Simpler Transconductance Op-amp Compensation

There is a practical difficulty involved in using the full-blown transconductance op-amp compensation scheme discussed above — because the pole and zero from HI are not independent. They will even tend to coincide if say Rf2 is much smaller than Rfl (i.e. if the desired output voltage is almost identical to the reference voltage). In that case, the pole and zero coming from HI will cancel each other out completely. Therefore, we can t proceed anymore, because we were counting on the zero from HI to change the open-loop gain from —2, to —1, just in time before it crossed over. 

Example Using a 300 kHz synchronous buck controller we wish to step down 25 V to 5 V. 

if the divider is not present (i.e. Vo = Vref), the gain of the stage Rfl/(Rfl + Rf2) above should be set equal to 1. Further, 

Note that by reintroducing C2, the computed crossover again occurs slightly earlier (by about 20%) — at around 80 kHz, instead of 100 kHz. The phase margin is now 36° (closer to the optimal). 

Also note that for this simpler compensation scheme to work, the ESR zero must lie between the LC pole frequency and the selected crossover frequency. 

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