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There is a step-down DC / DC converter that receives 5V input and 3.3V output It powers the Atmega328P microcontroller.

TLV1117 TI

For the firmware of the controller, I will use the USBASP programmer. It connects to the ISP connector (MISO / MOSI / SCK / RST / 3.3V / GND).

During programming, the 3.3V line will be powered from the programmer.

What will happen in this case with the down converter turned off? Power will be supplied from the bottom of the inverter. Is it necessary to put a diode on the 3.3V output?

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    \$\begingroup\$ The datasheet section 9.1.1 says " ... • (D) An external diode is recommended to protect the regulator if the input instantaneously is shorted to GND ..." and figure 9 shows this diode. Why do you suggest ignoring what the datasheet recommends. | "Put a diode on" is unclear - if you means "in series with" it will stop correct regulation. If you means as fig 9 - that's what T recommend. If ...? \$\endgroup\$
    – Russell McMahon
    Nov 26, 2019 at 10:24
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    \$\begingroup\$ Note that the diode of fig 9 will cause the regulator input circuitry to be powered by the programmer supply. If you wish to prevent this you will need to design the circuit accoridngly. \$\endgroup\$
    – Russell McMahon
    Nov 26, 2019 at 10:26
  • \$\begingroup\$ In normal mode, TLV1117 receives 5V and reduces it to 3.3V. If I want to program the controller, I will connect the programmer power from the 3.3V side. It turns out that the power will be supplied after the converter. It turns out that I am applying voltage in the opposite direction (from the 3.3V side). A series diode will create a voltage drop. One option is to disconnect the wire from the converter from the 3.3V side. But it is inconvenient. \$\endgroup\$
    – Delta
    Nov 26, 2019 at 10:29
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    \$\begingroup\$ @АлексГарисон no, you fit the diode as shown in figure 9. If you want any more information on reverse voltage protection for your regulator, a good link is here: rohmfs.rohm.com/en/products/databook/applinote/ic/power/… \$\endgroup\$
    – MCG
    Nov 26, 2019 at 11:40
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    \$\begingroup\$ @АлексГарисон I understood exactly what you meant. I said: 'look at fig 9' & 'a series diode is not suitable' & 'IF you do not want the unpowered 5V circuitry powered by 3V3 via the reverse diode you would need to do something else.' || That could be a transistor which turns off when 5V is absent. Both FET or bipolar have pros and cons there. eg bipolar - collector to regulator out, emitter to 3V3 load, base via resistor to 5V in. OKish but you get a Vce drop when on. USE a FET and VDds drop can be tiny but body diode conducts. Back to back FETs get annoying but work. \$\endgroup\$
    – Russell McMahon
    Nov 26, 2019 at 15:25

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Is it necessary to put a diode on the 3.3V output?

No. The regulator output will draw a few mA when voltage is back-fed into it from the USBASP, but this won't harm it.

However it could damaged if there is a large amount of uncharged input filter capacitance. To prevent this you should disconnect the power supply unit when programming, and only have a small capacitor (eg. 10uF) close to the regulator input.

A diode (preferably Schottky type for low voltage drop) between the power supply and regulator (before the 10uF capacitor) will also help to isolate it from the power supply capacitance, as well as protecting it from reverse voltage.

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  • \$\begingroup\$ Thanks for the answer! \$\endgroup\$
    – Delta
    Nov 27, 2019 at 15:44

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