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LT4293 Datasheet(PDF) 21 Page - Analog Devices |
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LT4293 Datasheet(HTML) 21 Page - Analog Devices |
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21 / 44 page  LTC9101-2/LTC9102 21 Rev. 0 For more information www.analog.com APPLICATIONS INFORMATION Figure 12. RESISTANCE PD PSE 0 10k 15k 9101-2 F12 19k 26.5k 26.25k 23.75k 150 (NIC) 20k 30k 33k IEEE 802.3 Signature Resistance Ranges Multipoint Detection The LTC9101-2/LTC9102 uses a multipoint method to detect PDs. False-positive detections are minimized by checking for signature resistance with both forced current and forced voltage measurements. Initially, two test currents are forced onto the channel (via the OUTn pin) and the resulting voltages are measured. The detection circuitry subtracts the two V-I points to determine the resistive slope while removing offset caused by series diodes or leakage at the port (see Figure 13). If the forced current detection yields a valid signature resis- tance, two test voltages are then forced onto the channel and the resulting currents are measured and subtracted. Both methods must report valid resistances to report a valid detection. PD signature resistances between 17k and 29k (typically) are detected as valid and reported as Detect Valid in the corresponding Detection Status reg- ister. Values outside this range, including open and short circuits, are also reported. If the channel measures less than 1V during any forced current test, the detection cycle will abort and Short Circuit will be reported. Table 8 shows the possible detection results. Detection is always performed on a per-channel basis. Figure 13. FIRST DETECTION POINT SECOND DETECTION POINT VALID PD 25k SLOPE 240 160 0V–2V OFFSET VOLTAGE 9101-2 F13 PD Detection Table 8. Detection Status MEASURED PD SIGNATURE (TYPICAL) DETECTION RESULT CONNECTION CHECK RESULT Incomplete or Not Yet Tested Detect Status Unknown Connection Check Status Unknown VPD < 1V Short-Circuit Invalid/Fault CPD > 2.7μF or VPD > 10V CPD Too High Invalid/Fault RPD < 17k RSIG too low Invalid/Fault 17k < RPD < 29k, Single-Signature PD Detect Valid Single-Signature 17k < RPD > 29k, Dual-Signature PD Detect Valid Dual-Signature RPD > 29k RSIG Too High Invalid/Fault RPD > 50k Open Circuit Invalid/Fault PSE Detected PSE Detected or Port is Precharged Invalid/Fault MOSFET Fault MOSFET Fault Detected Invalid/Fault More on Operating Modes The port’s operating mode determines when the LTC9101-2/LTC9102 runs a detection cycle. In manual mode, the port will idle until the host orders a detect cycle. It will then run detection, report the result, and return to idle to wait for another command. In semi-auto mode the LTC9101-2/LTC9102 autono- mously polls a port for PDs, but it will not apply power until commanded to do so by the host. The Detection/ Classification Status registers are updated at the end of each detection/classification cycle. In semi-auto mode, if a valid signature resistance is detected and classification is enabled, the port will clas- sify the PD and report that result as well. The port will then wait for at least 100ms, and will repeat the detection cycle to refresh the data in the Detection/Classification Status registers. The port will not turn on in response to a power-on com- mand unless the current detect result is Detect Valid. Any other detect result will generate a tSTART fault if a power-on command is received. Behavior in Auto mode is similar to semi-auto; however, after Detect Valid is reported and the port is classified, it is automatically powered on without host intervention. In |
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