LTC4267 isolated 5V will not start

Question

I've incorporated the LTC4267 isolated 5V supply as detalied in Design Note 361 (attached). The board will not start; I can see the initial voltage between VPORTP and VPORTN is sufficient to start the internal Rclass detection but that's all; it just repeats the initial ramp. Has anyone used this design and can anyone suggest what might be the problem? PDF

EHorsma · Accepted Answer

Hello, 
 Can you confirm that C5 or its equivalent is either removed or modified so that it connects between VPORTP and PGND? My best guess is that this design was intended for use with PSEs that support legacy detection, i.e. use the presence of a bulk capacitor as indication a PD is connected. This technique is not IEEE 802.3-compliant, but was more common twenty years ago in the early days of PoE. IEEE 802.3-compliant PSEs expect between 50nF to 120nF of port capacitance, or just C1 on DN361. Otherwise, can you share a waveform of the detection attempt and what type of PSE/Injector you are using? 
 On a side note, if you need more than 13W there are now several options available without requiring the battery circuit in DN361. 
 Best Regards, 
 Eric)

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LTC4267 isolated 5V will not start

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