6 Replies Latest reply: Mar 15, 2013 1:26 PM by ConalW

Hi,

We have asked question from a customer, who will adopt three of ADM3485 and communicate through maximum 23 meter.

They are going to use 100 Ohm termination, but want to know how much wattage of dummy load can be recommended at termination of each end.

Perhaps it is a issue of short circuit in accident and probably required wattage depends on how much current loaded at each end.

However, we can see thermal shutdown capability in the datasheet of ADM3485, but we don't know how it works in detail.

If thermal shutdown works well, it seems there only necessary to add dummy load, only the period before thermal shutdown actually works.

Could you please let us know exact condition of this shutdown, or time and load/current correlated data/chart ?

As well, if you instruct how do we estimate and decide wattage of dummy load, it would be very much appreciated.

Best Regards,

Hi,

Best Regards,

Hi usaghi,

The recommendations do depend on how the customer wants to decide the wattage.

If the decision is to choose a wattage for a short-circuit event between the bus lines, then the ADM3485E short circuit protection is specified in the datasheet to limit the current to 250 mW (page 3, Short-Circuit Output Current). However, from the typical plots in figure 18 & 19 on page 10, we can see that the output current is actually limited to about 100 mW in the typical case.

From P = I²R, and two Rt of 100 Ohm, this gives us (0.1)² x 50, or 0.5 W.

There may be other factors to consider though, for example, is there another RS-485 device on the bus that isn't the ADM3485E?

Regards,

Conal

Hi Conal,

Thank you very much for your answers and it brings one hypothetical answer that P = I²R = (0.1)² x 100/3 = 0.33W.

Because it has three terminals with all ADM3485, so I put 100/3 instead of 50, but please correct if I'm wrong.

One confusing is what you said at <from the typical plots in figure 18 & 19 on page 10, we can see that the output current is actually limited to about 100 mW in the typical case.>

I see datasheet Rev.E at 2011/11 which is same as the one on product web page. Here figure 18 & 19 are brief diagrams of test circuit, not plots.

For example, Figure 18 is <Differential Output Voltage Drivers with Varying Common-Mode Voltage>.

There is no other plot on this Rev.E datasheet which is directly refering to 100mW, despite I can see some plots showing tabletop type chart around 100mA so may need calculating.

Please clarify which plot you were refering, in order to derive 100mW.

Again, sorry for these fundamentals and thank you very much for your support and helps.

Best Regards,

Hi Usaghi,

Sorry, I had been looking at the ADM3485E datasheet (a similar part with enhanced ESD).

For the ADM3485 refer to page 10, figure 13 and 14.

Also, I made a typo, I meant the output current is typically limited to 100 mA (or in the case of the ADM3485, 120 mA might be a better typical figure). My calcuation based on this resulted in 0.5 W as per the rest of my post above.

As regards using 50 or 100/3, there should only be two termination resistors on the bus. 100/3 would correspond to 3 terminations resistors, resulting in a load outside the specification of the driver.

Regards,

Conal

Hi Conal,

Thank you very much for your confirmation. It becomes clear.

And also I have awared my misunderstanding in regards to number of termination.

I have a chart as follows provided from them, and there are only two termination registers, despite it has three tranceivers, so final answer must be 0.5W derived from R=50(100/2) for this case.

Confirmation again from you would be very much appreciated.

Best Regards,