Current load

Hi Shlomispira,

In order to help you in the best way, I would need to better understand the setup used during the test.

1) Could you share the schematic and connections used?

2) Is the driver enabled during the test?

3) Was the device transmitting data at the time of the measurements? If so, at what speed?

4) Were there any loads on the bus?

5) Moreover, does the 150 mA current measurement only refer to the surplus current of Vcc2? (See image below for explanation, it is an extract from the datasheet)

Waiting for your feedback, I will try to study the problem and understand the solution.

Thank you for your patience,

Andrea

The diver is enable. and the device transmitting data

115.2K

the load on the bus is a sensor with max current of 150mA . Typ 113mA. 

I'm using the VCC2 to power up the sensor . 

For debugging purposes, I connected a 33Ohs (simulate 150mA) resistor to the VCC2 without the sensor.

Only resistor and then I powered up the circuit and measured VCC2 3.8V.

For debugging purposes, I connected a 33Ohs (simulate 150mA) resistor to the VCC2 without the sensor.

Only resistor and then I powered up the circuit and measured VCC2 3.8V.

First of all, thanks for the schematic and the answers, they can help a lot.

Just to make sure that I understood correctly, to do the test you replaced the sensor with a 33 Ohm resistor, placed between pin 1 and pin 4 of JP22 (and therefore between Vcc2 and GND2), right?

And, at the same time, the device was transmitting data on the bus at a frequency of 115.2KHz?

At first glance I find no errors in your circuit, so my intention is to try to replicate the setup to help you as best as possible.

I will update you as soon as I have some results.

Thank you for your patience,

Andrea

J

AndreaRusso said:

Just to make sure that I understood correctly, to do the test you replaced the sensor with a 33 Ohm resistor, placed between pin 1 and pin 4 of JP22 (and therefore between Vcc2 and GND2), right?

Yes

AndreaRusso said:

And, at the same time, the device was transmitting data on the bus at a frequency of 115.2KHz?

Yes

Understood, thank you.

I should have the test board available at the beginning of next week. I will then conduct some tests as well and update you with the results.

Thanks for your patience.

Andrea

Hi Andrea,

Any progress?

shlomi, 

Hi Shlomi,

Unfortunately, not yet.

The components and boards were supposed to arrive yesterday, but unfortunately customs are delaying the shipment. I hope to receive everything as soon as possible and give you the test results.

In the meanwhile, I take this opportunity to tell you that if in those particular conditions, the LTM2885 should not be able to provide the right amount of current needed, I can help you find some alternative solutions, for example using two separate components for the isolation of the RS-485 channel and the iso-power.

Regards,

Andrea

Hi Andrea,

We already manufacture the boards.

We need solution on these boards

Regards,

Shlomi

Hi Andrea,

Any progress?

shlomi, 

Hi Andrea,

Any progress?

shlomi, 

Hi

Can I connect two VCC2 from two ICs together in parallel?

Hi Shlomi,

Firstly, I apologize for my very late response. I was not able to answer before since I was out of office for working reasons. Thanks for your patience.

As you will surely have understood, the graph (present in the datasheet, and reported below) shows the total current available at the secondary side (bus side) of the component. This quantity includes also the current needed to drive the bus, the load of the cable and all the circuitry present in the transmitter. Thus, with different setup, different configurations and conditions, the surplus current can vary from the shown value.

My understanding is that the surplus current for your application could not be enough for supply the sensor without a drop in the voltage.

So, I think that for a stable and safe functioning of all the system it could be better to do some modifications.

Answering to your last question, it should be possible to have two different ICs in parallel, but there could be some inconsistencies in the output voltages and output impedances which would make the design complicated.

Another way to resolve the problem could be to use an ISO Power only device to supply the secondary side + an isolated RS-485 transceiver for the data communication.

I hope this can help.

Thanks again for your patience,

Andrea