SLUAAJ0 February 2024 TPS51397A , TPS54308 , TPS54320 , TPS54350 , TPS54620 , TPS54622 , TPS54821 , TPS54824 , TPS563300 , TPS566231 , TPS566235 , TPS566238 , TPS568230 , TPS56C215 , TPS62933 , TPS62933F , TPS62933O
Figure 3-1 shows the PCM buck converter including second stage filter and hybrid sense.
Figure 3-2 is the overall control block diagram of PCM buck converter with second stage filter and hybrid sense. The variable notation refers to application note[5].
Among all the variables, the meanings of below ones are same as normal PCM buck converter without second stage filter:
The following transfer functions are new added or the meanings are changed from normal buck converter without second stage filter:
This application note does not introduce the derivation method of those transfer function expressions which have same meanings as normal PCM buck converter without second stage filter. The expressions are listed as Equation 1 to Equation 4.
where, Sn is the on-time slope of the sensed-current waveform and Se is the external ramp slope.
After adding second stage filter, the output impedance structure of buck converter changes from Figure 3-3 to Figure 3-4.
Since the output capacitors used in the design of a low-ripple design is generally low-ESR MLCC, to simplify the derivation of the loop model, the capacitor ESR effect is ignored here. From Figure 3-4, we can get the expression of the output impedance ZO of two stage filter as:
G2 reflects the low pass filter effects of second stage filter L2 and C2. The expression is:
The feedback signal VFB includes VO1 information coupled through feed-forward path and VO2 information through feedback voltage divider. According to the superposition theorem, we can get:
Based on the relation of Figure 3-2, it can be derived: