I am new to this forum. I am trying to design a MOSFET gate driver board of my own and I am thinking of using HCPL-3120 as the optocoupler with a VCC of +15 Volts and VEE of -8 Volts. I am using TMS320F28335 as the DSP controller. After referring the datasheet of HCPL-3120 (figure 25 in particular) I am unable to find a suitable open-collector/open-drain buffer IC to put in between the optocoupler and the DSP kit. I have 3 questions about this buffer IC.

Q1) Can you suggest a suitable 5 pin buffer IC for my design? Rather, how do I select a buffer IC for myself?

Q2) Can I give a floating input to my buffer? I mean, can I connect only the PWM output directly to my buffer without any ground?

Q3) I will generate an isolated 5 V supply using a 12 V transformer. Should the 2 grounds shown on the left of the optocoupler in the attached image be connected to the ground of my isolated 5 V supply?

HCPL-3120 datasheet - Figure 25 Recommended LED Drive and Application Circuit

Image source: Broadcom (Avago) - HCPL-3120 datasheet

  • 1
    \$\begingroup\$ Hi Ankit, just a note that Electronics Stack Exchange is not a "forum" - this site is specifically question-and-answer. Please ensure that all questions adhere to the site rules. Thank you and welcome. \$\endgroup\$
    – rdtsc
    Commented Jul 12, 2023 at 13:32
  • \$\begingroup\$ ankit kumar - Hi, To comply with this site rule, a post's author is required to include a link back to each original source webpage / PDF etc. for content copied or adapted from elsewhere (see the rule re: books etc). I saw you mentioned the source in the text, thanks for that, although an actual link is needed. In order to help you, I found what I believe to the source for that image & added it. For the future, please note it's your responsibility to add the link(s) :) As you're new here, please see the tour & help center for the main site rules & etiquette. Thanks. \$\endgroup\$
    – SamGibson
    Commented Jul 12, 2023 at 15:22

1 Answer 1

  1. You need a buffer with enough drive strength for the LED current. Open-drain buffers are 74xx07. At 5 V, the logic family with the highest drive strength is LVC, so use the (SN)74LVC1G07.

  2. Your PWM-generating device, the buffer, and the optocoupler input must use the same ground.

  3. Pins 1 and 4 of the optocoupler are unused; they are grounded only to prevent them from floating and acting as antennas. What matters is that pin 3 has the correct voltage (here, ground of the 5 V circuit).


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