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LM2734ZMKX bảng dữ liệu(PDF) 11 Page - National Semiconductor (TI) |
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11 / 9 page Calculating the LM2734Z Junction Temperature (Continued) The second method can give a very accurate silicon junction temperature. The first step is to determine RθJA of the appli- cation. The LM2734Z has over-temperature protection cir- cuitry. When the silicon temperature reaches 165˚C, the device stops switching. The protection circuitry has a hyster- esis of 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 PCB can be characterized during the early stages of the design by raising the ambient temperature in the given application until the circuit enters thermal shutdown. If the SW-pin is monitored, it will be obvious when the internal NFET stops switching indicating a junction temperature of 165˚C. Knowing the internal power dissipation from the above methods, the junction tempera- ture 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. Design Example 3: Operating Conditions Package SOT23-6 V IN 12.0V P OUT 2.475W V OUT 3.30V P DIODE 523mW I OUT 750mA P IND 56.25mW V D 0.35V P SWF 108mW Freq 3MHz P SWR 108mW I Q 1.5mA P COND 68.2mW I BOOST 4mA P Q 18mW V BOOST 5V P BOOST 20mW T RISE 8ns P LOSS 902mW T FALL 8ns R DSON 400m Ω IND DCR 75m Ω D 30.3% Using a standard National Semiconductor Thin SOT23-6 demonstration board to determine the RθJA of the board. The four layer PCB is constructed using FR4 with 1/2oz copper traces. The copper ground plane is on the bottom layer. The ground plane is accessed by two vias. The board measures 2.5cm x 3cm. It was placed in an oven with no forced airflow. The ambient temperature was raised to 94˚C, and at that temperature, the device went into thermal shutdown. If the junction temperature was to be kept below 125˚C, then the ambient temperature cannot go above 54.2˚C. T J -(R θJA xPLOSS)= TA The method described above to find the junction tempera- ture in the Thin SOT23-6 package can also be used to calculate the junction temperature in the LLP package. The 6 pin LLP package has a RθJC = 20˚C/W, and RθJA can vary depending on the application. RθJA can be calculated in the same manner as described in method #2 (see example 3). LLP Package The LM2734Z is packaged in a Thin SOT23-6 package and the 6–pin LLP. The LLP package has the same footprint as the Thin SOT23-6, but is thermally superior due to the ex- posed ground paddle on the bottom of the package. 20130374 No Pullback LLP Configuration RθJA of the LLP package is normally two to three times better than that of the Thin SOT23-6 package for a similar PCB configuration (area, copper weight, thermal vias). 20130370 FIGURE 7. Dog Bone www.national.com 11 |
Số phần tương tự - LM2734ZMKX |
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Mô tả tương tự - LM2734ZMKX |
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