LP3873 datasheet(14/24 Pages) TI

14 / 24 page

LP3873, LP3876

SNVS220E – NOVEMBER 2002 – REVISED APRIL 2013

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REVERSE CURRENT PATH

The internal MOSFET in LP3873 and LP3876 has an inherent parasitic 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

LP3873 and LP3876 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:

PD = (VIN−VOUT)IOUT+ (VIN)IGND

where IGND is the operating ground current of the device (specified under Electrical Characteristics).

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

The maximum allowable value for junction to ambient Thermal Resistance,

θJA, can be calculated using the

formula:

θJA = TRmax / PD

LP3873 and LP3876 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 TO263

package.

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

θHA≤ θJA − θCH − θJC.

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 TO-220 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 25 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.

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