The Need for Regulation

Along the way

1
The load has a spec. Operating window here is ideal 3.3 V ±5% (green band). Stay inside it.
2
DC setpoint error first. Vref and feedback-divider tolerance shift \(V_{\mathrm{set}}\) at idle — before any current changes.
3
Load is the stimulus (bottom chart). Step amplitude and slew rate set how hard the edge is: big ΔI and fast edges are harder than small, soft ramps.
4
Response time is the loop’s speed. A slow regulator can’t catch a fast, high-current edge — deeper dip and taller overshoot on release. Faster response shrinks the excursion.
5
Red = left the window. Accuracy near the green edge leaves less room for dynamics. Fast + large load with slow response is the classic fail.

Top chart = \(V_{\mathrm{OUT}}\) (the pass/fail). Bottom = \(I_{\mathrm{LOAD}}\) (bold step/release with finite slew).

Next: Part 2 — resistor divider as a soft source. Part 3 — stiffness as \(Z_{\mathrm{out}}(f)\).

Stay in band · load step & release

Top: \(V_{\mathrm{OUT}}\) vs load window · Bottom: \(I_{\mathrm{LOAD}}\) · harder loads = larger/faster ΔI vs slower response

DC setpoint error (% · Vref + divider) Response time (µs) Load step amplitude (mA) Load slew rate (mA/µs)

\(V_{\mathrm{OUT}}\) · blue in-spec · red out · green = ±5% load window

\(I_{\mathrm{LOAD}}\) · step up then release · slew sets edge speed

Status—

Setpoint—

Peak low / high—

Edge time · peak |ΔV|—

Time out of band—

\[ V_{\mathrm{set}} = V_{\mathrm{ideal}}\,(1 + \varepsilon_{\mathrm{DC}}) \]

→ —

\[ t_{\mathrm{edge}} = \Delta I / \mathrm{slew} \]

→ —

\[ \Delta V_{\mathrm{pk}} \propto \Delta I \cdot \dfrac{t_{\mathrm{resp}}}{t_{\mathrm{resp}}+t_{\mathrm{edge}}} \]

→ —

Toy model (not SPICE): larger or faster load edges raise peak excursion when response time is long. Fast loops (small \(t_{\mathrm{resp}}\)) ride out the same load with less |ΔV|. Trace is red while outside the green band.

Try it: “Easy load” stays green. Switch to “Hard load” (big + fast) with the same response time — deeper red valleys. Then speed up Response time and watch the excursion shrink. “Slow loop” fails even a moderate step.

Analog Intuition

Stay in band · load step & release