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Non-Linear or Fixed Frequency Control?

Here is the second of three short tech nuggets, by Rich Nowakowski from TI, before our “Ask the experts” session on Wednesday, July 23 at 1:00 p.m. ET (10:00 a.m. PT). Hopefully these brief blogs will help stir your curiosity and questions, which you can post on our chat site here either before or during the session.

DC/DC converters with non-linear control modes claim to have a very fast load-transient response-time with minimal output capacitance and no loop compensation. Such benefits may be appealing to many designers familiar with voltage or current-mode control architectures. Many non-linear control modes are on the market with slight variations, and more are sure to be developed, such as TI’s TPS53515 with D-CAP3TM offering improved output voltage accuracy. On the other hand, a fixed and synchronizable frequency with the ability to compensate the control loop may still desirable. Here is an article that discusses the frequency response model of the D-CAP2TM control mode.

Figure 1

D-CAP2 Simple Block Diagram

D-CAP2 Simple Block Diagram

Hysteretic control, which is basically non-linear control, has become an important control method due to its fast transient response. Normal hysteretic control requires a relatively high-ESR output capacitor. Adding ripple injection to hysteretic control allows the use of low-ESR ceramic output capacitors. A “fixed on-time with bottom detection having a ripple injection” control topology is shown in Figure 1.

This topology, which is a type of hysteretic control, became popular due to pseudo-fixed PWM frequency operation along with compatibility with low-ESR ceramic output capacitors. It is interesting that this control method behaves like linear control, showing similarity to a frequency response (Bode-plot) of voltage mode control while keeping wide loop bandwidth. The operation of D-CAP2 control is similar in concept to Figure 2. The difference is that the ripple injection circuit is integrated on silicon.

Figure 2

D-CAP2 Waveform

D-CAP2 Waveform

How is the control-mode becoming an important criterion for DC/DC converter selection?

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