Colm, I just got a question from a customer: The ADM2587E isolated DC/DC converter generates 3.3V isolated supply, can the part be powered from 5V?
The Vcc input supply voltage (non-isolated supply) operates over an input supply range of 3.0V to 5.5V (3.3 V to 5V supplies +/-10%).
This means a single part can be used for both 3.3V and 5V designs – Get’s designers bonus for with the purchasing department!
The isolated supply, generated internally by the isopower DC/DC converter, is 3.3V. This was chosen to keep the internally dissipated power lower (P=V * I; V kept at 3.3V instead of 5V).
OK, so if the isolated supply is 3.3V, can this transceiver drive an RS-485 bus with 5V powered transceivers at other nodes on the bus?
One of the benefits of the RS-485 specification is that it is not dependant on the supply voltage of the transceivers for correct operation.
RS-485 specifies a transmit and receive signalling window with maximum loads, so as long as the 3.3V component meets the envelope, it will be received correctly by the 5V receiver and vice-versa for the 5V device driving the 3.3V receiver.
Is the ADM2582/87E compliant to PROFIBUS?
If not, what could be a solution for PROFIBUS isolating signals and power?
The ADM2582E/87E is not profibus compliant. If a profibus compliant part is required, I would advise using the ADM2485. This part contains an integrated transformer driver that drives an external transformer which then gets rectified and regulated to provide an isolated supply. Please see attached circuit from the lab.
The ADM2486 is a signal isolated only profibus compliant part.
Can you explain why the ADM2582/87E is not compliant to PROFIBUS ?
PROFIBUS requires a greater minimum Vod (differential output voltage) from the driver compared to the standard RS-485 requirements. The minimum Vod required for Profibus is 2.1V, the ADM2582E/87E has a lower minimum Vod.
For comparison, the ADM2485 signal isolated RS-485 transceiver with integrated transformer driver is PROFIBUS compliant, as is the ADM2486 signal isolated RS-485 transceiver. These both have minimum Vod of 2.1V.
Additionally, for profibus repeaters, there is an additional isolated 100mA at 5V required by the isolated DC/DC converter on top of the profibus load itself. An additional isoPower isolated DC/DC converter, such as the ADuM5000, would provide this additional power when the repeater case is relevant.
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