← Wiki · Concept

Decoupling capacitors

Every chip needs a small capacitor right at its supply pin. It supplies the fast current spikes the chip draws, so the rail stays clean.

Why#

A digital chip draws its current in sharp pulses, every clock edge. The supply is far away, behind traces that have inductance, so it cannot follow those pulses; the voltage at the pin dips and the chip misbehaves or resets. A capacitor next to the pin delivers the pulse locally and is recharged between them.

The recipe#

  • 100 nF ceramic at every supply pin (or pin pair), as close as you can: under 2–3 mm of trace to the pin and a via straight to ground.
  • One bulk capacitor per rail where the power enters a block: 10 µF ceramic for a 3.3 V logic rail, more for radios and LEDs.
  • Follow the datasheet where it asks for more: Espressif asks for 10 µF
    • 100 nF on the module’s 3V3 and a supply that delivers at least 500 mA.
Example circuit: C1 (100 nF) sits on the chip's VCC pin with the shortest path to GND; C2 (10 µF) is the bulk capacitor where 5 V arrives.
C1 (100 nF) sits on the chip's VCC pin with the shortest path to GND; C2 (10 µF) is the bulk capacitor where 5 V arrives. Open the decoupling_chip example from this page in PCB Canvas: “Insert into my schematic”.

On the board#

In the schematic the capacitor is just “on the net”. Where it goes is a PCB decision: after Update PCB, drag the capacitor next to its pin and keep the loop VCC → capacitor → GND small. A solid ground pour under the chip helps more than anything else.

Pitfalls#

  • A capacitor placed “somewhere on 3V3” across the board does nothing for the chip that needs it.
  • Radios (ESP32 Wi-Fi) draw peaks of several hundred mA. Too little bulk capacitance shows up as brown-out resets the moment Wi-Fi starts.