Tag
power integrity
6 articles.
- Your 2.2 µF is 0.68 µF: DC bias and the capacitance you did not buy
Class 2 ceramics lose capacitance when you put voltage across them. Würth measured a 25 V X7R part losing 69 % of its value at its own rated voltage — and the datasheet headline never mentions it.
- Ferrite beads are not resistors, and the 100 Ω is not 100 Ω
A bead is only useful in the middle of its impedance curve, the rated impedance is one point at 100 MHz, DC bias collapses it, and putting one in front of a capacitor can build a filter that amplifies the noise you were removing.
- The via is the decoupling: 40 dB from layout alone
Würth built one board with the same ferrite and the same two 100 nF capacitors in every channel, and changed only the layout. The worst channel is more than 40 dB down on the next best. The capacitor was never the variable.
- θJA is a property of the test board, not of your board
The thermal resistance on the datasheet was measured on a JEDEC board that is not the one you designed. Here is what the number actually contains, and the five terms you control.
- Where the buck ripple comes from, and why more capacitance didn't help
The textbook formula ΔV = I/(8Cf) is the special case where ESR is zero. In the regime most real designs sit in, output ripple is I·ESR — independent of capacitance, duty cycle and switching frequency alike.
- Why 100 nF, and the frequency where it stops being a capacitor
The value is inherited, not derived. What actually decides whether a decoupling capacitor works is its inductance — and above a few tens of megahertz your 100 nF is an inductor.