LM2830_15 Datasheet, PDF (26/37 Pages) Texas Instruments – LM2830/-Q1 High-Frequency 1.0-A Load Step-Down DC-DC Regulator

LM2830, LM2830-Q1
SNVS454E – AUGUST 2006 – REVISED DECEMBER 2014
www.ti.com
Thermal Considerations (continued)
R_ΦJC 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:
R_JC=
T_J - T_C
Power
(30)
Therefore:
T_j = (R_ΦJC x P_LOSS) + T_C
(31)
From the previous example:
T_j = (R_ΦJC x P_INTERNAL) + T_C
T_j = 30°C/W x 0.189W + T_C
(32)
(33)
The second method can give a very accurate silicon junction temperature.
The first step is to determine R_θJA 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 R_θJA for any application can be characterized during the early stages of
the design one may calculate the R_θJA 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
R_θJA can be determined.
165° - Ta
R_6JA= _PINTERNAL
(34)
Once this is determined, the maximum ambient temperature allowed for a desired junction temperature can be
found.
An example of calculating R_θJA 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:
P_INTERNAL = 189mW
(35)
R_6JA=
165^oC - 144^oC
189 mW
= 111^o C/W
(36)
If the junction temperature was to be kept below 125°C, then the ambient temperature could not go above 109°C
T_j - (R_θJA x P_LOSS) = T_A
(37)
125°C - (111°C/W x 189mW) = 104°C
(38)
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