Compensation selection can be done with aid of WEBENCH to select compensation components or with the aid of the average Spice model to simulate the open loop modulator and power stage gain. Alternatively, the following procedure gives a good starting point.
While current mode control typically only requires Type II compensation, it is desirable to layout for Type III compensation to increase flexibility during design and development. Current mode control boost converters have higher gain with higher output impedance, so it is necessary to calculate the control loop gain at the maximum output impedance, estimated by Equation 58.
The transconductance of the TPS40210 current mode control can be estimated by Equation 59.
The maximum output impedance ZOUT, can be estimated by Equation 60.
At the desired crossover frequency (fL) of 30 kHz, ZOUT becomes 0.146 Ω.
The modulator gain at the desired cross-over can be estimated by Equation 62.
The feedback compensation network needs to be designed to provide an inverse gain at the cross-over frequency for unity loop gain. This sets the compensation mid-band gain at a value calculated in Equation 63.
To set the mid-band gain of the error amplifier to KCOMP, use Equation 64.
R4 = 18.7 kΩ selected.
Place the zero at 1/10th of the desired cross-over frequency.
C2 = 2200 pF selected.
Place a high-frequency pole at about five times the desired cross-over frequency and less than one-half the unity gain bandwidth of the error amplifier:
C4 = 47 pF selected.