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ISOW7842: ISOW7840 & ISOW7842

Gaurav Bhatia
Prodigy 20 points
Part Number: ISOW7842
Other Parts Discussed in Thread: ISOW7840, , ISOW7841

Tool/software:

Hi,

I have few queries related to ISOW784x series.

  1. I want to double check and get confirmation that ISOW7840 and ISOW7842 can support input configuration of Vcc as 3.3V and Viso to be 5V.
  2. My second question is related to ISOW7841. I see that it can support Vcc 3.3V and Viso 5V configuration, could you please help me understand current draw table on page 17. Icc max at 100Mbps is 80mA with no load, does this mean that the current draw is 80mA if you merely sending the 100Mbps signal into the channel but nothing connected on isolated channels side which would consume any power. 
  3. For ISOW7841, if I now connect load on the isolated side then Viso can provide maximum of 22mA at 100Mbps which is just to support powering external chips on isolated side while 80mA on Icc will support data channels at 100Mbps on both sides i.e. input and output side. Please help me understand that the current draw increase on Icc from 31ma (no load) to 80ma(100Mbps) is to support additional current requirement on all 4 input and all 4 output channels while it has maximum 22mA Viso power delivery capability.
  4. Absolute maximum current draw for ISOW7841 in operating condition would be roughly 137mA or less. Am i correct?
  5. Could you help me understand the total power budget equation on pg. 40 for my used case. Where Viso is 5V, Iiso 22mA (for 100Mbps), efficiency is 42%, Vcc is 3.3V and I am having difficultly that what would be my Iinpx. What would be maximum amount of current draw I can estimate to drive the external chip with all data channels operating at 100Mbps. 

    Please note that my application doesn't use 100Mbps, I am trying to understand the datasheet better.

Thanks,

 

 

  • 0
    TI__Expert 7585 points

    Hi Gaurav,

    Thanks for reaching out. I believe this is in continuation of a previous e2e post started by your colleague : https://e2e.ti.com/support/isolation-group/isolation/f/isolation-forum/1501721/isow7842-for-setup-application

    Regarding your questions, Please find my inputs below:

    Gaurav Bhatia said:
    I want to double check and get confirmation that ISOW7840 and ISOW7842 can support input configuration of Vcc as 3.3V and Viso to be 5V.

    Yes, as mentioned in datasheet this particular mode of operation comes with limited ILOAD Values.

    Gaurav Bhatia said:
    My second question is related to ISOW7841. I see that it can support Vcc 3.3V and Viso 5V configuration, could you please help me understand current draw table on page 17. Icc max at 100Mbps is 80mA with no load, does this mean that the current draw is 80mA if you merely sending the 100Mbps signal into the channel but nothing connected on isolated channels side which would consume any power. 

    The Table on Page17 states the ICC which is primary supply current consumption when 100Mbps Transmission is happening in 3V5 mode with a capacitive load of 15pF on all isolated channels and No ILOAD consumption from VISO pin. FYI, the Isolated channels are not meant to draw any higher current which >4mA. These are digital outputs and not meant to draw high current values for power consumption.

    Gaurav Bhatia said:
    For ISOW7841, if I now connect load on the isolated side then Viso can provide maximum of 22mA at 100Mbps which is just to support powering external chips on isolated side while 80mA on Icc will support data channels at 100Mbps on both sides i.e. input and output side. Please help me understand that the current draw increase on Icc from 31ma (no load) to 80ma(100Mbps) is to support additional current requirement on all 4 input and all 4 output channels while it has maximum 22mA Viso power delivery capability.

    The ICC increase from 31mA to 80mA is based on the increasing signal speed on all 4 IO Channels on both Primary and Secondary Side with any ILOAD requirement. If you wish to draw 22mA of ILOAD while transmitting at 100Mbps, the the ICC will increase 80mA + Some Extra ICC. This Extra ICC can be estimated by Figure7-5

    Gaurav Bhatia said:
    Absolute maximum current draw for ISOW7841 in operating condition would be roughly 137mA or less. Am i correct?

