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Original question:

ONET4201LD: 100 Mbps laser diode driver

ONET4201LD: laser diode configuration connection

Michael9q
Intellectual 565 points
Part Number: ONET4201LD
Other Parts Discussed in Thread: ONET4291VA, ONET1141L

Hello there,

we use laser driver ONET4201LD to drive laser diode (LD) with monitoring photodiode (MPD) - connection type A in TO56 chassis. However this connection does not allow us to place the LD chassis into conductive housing with GND potential. Resulting short circuit of VCC on LD chassis and housing with GND.

What LD configuration we should use with ONET4201LD to have GND potential on the LD housing (common pin 2 connected to LD chassis) and still be able to use MPD?

Can the ONET4201LD still drive other LD types? Could you please draw recommended circuit with GND on LD chassis, MPD and ONE4201LD?

Thank you a lot,

Best,

Michael

  • 0
    TI__Guru* 93670 points

    Hi Michael,

    I checked and this configuration - GND connection to pin 2 - is not supported. 

    Regards,Nasser

  • 0 in reply to Nasser Mohammadi
    Intellectual 565 points

    Hi Nasser,

    thank you very much for checking this for me. The only way I can think of now is that isolate laser diode chassis from the rest of the system.

    Is there a specific reason to have VCC on laser diode and to use ONET4201LD driver as sink?

    Can be the MOD- and MOD+ swapped to change polarity applied on the laser?

    Is the BIAS pin only sink and cannot be used as source?

    AC and DC coupling has nothing to do with correct?

    Do you have any other laser driver capable of driving LD with C pin configuration and capable of same dataspeed 100mbps?

    For example: https://www.ti.com/product/ONET4291VA ONET4291VA? 

    Thank you very much.

    Best Regards,

    Michael 

  • 0 in reply to Michael9q
    TI__Guru* 93670 points

    Hi Michael,

    ONET4201VA is AC coupled and it has back termination to VCC. On the other hand, ONET4201LD needs external VDD termination. Additionally, i could not find a validation report showing  we can ground cathode of the laser. Therefor i am hesitant to recommend this.

    Regards,Nasser

  • 0 in reply to Nasser Mohammadi
    Intellectual 565 points

    Hi Nasser,

    thank you for highly appreciated answers.

    Do you think that ONET4201LD with current mirrors at both LD and PD would solve the problem?

    The laser diode would be in C package:

    ONET1141L might be also solution even design datarate 11.3Gbps.

    Thank you,

    Michael

  • 0 in reply to Michael9q
    TI__Guru* 93670 points

    Hi Michael,

    It is possible. However, i haven't come across a case where current mirror was used. You can experiment/try.

    Regards,Nasser

  • 0 in reply to Nasser Mohammadi
    Intellectual 565 points

    Hi Nasser,

    it seems that the only TI laser driver capable to drive a laser diode with C type pinout and higher current ~ 50mA (bias + mod) is the ONET1141L. 

    1. Even though that the laser driver is not characterized under 1 Gbps, is there any concern why it should not work on 125Mbps?  
    2. At pin 15 - COMP of the ONET1141L could be used higher value capacitor (0.1uF) to run stable 125Mbps with APC?
    3. Is it possible to connect directly MOD+ to Laser diode anode?
    4. Can be used DC coupling for the laser instead of the AC coupling shown in application schematics?

    5. This is the connection we want to use. 

      Pin 1 is the Bias + modulation 

      Pin 2 is common GND

      Pin 3 is the PD

    Could you please confirm the desired connection?

    Thank you very much,

    Best Regards,

    Michael 

  • 0 in reply to Michael9q
    TI__Guru* 93670 points

    Hi Michael,

    APC loop has not been characterized at this low data rate. 

    Also, without using a current mirror - shown in your previous post - i am not sure if we can drive the laser in this mode.

    Device has not been designed or characterized for the application you've noted. 

    Regards,Nasser

  • 0 in reply to Nasser Mohammadi
    Intellectual 565 points

    Hi Nasser,

    the last comment is to the ONET1141L which should be capable of directly driving laser diode with anode from BIAS and MOD+.

    The option with current mirrors was for suggested for the ONET4201LD.

    Please see following schematics original (left), proposed (right). Could you please comment whether this schematics is theoretically functional?

    We are aware of APC loop is not characterized for the low data rate.

    Thank you for answer,

    Michael

      

  • 0 in reply to Michael9q
    TI__Guru* 93670 points

    Hi Michael,

    I think right hand side of the 0.1uF Cap needs to be biased to a supply - similar to EA bias on the left-hand side.

    Regards,Nasser

  • 0 in reply to Nasser Mohammadi
    Intellectual 565 points

    Hi Nasser,

    yes, that is the connection to bias. Please see in green:

    Does it make sense?

    Thank you,

    Michael 

  • +1 in reply to Michael9q
    TI__Guru* 93670 points

    Hi Michael,

    What you have shown seems to be workable. However, i have not seen ONET1141L being operated with a laser such as what you have shown.

    Regards,Nasser

  • 0 in reply to Nasser Mohammadi
    Intellectual 565 points

    Hi Nasser,

    thank you very much for your help here. I will upload any result then.

    Best regards.

    Michael