Power Tips: Current-Mode Control Simplifies Compensation of Buck LED Regulators

The block diagram below details a synchronous buck converter with the LED and sense resistor in series with the inductor. In this application, the full ripple current of the inductor flows through the LED. If less ripple current is desired, simply increase the inductor value or place a capacitor in parallel with the LED. Whether the output capacitor is present or not, the important point is to place the current sense resistor in series with the inductor current. Using it as a feedback element makes the controller's power stage gain relatively flat and compensation simple. The dominant pole that is often seen in the voltage mode power stage gain and is set by the inductor and output capacitor (or the ac impedance of the LED) disappears. The combination of current mode control and current regulation works like magic!

To reduce power dissipation and maintain high efficiency, a gain block is added to the signal from the sense resistor. The resistor divider R1/R2 allows external regulation of the LED current by varying the Vcontrol signal level. For a typical converter, the power stage gain is usually measured between the COMP pin and the Vsense input, and the internal error amplifier is excluded. In the circuit below, this is the sum of the gain from the sense resistor to COMP, the gain block, and the R1/R2 divider. The plot below labeled Vsense shows a simulation of these total gains. There is no change in the response vs. frequency due to the use of current feedback and current mode control. A similar response can be achieved without the gain block and R1/R2 divider. This is also possible if the sense resistor is placed on the ground side of the LED and its current sense signal is connected directly to Vsense.

To calculate the loop gain, just know the internal error amplifier response and add it to the power stage gain, Vsense. Since the internal error amplifier of the TPS54218 is a transconductance amplifier with a gm of 225uS (V/A), all that is needed for compensation is a capacitor between COMP and ground. The resulting total loop gain (V_COMP) is similar to what is plotted below. In terms of selecting capacitor values, first select the necessary error amplifier gain to obtain the desired bandwidth. Then use the following equation to calculate the value of the capacitor, where gainBW is the desired error amplifier gain (in dB) at the selected crossover frequency. Alternatively, the error amplifier gain can be determined by dividing its gm by the capacitor impedance at the target frequency.

Regulated current mode control simplifies stabilization of the control loop by eliminating the dominant pole in the power stage and reduces the number of compensation components. Keep in mind that this is a first order approximation of the loop gain and that slope compensation introduces a pole in the power stage which rolls off the power stage gain at higher frequencies. So don’t push the bandwidth too high and always verify the results in the lab.