Peak current-mode control samples the inductor's peak on every cycle, which makes it fast but forces you to add slope compensation above 50% duty cycle and leaves you vulnerable to noise on the current-sense pin. Average current-mode control (ACMC) takes a different approach: it closes a high-gain inner loop around the average inductor current, using a dedicated current-error amplifier (CEA) rather than a bare comparator.

The architecture has three players. First, the outer voltage loop generates a current-command signal (Icmd). Second, a sense resistor or current transformer produces a signal proportional to instantaneous inductor current. Third, the CEA — an op-amp with a compensation network — integrates the difference between Icmd and the sensed current, and its output drives a PWM comparator against a fixed sawtooth ramp. Because the CEA has large DC gain and low bandwidth relative to the switching frequency, the ripple gets averaged out and only the DC component of inductor current is regulated.

Why this matters: peak-mode control's sub-harmonic oscillation above D=0.5 comes from perturbation propagation cycle-to-cycle. ACMC's inner loop bandwidth is deliberately set well below Fsw/2, so cycle-to-cycle perturbations decay naturally. No slope compensation is required, and the noise immunity on the current-sense signal improves dramatically because the CEA filters it.

Real-world example: a 1 kW power-factor-correction (PFC) boost converter operating at 100 kHz. Input rectified line voltage swings from 0 V to 170 V peak, so duty cycle sweeps from near 100% down to about 30% every half-line-cycle. Peak-mode control would need aggressive slope compensation and still struggle with the current-sense noise near the zero-crossing (where signal amplitude is tiny). Every high-performance PFC controller (UCC28180, NCP1654, L4981) uses average current-mode control precisely because the CEA cleanly forces sinusoidal input current tracking regardless of duty cycle.

Design rule of thumb for the CEA compensation: set the crossover frequency of the inner current loop to roughly Fsw / 10. For a 100 kHz converter that's 10 kHz. Place a zero at the output-filter pole frequency (typically 1-2 kHz for a PFC) and a high-frequency pole at Fsw/2 to attenuate switching ripple. A typical CEA network: Rf = 10× Ri, Cf = 1/(2π · Fz · Rf), and Cp = 1/(2π · (Fsw/2) · Rf).

Trade-off: ACMC responds more slowly to load transients than peak-mode because there's an extra integration stage. For fast point-of-load converters, peak-mode still wins. For PFC, high-power AC-DC, battery chargers, and any application where clean current shaping matters more than microsecond transient response, ACMC is the right tool.