“This article explores why constant-on-time control (COT) is more effective than conventional current-mode control in DC/DC converters.
This article explores why constant-on-time control (COT) is more effective than conventional current-mode control in DC/DC converters.
Figure 1 shows the traditional current mode architecture of a DC/DC converter by comparing the sampled current (in red) with the output of the error amplifier in the voltage feedback loop (in blue) to generate the control MOSFET PWM pulses.
In traditional control architectures, two factors affect the transient response performance of output load changes.
The first factor is the error amplifier. In the voltage feedback loop, the error amplifier of the compensation network acts as a low-pass filter, thereby lengthening the response time of the converter to output voltage changes.
Figure 1: Current Mode DC/DC Architecture Diagram
Figure 2 shows the effect of error amplifier delay on the loop transient response. In this example, the load current rises rapidly from 0A to 20A, and as you can see from the bottom curve, VOUT drops significantly before recovering. The green curve is the output of the error amplifier, which reaches its maximum value two cycles after the maximum undershoot occurs. This delay is caused by the low-pass filter of the error amplifier.
Figure 2: Effect of Error Amplifier Delay on Output Undershoot
The second factor is the switching cycle delay caused by the internal clock, which feeds pulse-width modulation (PWM) control back to the output MOSFETs. In the continuous current control mode of operation, since the control frequency is fixed, the turn-on timing of the MOSFETs is determined by the clock period. Even with a PWM duty cycle to control the on-time of the high-side MOSFET, it cannot turn on again until the next clock cycle begins. Once the top MOSFET is turned off, the load current increases from 0A to 20A (see Figure 3). The output of the error amplifier rises quickly to respond, but the high-side MOSFET must wait until the next clock cycle to turn on. During this period, the output voltage continues to drop. The shaded area is the difference between the load current and the Inductor current. This insufficient current must be supplied by the output capacitor and will cause the output voltage to undershoot.
Figure 3: Effect of Internal PWM Clock Period on Output Undershoot
Compared with traditional voltage/current mode control, the constant-on-time control (COT) structure is very simple, it samples the output voltage through a feedback resistor (see Figure 4), and then directly compares the output voltage ripple valley value to the reference voltage , which generates a fixed on-time pulse to turn on the high-side MOSFET. When the on-time pulse expires, the upper MOSFET turns off (and the lower MOSFET turns on).
Figure 4: Constant On-Time DC/DC Architecture
The COT architecture eliminates the need for the compensation network in traditional voltage/current mode DC/DC control, and the design of the converter is simpler because there are fewer components and there is no need to spend a lot of time adjusting the compensation values. Reliable COT operation requires that the output voltage ramp at the feedback node be large enough to ensure frequency-jitter-free operation. Therefore, the ramp should be larger than the noise of the feedback input in any stochastic system.
If the output capacitor has sufficient equivalent series resistance (ESR), the feedback ramp voltage generated by this ESR dominates the smaller series resistance of the inductor. In this case, a simple resistor divider network is sufficient (see Figure 5), which can often be used for electrolytic or POSCAP capacitors.
Figure 5: COT Feedback Input Ramp Voltage from Output Capacitor ESR
Figure 6: COT Feedback Input Ramp Voltage from Inductive Ramp Conversion Circuit
If low-ESR ceramic capacitors are preferred, an additional “ramp generator” circuit can be used to generate the desired feedback ramp voltage (see Figure 6).
It is important to note that the feedback voltage is fed directly into the comparator to drive the timing control block, and there is no error amplifier or internal fixed frequency to cause delays that affect transient response time.
The COT control architecture uses the reference comparator output to trigger the timing pulse generator instead of using a fixed frequency clock. The frequency of pulses is determined by the output load current. In continuous conduction mode with stable output current requirements, the COT control operates at an approximately fixed frequency. However, during the transition of the load current from low to high, the COT pulse generator outputs high-frequency pulses to minimize output undershoot. Once the normal output voltage is reached, the pulse frequency is reduced to the level required to maintain a stable regulated output voltage.
In addition, the transient response time of COT control is twice as fast as that of conventional voltage or current mode control. The lower undershoot makes it easier to meet load voltage tolerance specifications. This also means that compared to voltage or current mode converters, converters based on COT control mode require less output capacitance to meet a given load transient response, saving both space and cost.
FIG. 7 is a comparison diagram of current control and COT control modes. For the same load current boost converter, COT control has a faster switching speed, reducing the gap between the inductor and the output current, thereby further reducing the output undershoot.
Figure 7: Comparison plot of transient response to load jump (current mode vs. COT control mode)
Another advantage of the COT variable frequency control structure is that at light loads, the pulse frequency is further reduced to maintain high efficiency. Because the pulses are only issued when the output load requires it, smaller internal switching losses can be achieved compared to voltage or current mode architectures with permanent switching clocks. This means that COT-based DC/DC converters have very high efficiency under light or no-load conditions and are the best choice for battery powered devices/devices with power saving modes.
To sum up, COT control is superior to traditional current and voltage mode control solutions due to its advantages of fast transient response, high efficiency, few components, and simple design.