R6JA=

165

oC - 144oC

189 mW

= 111

o C/W

R6JA=

165°- Ta

PINTERNAL

RJC=

TJ - TC

Power

LM2830, LM2830-Q1

SNVS454E – AUGUST 2006 – REVISED DECEMBER 2014

www.ti.com

Thermal Considerations (continued)

RΦJC is approximately 30°C/Watt for the 6-pin WSON 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:

(30)

Therefore:

Tj = (RΦJC x PLOSS) + TC

(31)

From the previous example:

Tj = (RΦJC x PINTERNAL) + TC

(32)

Tj = 30°C/W x 0.189W + TC

(33)

The second method can give a very accurate silicon junction temperature.

The first step is to determine RθJA of the application. The LM2830 device has over-temperature protection

circuitry. When the silicon temperature reaches 165°C, the device stops switching. The protection circuitry has a

hysteresis of about 15°C. Once the silicon temperature has decreased to approximately 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 monitored,

it will be obvious when the internal PFET stops switching, indicating a junction temperature of 165°C. Knowing

the internal power dissipation from the above methods, the junction temperature, and the ambient temperature

RθJA can be determined.

(34)

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 Texas Instruments LM2830 WSON demonstration

board is shown below.

The four layer PCB is constructed using FR4 with ½ oz copper traces. The copper ground plane is on the bottom

layer. The ground plane is accessed by two vias. The board measures 3-cm × 3-cm. 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:

PINTERNAL = 189mW

(35)

(36)

If the junction temperature was to be kept below 125°C, then the ambient temperature could not go above 109°C

Tj - (RθJA x PLOSS) = TA

(37)

125°C - (111°C/W x 189mW) = 104°C

(38)

26

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