1 Constant-on-Time (COT) Control
Understanding the fundamental operating principles is important as we delve into the design considerations of Constant-On-Time (COT) converters. Figure 1-1 illustrates the block diagram of COT control in a buck converter application. The buck converter is comprised of the High-Side (HS) and Low-Side (LS) switches, typically MOSFETs, along with the output filter (inductor and output capacitors), input capacitors, and the control stage.
The control stage includes comparators, an SR flip-flop (SR latch), a Constant ON time generator, and a Minimum OFF time generator. It is important to note that on-time and off-time are always referenced to the high-side switch operation. Moreover, the Constant ON time generator consists of a threshold comparator, a constant current source (IT), a capacitor (CTON), and a capacitor discharge switch (QN). The interaction between the constant current source and capacitor value determines the generation of TON (on-time).