The customer is facing the issue dropping 3V instead of 3.3V as ADM2582EBRWZ’s VISOOUT(VISOIN) during Half-Duplex RS-485. AT this moment, RT is 120ohm and the external load is about 5mA.
Could you please tell me how to improve this asap?
Have the customer placed a ferrite for EMI suppression between the VISOOUT and VISOIN pins?
Do you have any details on the schematic circuit?
Thank you very much for your support.
Please find the attached document. Even if you ignore Japanese, perhaps you can understand the document meaning.
In this schematic it looks like two LEDs are being powered from the ADM2867E's VISOOUT, drawing about 7mA of current each. This is likely the reason the VISOOUT is dropping below 3V. The ADM2587E isn't rated a given VISO level when additional current is drawn from the VISOOUT pin for other circuitry. Any additional current drawn from the VISOOUT pin will decrease the VISOOUT voltage through IR losses.
The BLM15HD182SN1 ferrites have a maximum DC resistance of 2.2ohms which will contribute to this. Drawing an additional 14mA through these two ferrites to power two LEDs will result in an additional IR drop of up to 60mV.
The situation could be improved by reducing the current through the LEDs, by increasing the value of R169 and R171, or switching over to lower power LEDs and adjusting R169 and R171 accordingly.
I've already mentioned the external load about 5-7mA. It means ADM2587E supplies only one LED. Then I think it has another reason. What do you think about that ?
Any information on the datarate the customer is using the ADM2582E at? Higher frequency operation will draw more current from the VISO pin, and will increase resistive drop on the VISO level.
The ADM2582E VISO supply is rated as a typical 3.3V output, and this is without the EMI suppression ferrites in place. Because of this we can't guarantee that under some conditions the VISO will not drop below 3V. This is especially the case when an additional external current load is drawn from VISO.
I think the likely explanation is even a small external load of 5-7mA plus the normal ADM2582E operating current through the DC resistance of the ferrites is causing the drop in VISO.
I'd like to confirm what you mean. If I recognize +3.3V on VISO pin without the external load and ferrites, is the IC no problem ? In other words, if VISO become +3.0V with ferrite, is the IC no problem ? The customer measures VISO as +3.3V instead of +3.0V after replacing ferrites for 0 ohm resisters.