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поискавой системы для электроныых деталей |
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поискавой системы для электроныых деталей |
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BCR402 датащи(PDF) 6 Page - Infineon Technologies AG |
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BCR402 датащи(HTML) 6 Page - Infineon Technologies AG |
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6 / 8 page  AN 066 Rev D 6 / 8 16-Jan-2004 Applications Note No. 066 Silicon Discretes 4. Temperature Compensation, Negative Temperature Coefficient of BCR402R and Protection of LEDs from Thermal Runaway For this section, please refer to the internal schematic diagram of the BCR402R shown in Figure 8 below. Note that, as a rough rule-of-thumb, the base- emitter forward junction voltage of a Silicon PN junction has a temperature coefficient of something like –2.3 mV / °C. In other words, as temperature increases, the emitter-base voltage of the internal PNP transistor in BCR402R decreases at a rate of about 2 millivolts per degree C. Figure 8. BCR402R Internal Schematic (inside box formed by dotted line) Ordinarily, this temperature coefficient could create the following problem: for a given operating point, as we go from cold to hot, the PNP transistor’s VBE will decrease, making the transistor “turn on harder”, thereby making the BCR402R provide more current from pin 2 (IOUT) as we get hotter, creating the potential to have a thermal runaway condition in our LED display circuit. To prevent this, two internal series diodes are placed between the +VS node and the internal PNP transistor’s base. Since each of these two internal diodes also has a temperature coefficient of the same value (- 2.3mV / °C), as we go hotter, the PNP transistor’s base voltage will rise, tending to “throttle back” the PNP transistor. If only one diode junction were used, we would more or less evenly temperature compensate for the PNP transistor’s B-E junction and maintain a fairly constant BCR402R output current over temperature. The addition of the second diode “over-compensates” or actually causes the BCR402R to “source” slightly less current as one goes hot – providing negative feedback and thereby preventing a potentially dangerous thermal runaway condition for the LEDs in the display circuit. This is especially useful for LED displays subjected to wide temperature variations, e.g. those found in automotive applications. 5. PCB Layout Details The top view of the Application Board is given in Figure 9 on the next page. An enlarged picture including component placement is provided in Figure 11 on page 7. Note that there are two positions in which to place R1, in order to accommodate either 0402 or 1208 case size resistors. Open connections to ground on the left of the LED stack make it easy to modify the board for use with less than four LEDs. Figure 10 on the next page shows an enlarged view of the PCB section where the LED driver is placed. The top and bottom metal layers are shown. This reveals the heat sink (metal area) for the BCR402R on the backplane or bottom metal layer of the PC board. The PC board footprint was designed to fit the SOT143R package used by BCR401R and BCR402R, but it will also accommodate the SC74 package as used by the BCR405U without modification. Please note that this particular PCB does not accommodate the “High Voltage” application described in Section 3. |
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