2.1 Advancements in T_ON Generator

In ACOT, as illustrated in Figure 2-1, the T_ON generator incorporates additional feedback from both the input voltage and the switching (SW) node voltage. Specifically, the constant current source becomes a function of the input voltage (IT = K1 • V_IN, where K1 = constant) and the threshold of the T_ON generator comparator becomes a function of the output voltage (V_TH = K2 • V_O, where K2 = constant). These enhancements enable ACOT to function similarly to a constant-frequency converter and alleviate frequency variation issues.

For a basic COT, T_ON (On time) can be expressed as:

$T_{ON}=\frac{V_{TH}\cdot C_{TON}}{I_T}$

When it comes to ACOT, substituting VTH = K1 • V_O and I_T = K2 • V_IN into Equation 2-1, it results the following:

$T_{ON}=\frac{(K1\cdot V_O)\cdot C_{TON}}{{K2\cdot V}_{IN}}=\frac{K\cdot V_O\cdot C_{TON}}{V_{IN}}$

Where:

K = K1/K2

Additionally, for a buck converter, it is known that (T_SW = Switching period):

$T_{ON}=\frac{V_O}{V_{IN}}\cdot T_{SW}$

Combining Equation 2-2 and Equation 2-3, results in:

$\frac{V_O}{V_{IN}}\cdot T_{SW}=\frac{V_O}{V_{IN}}\cdot C_{TON}\cdot K$

Hence:

$T_{SW}=C_{TON}\cdot K$

This derivation signifies that ACOT maintains a constant frequency (theoretically, in steady state condition) regardless of input or output voltage variations, as the switching period is solely dependent on the T_ON generator capacitor and a few constants. However, a slight frequency variation is to be expected due to many factors, such as T_ON generator component tolerances, duty cycle variations due to power stage losses, line transients, load transients, etc.