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поискавой системы для электроныых деталей |
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поискавой системы для электроныых деталей |
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LTC4264IDE датащи(PDF) 9 Page - Linear Technology |
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LTC4264IDE датащи(HTML) 9 Page - Linear Technology |
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9 / 24 page  LTC4264 9 4264f APPLICATIONS INFORMATION DETECTION V1 CLASSIFICATION UVLO TURN-ON UVLO OFF POWER BAD UVLO OFF UVLO ON UVLO TURN-OFF τ = RLOAD C1 PWRGD TRACKS VIN DETECTION V2 –10 TIME –20 –30 –40 –50 –10 TIME –20 –30 –40 –50 –10 TIME –20 –30 –40 –50 20 10 ICLASS ILIMIT_HIGH ILIMIT_LOW dV dt ILIMIT_LOW C1 = POWER BAD POWER BAD POWER BAD TIME TIME POWER GOOD POWER GOOD DETECTION I1 CLASSIFICATION DETECTION I2 LOAD, ILOAD (UP TO ILIMIT_HIGH) 4264 F03 ICLASS DEPENDENT ON RCLASS SELECTION ILIMIT_LOW = 300mA, ILIMIT_HIGH = 750mA I1 = V1 – 2 DIODE DROPS 25k Ω ILOAD = VIN RLOAD I2 = V2 – 2 DIODE DROPS 25k Ω GND PSE IIN RLOAD RCLASS VOUT C1 GND RCLASS PWRGD PWRGD LTC4264 VOUT VIN Figure 3. Output Voltage, PWRGD, PWRGD and PD Current as a Function of Input Voltage SERIES DIODES The IEEE 802.3af-defined operating modes for a PD refer- ence the input voltage at the RJ45 connector on the PD. The PD must be able to handle power received in either polarity. For this reason, it is common to install diode bridges BR1 and BR2 between the RJ45 connector and the LTC4264 (Figure 4). The diode bridges introduce an offset that affects the threshold points for each range of operation. The LTC4264 meets the IEEE 802.3af-defined operating modes by compensating for the diode drops in the threshold points. For the signature, classification, and the UVLO thresholds, the LTC4264 extends two diode drops below the IEEE 802.3af specifications. Note that the voltage ranges specified in the LTC4264 Electrical Specifications are referenced with respect to the IC pins. The LTC4264 threshold points support the use of either traditional or Schottky diode bridges. DETECTION During detection, the PSE will apply a voltage in the range of –2.8V to –10V on the cable and look for a 25k signature resistor. This identifies the device at the end of the cable as a PD. With the PSE voltage in the detection range, the LTC4264 presents an internal 25k resistor between the GND and VIN pins. This precision, temperature-compensated resistor provides the proper characteristics to alert the PSE that a PD is present and requests power to be applied. The IEEE 802.3af specification requires the PSE to use a ΔV/ΔI measurement technique to keep the DC offset voltage of the diode bridge from affecting the signature resistance measurement. However, the diode resistance appears in series with the signature resistor and must be included in the overall signature resistance of the PD. The LTC4264 compensates for the two series diodes in the signature path by offsetting the internal resistance so that a PD built with the LTC4264 meets the IEEE 802.3af specification. In some designs that include an auxiliary power option, such as an external wall adapter, it is necessary to control whether or not the PD is detected by a PSE. With the LTC4264, the 25k signature resistor can be enabled or disabled with the SHDN pin (Figure 5). Taking the SHDN |
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