages) TI1

9 / 19 page

LP3891

www.ti.com

SNVS235D – SEPTEMBER 2003 – REVISED APRIL 2013

SHUTDOWN OPERATION

Pulling down the shutdown (S/D) pin will turn-off the regulator. Pin S/D must be actively terminated through a

pull-up resistor (10 k

Ω to 100 kΩ) 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.

POWER DISSIPATION/HEATSINKING

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:

PD = (VIN−VOUT)IOUT+ (VIN)IGND

(1)

where IGND is the operating ground current of the device.

The maximum allowable temperature rise (TRmax) depends on the maximum ambient temperature (TAmax) of the

application, and the maximum allowable junction temperature (TJmax):

TRmax = TJmax− TAmax

(2)

The maximum allowable value for junction to ambient Thermal Resistance,

θJA, can be calculated using the

formula:

θJA = TRmax / PD

(3)

These parts are available in TO-220 and DDPAK/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

θJA calculated above is ≥ 60 °C/W

for TO-220 package and

≥ 60 °C/W for DDPAK/TO-263 package no heatsink is needed since the package can

dissipate enough heat to satisfy these requirements. If the value for allowable

θJA falls below these limits, a heat

sink is required.

HEATSINKING TO-220 PACKAGE

The thermal resistance of a TO-220 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

θJA will be same as shown in next section for

DDPAK/TO-263 package.

The heatsink to be used in the application should have a heatsink to ambient thermal resistance,

θHA≤ θJA − θCH − θJC.

(4)

In this equation,

θCH is the thermal resistance from the case to the surface of the heat sink and θJC is the thermal

resistance from the junction to the surface of the case.

θJC is about 3°C/W for a TO220 package. The value for

θCH depends on method of attachment, insulator, etc. θCH varies between 1.5°C/W to 2.5°C/W. If the exact value

is unknown, 2°C/W can be assumed.

HEATSINKING DDPAK/TO-263 PACKAGE

The DDPAK/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 19 shows a curve for the

θJA of DDPAK/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.

Submit Documentation Feedback

Product Folder Links: LP3891

LP3891 数据表(PDF) 9 Page - Texas Instruments

LP3891 数据表(HTML) 9 Page - Texas Instruments

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

数据表下载