Tag
power integrity
13 articles.
- AWG wire gauge and current: what actually sets the limit
Why a wire's current rating belongs to its insulation rather than its copper, what the ampacity charts really measure, and why voltage drop usually binds first.
- Switch-node ringing: measure it, then size the snubber
Two scope readings give the loop inductance and the parasitic capacitance. From those the snubber values follow, and so does the reason to fix the layout first.
- Choosing a buck inductor: ripple, saturation and heat
Size the inductance from the ripple target, then check the peak against saturation and the RMS against heating — two different ratings that fail differently.
- How long will the battery last? Estimating it properly
Capacity over average current is the easy part. Here is what the cell's rated drain, its internal resistance and the cutoff voltage do to that answer.
- Measuring supply ripple: the ground clip is the antenna
A 3-inch probe ground lead turns a switching spike into ringing that is not on the rail. Tip-and-barrel, coax, bandwidth limits, and the null test.
- Buck converter PCB layout: find the hot loop first
The two switch-state current loops differ by one small loop carrying chopped current. Its area sets the EMI, and the input capacitor's placement sets its area.
- Why an LDO oscillates: the output capacitor's ESR tunnel
The ESR zero that keeps a PMOS LDO stable, the tunnel where too little and too much both oscillate, why ceramics broke old parts, and what C_NR and C_FF trade.
- Your 2.2 µF is 0.68 µF: DC bias and the capacitance you did not buy
Ceramics lose capacitance under DC voltage. Würth measured a 25 V X7R part losing 69 % of its value at its rated voltage, and the datasheet never mentions it.
- Ferrite beads are not resistors, and the 100 Ω is not 100 Ω
A bead is useful in the middle of its curve, the rated impedance is one point at 100 MHz, DC bias collapses it, and before a capacitor it can amplify noise.
- 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 40 dB down.
- 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 yours. 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 ΔV = I/(8Cf) is the special case where ESR is zero. Where most real designs sit, output ripple is I·ESR, independent of capacitance and frequency.
- Why 100 nF, and the frequency where it stops being a capacitor
The value is inherited, not derived. What decides if a decoupling capacitor works is its inductance, and above a few tens of megahertz 100 nF is an inductor.