Hi team, I'm facing some difficulty trying to understand this circuit. It involves an error amplifier from UC2526. I want to evaluate its transfer function. However, as stated in the UC2526's datasheet, this is an Operational Transconductance Amplifier (OTA). I've read some explanations about this type of amplifier, but I'm still unsure how to work with it. Can you help me?
Lucas,
The output of the OTA is a current as opposed to the output of a traditional OA which is a voltage. The image you've attached shows C1 and R4 providing negative feedback such as what we use to adjust gain and frequency response when compensating a traditional voltage op amp. For an OTA (also commonly called a trasconductance amp or gm amp), there is no output-to-input feedback because the gain and frequency response are adjusted by R5 and C2 as you have shown in your schematic. Remove C1 and R4 and you are good to go.
For a traditional voltage OA, the feedback to input impedance ratio determine the closed loop DC gain. For an OTA, the large signal gain is also adjusted and determined by external components around the amplifier but since there is no feedback, the gain is relying upon the OTA transconductance
Figure 6 of the UC2526 data sheet shows the connections for sensing/regulating either a positive or negative voltage and I've drawn in red to show how the compensation would be connected for the OTA. For a more thorough explanation, refer to Demystifying Type II and Type III Compensators Using OpAmp and OTA for DC/DC Converters.
Regards,
Steve
Lucas,
Error amp gain vs freq given in the data sheet as shown in the graph below. The equivalent transconductance is in the range of 1.1<gm<1.5 but this is not published in the TI data sheet. With the open loop gain and gm transconductance, you should have all you need to compensate the control loop.
Steve
