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I have implemented DC-DC converter with mic5158 (schematic like in datasheet, no current limit, adjustable output), all work fine, except mosfet heating while medium load.

After investigation of this problem, i have found that gate-source voltage always keep near threshold (~4v). So mosfet always partially open instead of close-open cycle. With higher load voltage raising, so charge pump working.

I wondeder why Vgs does not fall below threshold and not raising to full-open mosfet level.

Any idea how to make mosfet fully open-close to avoid heating?

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2 Answers 2

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Your regulator is a linear regulator. This means that the voltage across the MOSFET will change as required to keep output volts at 3.3 volts.

If the MOSFET turned on fully, you would output the full supply voltage, not 3.3 volts.

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  • \$\begingroup\$ Thank you! This is what i missed from datasheet :) \$\endgroup\$
    – HardRock
    Commented Jul 29, 2020 at 16:23
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I have implemented DC-DC converter with mic5158

The MIC5156/5157/5158 is not a DC to DC converter but an LDO voltage regulator therefore, whatever voltage is needed on the MOSFET's gate \$\color{red}{\text{is}}\$ the Goldilocks value for precise operation of this linear circuit to produce the required output voltage.

Any idea how to make mosfet fully open-close to avoid heating?

Use a switching converter such as a buck regulator.

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  • \$\begingroup\$ Can you suggest switching converter IC with external mosfet please? (I need high current application) \$\endgroup\$
    – HardRock
    Commented Jul 29, 2020 at 16:21
  • \$\begingroup\$ Any suggestion can only be made on specifications and "high current" is not sufficient to go on. Max and min supply voltage, max output current and nominal output voltage levels are required as a minimum. Also, you need to state why you need an external MOSFET. \$\endgroup\$
    – Andy aka
    Commented Jul 29, 2020 at 16:29
  • \$\begingroup\$ Input 7.4 - 36v (12 - 24v in most common case) , output regulated 3.3 - 6v (5v in most common case). Current up to 32A, but for the first time, around 15A will be enough. External mosfet because i don't know another way to feed such current. All DC-DC step down that i hav found before was 3A max (LM2596 for example) \$\endgroup\$
    – HardRock
    Commented Jul 29, 2020 at 17:13
  • \$\begingroup\$ @HardRock I've setup Analog devices to list parts that are compatible. You can do the same with all the major players - analog.com/en/parametricsearch/11433#/… \$\endgroup\$
    – Andy aka
    Commented Jul 29, 2020 at 17:20

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