7.3.1 The Adaptive On-Time Control and PWM Operation

The main control loop of the TPS56637 is 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 quasi-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 TPS56637 also includes an error amplifier that makes the output voltage very accurate. No external current sense network or loop compensation is required for DCAP3™ control topology.

At the beginning of each cycle, the high-side MOSFET is turned on. This MOSFET is turned off after internal one-shot timer expires. This one-shot duration is set proportional to the output voltage, V_OUT, and is inversely proportional to the converter input voltage, V_IN, to maintain a pseudo-fixed frequency over the input voltage range, hence it is called adaptive on-time control. When the feedback voltage falls below the reference voltage, the one-shot timer is reset and the high-side MOSFET is turned on again . An internal ripple generation circuit is added to reference voltage for emulating the output ripple, and this enables the use of very low-ESR output capacitors such as multi-layered ceramic caps (MLCC).