    137mA Current is what will be drawn from Primary Supply in case of Short Circuit on VISO. The ILOAD from VISO can be as high as 130mA in different Voltage and conditions and the equivalent ICC can go as high as 220mA., refer Figure 7-5.

    Gaurav Bhatia said:
    Could you help me understand the total power budget equation on pg. 40 for my used case. Where Viso is 5V, Iiso 22mA (for 100Mbps), efficiency is 42%, Vcc is 3.3V and I am having difficultly that what would be my Iinpx. What would be maximum amount of current draw I can estimate to drive the external chip with all data channels operating at 100Mbps. 

    The Iinpx is nothing but the 31mA and 80mA numbers that we discussed above. The Iinpx current is independent of the ILOAD. The ILOAD(IISO) is a result of ICC and Efficiency, For the Example case that you described, the total ICC would be 80mA + 85mA of Currrent (Assuming 22mA if getting extracted as ILOAD).

    85mA can be extracted from the Figure7-5 which accounts for efficiency of 42% in 3V5 mode.

    Regards
    Varun

  • 0 in reply to Varun Kumar
    Prodigy 20 points

    Hi Varun,

    The datasheet doesn't explicitly mentions that ISOW7842 and ISOW7840 can be used for 3.3V Vcc and 5V Viso. There is table on pg17 which is missing these two part numbers(ISOW7840 & ISOW7842) in 7.12 section. This section (7.12) only mentions about ISOW7841. I also don't see any Characteristics graphs mentioning about 3.3V Vcc to 5V Viso in 7.22. All graphs for ISOW7840 and ISOW7842 doesn't have 3.3V Vcc and 5V Viso configuration captured. Is the datasheet not correct or missed out graphs and tables for ISOW7840 & ISOW7842 or these parts are not compatible for 3.3V Vcc.

    I am sorry but the reason why I am asking all this again is to double check as I used ISOW7840 and ISOW7842 in one of my design which worked with 5V Vcc and 5V Viso but when we switched to different configuration i.e. 3.3V Vcc and 5V Viso then the same design doesn't work. I am trying to identify where the fault lies in my system. Due to IP concerns I can't share the schematic snippets but what I can share is that the isolated side of the isolator is exactly same design and it is driving same load as previous design.

    I do see in figure 7.5 of datasheet for ISOW7841 that it has load current dropping from 130mA (for 5V Vcc & 5V Viso) compared to 40mA (for 3.3V Vcc & 5V Viso). After reading your response, I did run my design using ISOW7842 which was drawing current of 43mA on Icc at 3.3V which didn't work, so my application data rate as well as isolated load side is not power hungry. When I run same design with 5V Vcc & 5V Viso using ISOW7842 which works where the current consumption Icc is still similar 41mA.

    Please advise, thanks.

  • 0 in reply to Gaurav Bhatia
    TI__Mastermind 29185 points

    Hello Gaurav, 

    Varun is out of office today. Please allow another day and Varun will get back to you soon. 

    Best,
    Andrew

  • 0 in reply to Andrew J
    Prodigy 20 points

    Thanks Andrew for the information. I will wait for his response.

  • 0 in reply to Gaurav Bhatia
    TI__Genius 17555 points

    Hello,

    Thanks for reaching out. 

    Our team is on TI Holiday. Please allow our team until Monday next week (4/21/2025) to reply.

    Regards,
    Aaditya V

  • 0 in reply to Gaurav Bhatia
    TI__Expert 7585 points

    Hi Gaurav,

    Thanks for the detailed inputs on the history and apologies for confusion on ISOW7841 and ISOW7842. Now the situation is clear to me.

    Unfortunately Yes, it's only the ISOW7841 which is guaranteed to work for 3.3V Vcc and 5V Viso for 40mA of ILOAD. We do not guarantee this mode for any other channel configuration.

    Let me know if you have any further questions on ISOW7841. 

    Regards
    Varun