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Why add diode D7 in parallel with R20 on the LT4430 OPTO pin? Is there any reason excluding the diode and increasing R20 would be ill advised?

Voltage Regulator

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  • \$\begingroup\$ Besides what Curd said about series connection, you do have a point. A diode in series do look strange. I would expect the answer lies in the datasheet or application note for U2, LT4430. \$\endgroup\$
    – winny
    Commented Mar 29, 2018 at 7:23
  • \$\begingroup\$ There is no need for a diode, you can neglect it and there is no special mention about a diode at opto pin of LT4430 in a datasheet. \$\endgroup\$
    – CNA
    Commented Mar 29, 2018 at 7:28
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    \$\begingroup\$ It does not seem to be the case in circuit shown (thus probably not an answer), but in general it is good to keep on mind rather low maximal reverse voltage of LEDs (typically 5V). Therefore, in some situations protection against reverse voltage is advisable. \$\endgroup\$
    – Martin
    Commented Mar 29, 2018 at 7:39
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    \$\begingroup\$ Assuming it was deliberate: a) It increases the voltage at which the opto can turn on from 1.8V to 2.5V approx, which might affect undefined behaviour from U2 with very low voltages (i.e turns opto on at 2V - switchmode stalls). b) might slow the LED turn off (but photo-transistors are slow anyway) \$\endgroup\$
    – Henry Crun
    Commented Mar 29, 2018 at 8:21

1 Answer 1

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If you look at the datasheet there is one possible explanation

enter image description here

0.9V is quite high for a "low output". If you then considered a CNY17 optcoupler at high temperature. this could be enough to permit ~ 400uA to flow... doesn't sound much does it? but IF Vcc is relatively high, the dark current of the phototransistor would be higher and thus there could be the possibility of an unwanted output change.

Without knowing specifics of this PARTICULAR implementation (operating temperature etc) it would be safe to assume the extra diode is there to mitigate this "low voltage" output

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    \$\begingroup\$ +1 The CTR can be as high as 600% for that opto, and it takes about 2.5mA at the output to hit the switcher reference (1.22V/475\$\Omega\$). The CTR drops at high temperature but it's still a bit close for comfort. \$\endgroup\$ Commented Mar 29, 2018 at 10:43

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