Galvanic Isolators

Hi Lucas,

I have updated the CAN and USB isolators in my design and attached is those files.

Kindly review the circuit and share your comments on this.

Manjunath,

You may want to use termination resistors four CAN transceivers. Resistors equal to the characteristic impedance of the line should be used to terminate both ends of the cable to prevent signal reflections. Generally 120ohms is a common value.

The datasheet also recommends a bulk capacitance of typically 4.7 μF that should be placed near the VCC2 supply pin.

Also, how are you isolating the DC power supplies?

Hi Lucas,

I forgot to add Power isolation in my design,

I was just looking into ISO1042BQDWRQ1  component reference design in TI.com website and found the below one reference design with Power isolation in-it.

tidrxy2a.pdf

Below is the functional block diagram of our product.

If I have to give Power isolation for V, 3.8V and 3.3V supplies, How should I acheive that,

Please support on this.

Manjunath,

Generally we recommend using the SN6505 transformer driver. You could create an isolated 5V supply with it and then use that for Vcc2 of your CAN devices. You can also use LDOs to create your lower voltage supplies from the isolated 5V. Please be sure to implement proper isolation design in your PCB.

Respectfully,

Lucas

Hi Lucas,

In our design we have 3 different power supplies levels : 5V, 3.8V & 3.3V

Each power supply should be independent to each other? I mean to say, In our design Primary VIN supply is given to 5V and then from 5V we converting to 2 different voltage levels that is 3.3V and 3.8V. 

SOC uses 3.8V and requires maximum 2.5A of current at robust transmission,

3.3V components requires 1A at max

Available transformer driver SN6505 provides only 1A out. So How to isolate 3.8V supply, or Isolation should be given only for CAN and RS485 which is the most effected,

Request you to make me understand in this case.

In the attached reference design, The isolated supply is given to VCC1, Please check and confirm

Manjunath,

I don't know the complete definition of your system, but you only have to isolate the CAN Vcc2 power if all you need to do is have isolated CAN. The other voltage rails can come from the non-isolated 5V supply in this case. Below is a reference design on how to isolate the CAN transceiver. This same thing will have to happen for isolated RS485 as well. You may be able to use the same isolated 5V rail for your RS485 too depending on your isolation requirements and PCB design constraints.

Respectfully,

Lucas

Hi Lucas,

Thanks understood,

based on the reference design, I have made a block diagram, Kindly check this.

I had given individual Power isolation for both RS485 and CAN,( maximum power consumption of RS485 and CAN Isolated transceiver are 75mA*2=150mA and it will vary with respect to datarate, Since TPS76350 IC provides only 150mA output)

Please check this design.

and also in the reference design attached, ISOlated supply is given to VCC1, Please check this also.

Sorry, 150mA is the output current of TPS76350Q

Manjunath,

While the TPS76350 is the example LDO from the datasheet in the SN6505, you can use an LDO with higher current capability like the TLV1117. If you could power both your transceivers from just one SN6505/transformer/LDO combination then this could save you money, complexity, and board space. Otherwise, your block diagram looks good.

Respectfully,

Lucas

Lucas,

In my design 5V is used only for CAN and RS485 ranscivers. So meanwhile I found below schematic.

Where it uses "760390015" transformer for converting 3.3V to 5V, Please suggest?

So I can also use the above 475uH transformer ?

I mean to say, If I can use a 1:2 transformer instead of 1:1:1, I can remove 5V power supply.

"760390015" transformer provides a current out 350mA SO I think this is sufficient for CAN and RS485 Transceivers?

and also Isolated GND pin of RS485 and CAN can be connected together right? since single supply is used for both the transceivers?

Manjunath,

The ISO1042 still needs 5V on Vcc2. You can power the Vcc1 of both transceivers with 3.3V.  If all you are looking to do is isolate the bus side of your system from the rest of it then yes your transceivers can share the same GND2. 

Respectfully,

Lucas

Hi Lucas,

We can not use TLV1117 LDO because its drop out voltage is 1.4max at 0.8V output voltage, The output from the ISN6505/transformer combination will be 5V right?

Hence the drop out voltage should be very less?

TPS76350QDBVRQ1 LDO has 300mV dropout voltage@ 150mA.

Manjunath

with Low drop out and current with mini 300mA and 5V output, " TPS76850QPWPRQ1" this is the only part I got. but the cost is slightly expensive.

Please check and confirm.

Kindly reply, im waiting for your response

Manjunath,

If you use a transformer with a turns ratio high enough to achieve >6.4V then the TLV1117 should be ok. For example, the DA2304-AL transformer is one of the recommended transformers for 3.3V to 5V application in the SN6505 datasheet. This transformer has a turns ratio of 1:2.2

2.2 * 3.3V = 7.26V

This outputs an unregulated 7.26V that is well above the minimum 6.4V for the LDO I recommended. 

Respectfully,

Lucas

Hi Lucas,

I'm using 5V to 5V Isolated transformer actually,Please find it is attached schematic,

CAN-Isolator_1.pdf

in this case, to increase the current from the U12 "TPS76350QDBVRQ1"(its output current is 150mA max), 5V isolated supply will be shared with CAN, RS485, 2 GPIO's and 2 ADC parts, Hence need to increase the current output from the LDO)

I have selected "LP38693MP-5.0/NOPB" LDO with Vi-Vo=250mV, Iout-500mA.

Please find the attached updated file and comment on the same.

Manjunath

Please find the updaed schematic and review the same.

6318.CAN-Isolator_1.pdf

Manjunath

The LP38693MP-5.0 should be fine.

C57 and C64 are a much larger value than what the datasheet recommends. The datasheet recommends 0.1uF capacitors with an additional 4.7uF capacitor close.

C52 is disconnected. Is this purposeful? 

Hi Lucas,

In the reference design, It is mentioned 10uF, Hence I have used the same,

As per the datasheet, C57 and C64 values are changed to 0.1uF and 4.7uF is added.

C52 is connected now, earlier by miss it was not connected,

In the attached Schematic, I have 3 sections,

Isolated 5V Power, CAN Isolator sections and RS485 sections.

Please review with overall circuit and support on the same.

6663.CAN-Isolator_1.pdf

Thanks

Manjunath

Manjunath,

The isolated transceivers and the isolated DC/DC look good. However, I am not able to comment on the voltage translator or CAN controller. You will have to seek support on those in the relevant forum and/or from the relevant manufacturer. 

Respectfully,

Lucas