LM2830XMF datasheet(14/24 Pages) NSC

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Thermal Definitions (Continued)

Thermal impedance from the silicon junction to the ambient

air is defined as:

The PCB size, weight of copper used to route traces and

ground plane, and number of layers within the PCB can

greatly effect RθJA. The type and number of thermal vias can

also make a large difference in the thermal impedance.

Thermal vias are necessary in most applications. They con-

duct heat from the surface of the PCB to the ground plane.

Four to six thermal vias should be placed under the exposed

pad to the ground plane if the LLP package is used.

Thermal impedance also depends on the thermal properties

of the application operating conditions (Vin, Vo, Io etc), and

the surrounding circuitry.

Silicon Junction Temperature Determination Method 1:

To accurately measure the silicon temperature for a given

application, two methods can be used. The first method

requires the user to know the thermal impedance of the

silicon junction to top case temperature.

Some clarification needs to be made before we go any

further.

RθJC is the thermal impedance from all six sides of an IC

package to silicon junction.

RΦJC is the thermal impedance from top case to the silicon

junction.

In this data sheet we will use RΦJC so that it allows the user

to measure top case temperature with a small thermocouple

attached to the top case.

RΦJC is approximately 30˚C/Watt for the 6-pin LLP package

with the exposed pad. Knowing the internal dissipation from

the efficiency calculation given previously, and the case

temperature, which can be empirically measured on the

bench we have:

Therefore:

j =(R

ΦJC xPLOSS)+ TC

From the previous example:

j =(R

ΦJC xPINTERNAL)+ TC

j = 30˚C/W x 0.189W + TC

The second method can give a very accurate silicon junction

temperature.

The first step is to determine RθJA of the application. The

LM2830 has over-temperature protection circuitry. When the

silicon temperature reaches 165˚C, the device stops switch-

ing. The protection circuitry has a hysteresis of about 15˚C.

Once the silicon temperature has decreased to approxi-

mately 150˚C, the device will start to switch again. Knowing

this, the RθJA for any application can be characterized during

the early stages of the design one may calculate the RθJA by

placing the PCB circuit into a thermal chamber. Raise the

ambient temperature in the given working application until

the circuit enters thermal shutdown. If the SW-pin is moni-

tored, it will be obvious when the internal PFET stops switch-

ing, indicating a junction temperature of 165˚C. Knowing the

internal power dissipation from the above methods, the junc-

tion temperature, and the ambient temperature RθJA can be

determined.

Once this is determined, the maximum ambient temperature

allowed for a desired junction temperature can be found.

An example of calculating RθJA for an application using the

National Semiconductor LM2830 LLP demonstration board

is shown below.

The four layer PCB is constructed using FR4 with 1⁄2 oz

copper traces. The copper ground plane is on the bottom

layer. The ground plane is accessed by two vias. The board

measures 3.0cm x 3.0cm. It was placed in an oven with no

forced airflow. The ambient temperature was raised to

144˚C, and at that temperature, the device went into thermal

shutdown.

From the previous example:

INTERNAL = 189mW

If the junction temperature was to be kept below 125˚C, then

the ambient temperature could not go above 109˚C

j -(R

θJA xPLOSS)= TA

125˚C - (111˚C/W x 189mW) = 104˚C

LLP Package

For certain high power applications, the PCB land may be

modified to a "dog bone" shape (see Figure 6). By increasing

the size of ground plane, and adding thermal vias, the RθJA

for the application can be reduced.

20197468

FIGURE 4. Internal LLP Connection

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