LP38513 datasheet(12/20 Pages) TI1

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LP38513

SNVS361C – JULY 2007 – REVISED MARCH 2009

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POWER DISSIPATION/HEAT-SINKING

A heat-sink may be required depending on the maximum power dissipation (PD(MAX)), maximum ambient

temperature (TA(MAX))of the application, and the thermal resistance (θJA) of the package. Under all possible

conditions, the junction temperature (TJ) must be within the range specified in the Operating Ratings. The total

power dissipation of the device is given by:

PD = ( (VIN−VOUT) x IOUT) + ((VIN) x IGND)

where

•

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

(1)

The maximum allowable junction temperature rise (

ΔTJ) depends on the maximum expected ambient

temperature (TA(MAX)) of the application, and the maximum allowable junction temperature (TJ(MAX)):

ΔTJ = TJ(MAX)− TA(MAX)

(2)

The maximum allowable value for junction to ambient Thermal Resistance,

θJA, can be calculated using the

formula:

θJA = ΔTJ / PD(MAX)

(3)

LP38513 is 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 heat-sink 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.

HEAT-SINKING THE 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 the same as shown in the next section for

the DDPAK/TO-263 package.

The heatsink to be used in the application should have a heat-sink 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. The rated

θJC is about 3°C/W for a 5-lead TO-22

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.

HEAT-SINKING THE DDPAK/TO-263 PACKAGE

The DDPAK/TO-263 package uses the copper plane on the PCB as a heat-sink. The tab of this package is

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.

Figure 25.

θJA vs Copper (1 Ounce) Area for the DDPAK/TO-263 package

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