4 Stabilizing the Control Loop and Ripple Injection

Unlike traditional PWM controllers that rely on an error amplifier and ramp signal, ACOT control requires a sufficient ripple component at the feedback node to reliably trigger the comparator and maintain stable switching operation. This ripple enables the control circuitry to adjust the duty cycle of the power switch, facilitating precise regulation. In simpler terms, COT, ACOT, or any other form of hysteretic control relies on the ripple in the feedback as the ramp signal to the comparator for PWM generation.

For Microchip ACOT buck converter devices, it is advisable to maintain a ripple ramp at the feedback with a peak-to-peak amplitude within the range of 20 – 200 mV (or as specified in the device data sheet), and the ripple waveform should be in phase with the inductor current. Throughout this application note, this peak-to-peak amplitude will be denoted as ΔV_FB.

Given the significance of this ripple, various possibilities to ensure adequate ripple in the feedback will be examined. While some situations may naturally provide sufficient ripple without the need for additional components, there is a high likelihood that extra ripple will need to be introduced with the assistance of additional components when output capacitors with very low ESR are used. This process is termed “Ripple Injection” as it involves introducing ripple when an adequate amount is not naturally present in the feedback signal.