Original question:
Dear Sir/Ms.
My customer is asking how to measure Loop Gain. I have discussed with AE through Email before, and got AE's answer. But my client raised a new question as follows.
Q1: Customers want to know that GAIN / PHASE cannot be measured in CC mode?
Q2: Assuming that the loop is not really unstable, besides COMP and capacitor, can TYPE 2 CONTROL METHOD be used?
Q3: Do you have an excel file for calculating the pole zero of this BUCK?
Best Regards,
Kami Huang
Q1 > A1: It isn't that it can't be measured, but that it is a challenging and considering it is inherently stable because it is a single pole system. There is a Simplis model if you want to perform specific bode plot for desired components selected. Here is theoretical way you could perform the measurement, but again it is a single pole system that is inherently stable.
Q2 > A2: If you try to implement a type 2 system you might be able to get slightly wider bandwidth but then you will have large overshoots during PWM dimming, which can be very detrimental to LEDs. What bandwidth do you need? This is a closed loop system and will not be ideal for ultra fastest responses. You would likely want a hysteretic converter. This is design to be fast enough to light matrix manager applications where their isn't a need for exceptionally fast transient responses. It offers much better regulation at the cost of transient response. We recommend using smaller COMP cap values for faster responses.
Q3 > A3: No, we do not. Again, this is a single pole system and is inherently stable and the plots in the datasheet give you enough information about the gain and phase. We also have the Simplis model if you want to do more complex analysis for higher order effects to the control loop.
If you want to take this offline, we can discuss the application in greater detail so you can make the correct choice in your LED driver.
-Francis Houde
