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SN75185: Application issue compare with GD65232 --- DTR signal delay

Kay Chen53
Expert 2225 points
Part Number: SN75185
Other Parts Discussed in Thread: GD65232

Hi greeting!

I'm helping customer to debug a issue about their application use SN75185DBR

The VCC +/- is from ATX power +12V/-12V ; the other side is the RS-232 connector with cable hook up another device 

The issue now is they use SN75185 and find the DTR signal will be delay about 2ms after +3.3V ATX power , this is almost the same timing with -12V comes out and will cause system fail to power-on

But if they use the GD65232, it's ok to power-on 

Their schematic and test waveform could refer the file as I attached here. Could you help to provide some idea to us to know:

•Whether the diode is needed or not? (D24, D42), could be replaced by 0 ohm?
•What reason will cause DTR signal delay about 2ms

Thanks again. 

Issue of SN75185DBR.pptx

  • 0
    TI__Mastermind 25705 points

    Kay,

    Your first question is easier to answer. D24 and D42 could be replaced by 0Ohm resistors but there will cause potential issues. For example, if Dy is shorted to a voltage higher than Vdd, say Dy is 15V and Vdd is 12V, the current could flow back to the power supply. If the supply is low and has low impedance to ground, the current could be huge. If the diodes are there, the backward flow can be stopped. So please consider the use cases carefully before making the decision.

    I can try to test the parts and see if I can dig out more information about power up timing. I will update you when I get more information on it.

    Regards,

    Hao

  • 0 in reply to Hao L
    Expert 2225 points
    Hi Hao:

    thanks, I'd like to update some question about SN75185, we could find the ESD protection inside the function block ( exceeds 10kv on RS-232 pins & 3.5kv on all other pins (HBM) ), but we couldn't find the detail description about it. Would you help us to find some more detail spec about this ESD equivalent data (like Cin, or else?)
  • 0 in reply to Kay Chen53
    TI__Mastermind 25705 points
    Kay,

    Other than Cin, what other information do you want to get? It's usually not practical to give spec on internal ESD cells. But I'll try to get as much as I can.

    Regards,
  • 0
    TI__Mastermind 25705 points
    Kay,

    After looking at these two parts, I have some questions for you.
    1) Why did the customer use 3.3V supply? These two parts both require 5V Vcc.
    2) Is there any pull up resistor from the DTR (pin15) to Vcc on customer's board? If the pin is left floating, the voltage level would be around 1.4V (two diodes above ground) after Vdd and Vss are provided. If so, maybe the customer can try to use stronger pullup for SN75185 than that of GD65232.

    Can you please check with the customer? Thanks,
  • 0 in reply to Hao L
    Expert 2225 points
    Hi Hao:

    thanks again.
    here are some update we've done so far.

    1. the timing of 5V & 3.3V is the same, so customer use the timing of 3.3V to compare with the DTR signal
    2. we're also trying to change the pull-hi resistance to 330ohm, and customer side is under verify this solution whether is workable or not.
    3. is there any document could know the C(ESD)= ?pF whether may cause the delay or else effect

    Thanks again.
  • 0 in reply to Kay Chen53
    TI__Mastermind 25705 points

    Kay,

    I still don't suggest to use 3.3V for evaluating. The timing may look same in both cases, but internally some circuitry might not be fully turned on with 3.3V supply.

    Unfortunately it's not practical to have a spec for ESD cells. But I can measure the Cin of both devices to see if there is any difference.

    Regards,