Every switching regulator we've covered so far — voltage-mode, peak current-mode, average current-mode — needs a compensation network. You calculate the plant transfer function, place poles and zeros, tune a Type II or Type III network, and hope your ESR estimate was right. Hysteretic control throws all of that away. There is no error amplifier, no compensation cap, no bandwidth limit. The output ripple is the feedback signal.

The idea: feed the output voltage into a comparator with hysteresis. When Vout drops below the lower threshold, turn the high-side switch on. When Vout rises above the upper threshold, turn it off. The output naturally oscillates between the two thresholds. That's it — a buck regulator built from a comparator, a MOSFET, an inductor, and a cap.

Why it's fast: There's no loop bandwidth to speak of. Response to a load step is limited only by comparator propagation delay and inductor slew rate (di/dt = V_L/L). A hysteretic buck can respond to a 10 A load step in under 100 ns, where a compensated PWM regulator might take 5–20 µs. That's why hysteretic control dominates CPU V_core and DDR termination rails, where transient response beats efficiency and EMI concerns.

The catch — variable frequency: Switching frequency depends on Vin, Vout, L, C, and the ripple window. For a buck:

• t_on ≈ ΔV·L / [(Vin − Vout)·ESR]
• t_off ≈ ΔV·L / (Vout·ESR)
• f_sw = 1 / (t_on + t_off)

Notice ESR in the denominator — hysteretic control needs output capacitor ESR to generate a clean ripple signal in phase with the inductor current. Use a low-ESR ceramic-only output cap and the ripple becomes 90° out of phase (dominated by capacitive integration), the comparator sees a delayed signal, and the loop chatters or double-pulses. Fix: add a small series resistor to a sense cap, or use ripple injection (an RC network from the switch node to the feedback pin) to synthesize an artificial ripple.

Rule of thumb: Set the hysteresis window to 1–2% of Vout. For a 1.2 V rail, that's 12–24 mV. Combined with ~50 mΩ ESR and a 1 µH inductor, you'll land near 500 kHz–1 MHz switching.

Real-world example: The LTC1148 and later LTC3878 use hysteretic (constant on-time, technically) control for laptop V_core. Intel's VRD/VRM specs explicitly permit hysteretic controllers because meeting a ±50 mV window during a 100 A/µs load step is nearly impossible with fixed-frequency PWM. The tradeoff: variable f_sw spreads EMI into forbidden bands, so hysteretic isn't allowed in cellular or radio-adjacent supplies without a shielded enclosure.