The main control loop of the buck is an adaptive on-time pulse width modulation (PWM) controller that supports a proprietary DCAP3 mode control. The DCAP3 mode control combines adaptive on-time control with an internal compensation circuit for pseudo-fixed frequency and low external component count configuration with both low-ESR and ceramic output capacitors. It is stable even with virtually no ripple at the output. The TPS56424x also includes an error amplifier that makes the output voltage very accurate.
At the beginning of each cycle, the high-side MOSFET is turned on. This MOSFET is turned off after an internal one-shot timer expires. This one-shot duration is set proportional to the output voltage, VOUT, and is inversely proportional to the converter input voltage, VIN, to maintain a pseudo-fixed frequency over the input voltage range, hence it is called adaptive on-time control. The one-shot timer is reset and the high-side MOSFET is turned on again when the feedback voltage falls below the reference voltage. An internal ripple generation circuit is added to the reference voltage to emulate the output ripple, enabling the use of very low-ESR output capacitors such as multilayered ceramic capacitors (MLCC). No external current sense network or loop compensation is required for DCAP3 control topology.