SLUSEV2C June 2022 – March 2023 UCC28C50-Q1 , UCC28C51-Q1 , UCC28C52-Q1 , UCC28C53-Q1 , UCC28C54-Q1 , UCC28C55-Q1 , UCC28C56H-Q1 , UCC28C56L-Q1 , UCC28C57H-Q1 , UCC28C57L-Q1 , UCC28C58-Q1 , UCC28C59-Q1
PRODUCTION DATA
To compensate a peak-current-mode controller it’s very common to use a Type-II compensator. The Type-II compensator introduces a pole at DC, a relatively low frequency zero (fZ,COMP), and a higher frequency pole (fP,COMP). The pole at DC forces the system to have high gain at very low frequency and zero stead-state error.
The low frequency zero is formed by R18 and C19
The higher frequency pole is formed by R18 and C20
The mid-frequency gain of the compensator is given by
First, select a crossover frequency, 625 Hz. Then, use the frequency response of the plant (control-to-output) at low input voltage to determine how much gain the compensator must add to increase the crossover to the desired bandwidth. In Figure 9-5 the plant is measured to be -23.3 dB at 625 Hz. Therefore, the error amplifier must have a mid-frequency gain of 14.6 V/V.
Choose (R17+R19) = 22.5 kΩ, and solve for R18
Select a standard value for R18, like 324 kΩ.
Now that we know R18, it’s fairly straightforward to set fZ,COMP = fPOLE and solve for C19
Likewise, set fP,COMP = fZERO and solve the following for C20
Finally, choose standard capacitor values, C19 = 22 nF and C20 = 100 pF. Notice that a slightly lower value was used for C19 than calculated. This was done to have a faster “reset” time during after a load transient response. If the complete loop is found to have too little phase margin then C19 can be increased at the cost of slower reset time.