Shown is a basic circuit diagram to power a MCU. I am using the recommended caps for the test circuit for the regulator.

  • Should I put a diode between the +ve terminal of the power source and the input of the LDO just to be safe?
  • Will this circuit be suitable to power and program the MCU?
  • Are the decoupling caps correct or suitable?


simulate this circuit – Schematic created using CircuitLab

  • 1
    \$\begingroup\$ Where are the decoupling caps physically located? Where are the bulk caps? \$\endgroup\$ Mar 18 '14 at 4:45
  • \$\begingroup\$ @IgnacioVazquez-Abrams I know the caps should be physically located as close as possible to the reg. Where should the bulk caps go? Close to the Power supply right? \$\endgroup\$
    – Jed
    Mar 18 '14 at 4:51
  • \$\begingroup\$ You need decoupling caps close to both the reg and the MCU, and the bulk caps need to be close to both the MCU and the input of the reg. \$\endgroup\$ Mar 18 '14 at 4:51
  • \$\begingroup\$ Ahh so 4 caps k. Is it ok if the lines between the bulk and decoupling caps are say...2cm long? ill edit the circuit diagram \$\endgroup\$
    – Jed
    Mar 18 '14 at 4:54
  • \$\begingroup\$ Bulk is a little less sensitive to distance issues than decoupling. \$\endgroup\$ Mar 18 '14 at 4:55

Since you're throwing away most of the 9V, a diode in series won't hurt efficiency, and could prevent damage if someone touches the input voltage in reverse, so definitely put it in.

Keep the leads short, as @Ignacio suggests. There should also be decoupling across the micro-controller pins directly as well if the two are separated by more than some mm.

If the 9V is coming from a battery, I would also suggest a largish aluminum capacitor across the regulator input, 220uF to 1000uF rated at 10V or more. If the leads are short (an inch or so), that can replace the 330nF. If it's an AC adapter supplying the 9V, there's already a big cap in there, so the 330nF is fine.

  • \$\begingroup\$ And make sure that none of the decoupling is actually in the power path, otherwise their effectiveness will be reduced. \$\endgroup\$ Mar 18 '14 at 5:02
  • \$\begingroup\$ @IgnacioVazquez-Abrams what he said, and if you want to get the most from an on-chip ADC then start by increasing the ceramic caps to 10uF/X5R or X7R (and really watch the layout). \$\endgroup\$ Mar 18 '14 at 5:06
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    \$\begingroup\$ @Ignacio Vazquez-Abrams. Why decoupling caps put in the power path would reduce their effectiveness? \$\endgroup\$
    – Blup1980
    Mar 18 '14 at 5:57
  • \$\begingroup\$ reverse-biased diode ACROSS the rails, rather than inline, will introduce no drop and still provide protection. Commonly known as the idiot diode ;) \$\endgroup\$
    – John U
    Mar 18 '14 at 10:09
  • \$\begingroup\$ @IgnacioVazquez-Abrams What do you mean "in the power path" ? \$\endgroup\$
    – Jed
    Mar 18 '14 at 23:58

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