Constant On-Time Control
Constant On-Time Control (COT) operates by comparing the output voltage directly against a hysteresis band rather than a constant reference. When the output voltage falls below a lower threshold or rises above an upper threshold, the switch turns on or off accordingly. This leads to a very fast response to load changes. Its variable switching frequency can be a challenge for Electromagnetic Interference (EMI) considerations.
COT control is a slight variation of hysteretic control. It has a fast transient response. The on-time is generated by a one-shot on-timer. The off-time is terminated when the feedback voltage falls below the reference low threshold. It is a ripple-based controller, and the frequency will be modulated.
COT Control Details
Unlike traditional voltage- or current-mode control, COT control provides a way to eliminate the compensation loop. It removes the error amplifier and produces a Pulse-Width Modulation (PWM) signal with a fixed ON-time every time the feedback (FB) falls below VREF. COT results in a fast transient response without delays from the compensation loop or internal clock. The on-time of the high-side switch is fixed at given input and output voltages. Once the on-time expires, the high-side switch is turned off, and the low-side switch is turned on. There is a small dead time between the cycles of the switches to prevent shoot-through current.
COT Control Issues
There are two practical problems with COT control.
- Large switching frequency variation: The first point is that because the TON is fixed, the switching frequency has a strong dependency on the input and output voltages, as well as the load current. The switching frequency is approximately inversely proportional to the input voltage.
- Poor line regulation: The average value of the FB voltage is equal to VREF plus one-half the peak value of the FB ripple. FB ripple and VFB(avg) change as the inductor current changes, as shown in the right figure. This means that at high input voltage, the average voltage across the FB is greater, leading to worse line voltage regulation. The ripple of FB is above VFB, so the DC operating voltage point is higher than the required VOUT. Therefore, the DC regulation is not very good.
