LP38798

SNOSCT6A – MARCH 2013 – REVISED MAY 2013

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The thermal dissipation of the 12-lead WSON package is directly related to the printed circuit board construction

and the amount of additional copper area connected to the DAP.

The DAP (exposed pad) on the bottom of the 12-lead WSON DNT0012B package is connected to the die

substrate with a conductive die attach adhesive. The DAP has no direct electrical (wire) connection to any of the

twelve pins. There is a parasitic PN junction between the die substrate and the device ground. As such, it is

strongly recommend that the DAP be connected directly to the ground at device leads 6 and 7 (i.e. GND).

Alternately, but not recommended, the DAP may be left floating (i.e. no electrical connection). The DAP must not

be connected to any potential other than ground.

For the LP38798SD in the 12-Lead WSON DNT0012B package, the junction-to-case thermal resistance rating,

The junction-to-ambient thermal performance,

package, using the JEDEC JESD51 standards, is summarized in the following table:

LP38798SD-ADJ (12-lead WSON DNT0012B) Thermal Performance

Board

Thermal

Type

Vias

JEDEC

2-Layer

None

138°C/W

45.9°C/W

5°C/W

JESD 51-3

1

51°C/W

26.1°C/W

2

45°C/W

24.7°C/W

JEDEC

4-Layer

4

39°C/W

18.0°C/W

5°C/W

JESD 51-7

6

36°C/W

14.9°C/W

8

34°C/W

13.2°C/W

Estimating the Junction Temperature

The EIA/JEDEC standard (JESD51-2) provides methodologies to estimate the junction temperature from external

measurements (

surface of the package) when operating under steady-state power dissipation conditions. These methodologies

have been determined to be relatively independent of the copper thermal spreading area that may be attached to

the package DAP when compared to the more typical

Package Thermal Metrics' , Literature Number SPRA953B, for specifics.

Board Layout

The dynamic performance of the LP38798 is dependant on the layout of the PCB. PCB layout practices that are

adequate for typical LDO's may degrade the PSRR, noise, or transient performance of the LP38798.

Best performance is achieved by placing all of the components on the same side of the PCB as the LP38798,

and as close as is practical to the LP38798 package. All component ground connections should be back to the

LP38798 analog ground connection using as wide, and as short, of a copper trace as is practical. The connection

trace length on the FB pin will act as an antenna.

Connections using long trace lengths, narrow trace widths, and connections through vias should be avoided.

These will add parasitic inductances and resistance that results in inferior performance especially during transient

conditions.

A Ground Plane, either on the opposite side of a two-layer PCB, or embedded in a multi-layer PCB, is strongly

recommended. This Ground Plane serves two purposes : 1) Provide a circuit reference plane to assure accuracy,

and 2) provides a thermal plane to remove heat from the LP38798 through thermal vias under the package DAP.

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