Page 28 - PEN Ebook March 2021

Design Design

Table 1: Summary of Effect of Switching Frequency Increasing on BW/PM.
the requirement set by Equation (10). Once again, MPQ4430 BODE PLOT AT
since the switching frequency is configurable in DIFFERENT SWITCHING
this part, the maximum configurable switching FREQUENCIES

frequency is considered in Equation (10). Consider- Figure 4 shows the bode measurement of the
ing the maximum switching frequency of 2.5MHz, MPQ4430 as the switching frequency — and con-
the target COMP-P frequency (f COMP-P ), must be set sequently the inductor value — change.
close to f / 2 = 2.5MHz / 2 = 1.25MHz.
SW
There are a couple of important observations;
For this part, R COMP and C are 460kΩ and 0.2pF, Table 1 summarizes the results.
HF
respectively. This results in f COMP-P being 1.7MHz,

which is close enough to the 1.25MHz target. First of all, as seen in the bode measurements, at Figure 6: Bode Diagram of MPQ4430 at 2.5MHz With and Without Feed-Forward Capacitor.
higher switching frequencies the PM is improved.
Lastly, C is compared with C COMP to ensure the This is because, based on Equation (2), the pole switching frequencies is by adding a feed-for- CONCLUSION
HF
requirement set by Equation (11) is met. In this due to the inductor (ω ) is pushed further out as ward capacitor (C ) to the feedback network (see This article presented a systematic approach
L
FF
part, C and C COMP are 0.2pF and 52pF, respective- the switching frequency increases. This causes Figure 5). to evaluate the capability of the internal com-
HF
ly. Therefore, C is approximately 0.3% of C COMP . less negative phase at the BW frequency and an pensation networks based on the application
HF
So, the requirement for the C vs. C COMP value (C increase in the PM, which further confirms that Adding a feed-forward capacitor can great- switching frequency. The proposed evaluation
HF
HF
< 4% x C COMP ) is also met. setting the COMP-Z frequency based on the min- ly improve the BW and PM of the system. The technique involved three basic checks to ensure
imum switching frequency is a frequency response of the MPQ4430 was taken that the internal compensation network is prop-
wise decision. at a 2.5MHz switching frequency, both with and erly designed for an application with a known
without a 20pF feed-forward capacitor. With the or configurable switching frequency. In certain
Note that, since R COMP and C COMP additional capacitor, BW and PM improved. cases, adding external knobs can further improve
are fixed, and the increase in the transient performance of the system. These
the switching frequency only principles were applied to the MPQ4430, which
affects the second pole in the verified the effectiveness of the technique.
power stage, the BW is rela-

tively fixed. Therefore, it may
be desirable to increase the
BW as the switching frequency For More Information
increases. This can be accom-
plished by adding external ▶ Monolithic Power Systems
knobs.

▶ Buck Regulators
An effective way to increase
Figure 5: Adding a Feed-Forward Capacitor to the Compensation Network to Improve
Transient Response. the loop BW and PM at higher

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