For applications with fast transient requirements, it’s important to design the supply with high bandwidth and sufficient stability margin.
Power supply loop compensation design is usually viewed as a difficult task, especially for inexperienced supply designers. Practical compensation design typically involves numerous iterations on the value adjustment of the compensation components. This is not only time consuming, but is also inaccurate in a complicated system whose supply bandwidth and stability margin can be affected by several factors. The buck step-down converter is used as the typical example, but the concepts can be applied to other topologies.
For switching mode power converters, such as an LTC3851 or LTC3833 current-mode buck supply, a fast way to determine whether the unstable operation is caused by the loop compensation is to place a large, 0.1μF, capacitor on the feedback error amplifier output pin (ITH) to IC ground. (Or this capacitor can be placed between the amplifier output pin and feedback pin for a voltage mode supply.) This 0.1μF capacitor is usually considered large enough to bring down the loop bandwidth to low frequency, therefore ensuring voltage loop stability. If the supply becomes stable with this capacitor, the problem can likely be solved with loop compensation. An over-compensated system is usually stable, however, with low bandwidth and slow transient response. Such design requires excessive output capacitance to meet the transient regulation requirement, increasing the overall supply cost and size.
To simplify and automate the switching mode supply design, Linear Technology has developed LTpowerCAD design tool. It helps users to select a power solution, design power stage components, and optimise supply efficiency and loop compensation. For a given Linear Technology voltage mode controller such as the LTC3861, its loop parameters are modeled in the design tool. For a given power stage, users can place the pole and zero locations (frequencies), then follow the program guide to put in real R/C values and check the overall loop gain and load transient performance in real time. After that, the design can also be exported to an LTspice simulation circuit for a real time simulation.
In summary, for applications with fast transient requirements, it is very important to design the supply with high bandwidth and sufficient stability margin. This is typically a time consuming process. The LTpowerCAD design tool can be used to make supply loop design and optimisation a much simpler task.
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