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You definitely need to consider these additional capacitors in
SwitcherPro because they can significantly impact your stability. Not
only do you need to know the sum of your bulk capacitors (which will
impact your LC corner frequency) but you also need to know the
equivalent ESR of these caps because that will impact your ESR zero
frequency. I would do the following:
1) Click on the inductor in your design and select "Lock".
2) Click on one of the components in the feedback loop (like C8), the
"What If Analysis" will pop up. Click "Lock Components" and "Save
Changes".
3) Replace C2 with one of the 1500uF capacitors in SwitcherPro (like the Sanyo 6SEPC100M)
4) Click on "What If Analysis"
3) Click on "Power Stage" tab at the top
4) Replace "Resistance" value next to "Output Cap1" with your equivalent ESR.
You will not be able to change the "Output Cap1" capacitance in this
form because this form (unfortunately) does not allow values >
1000uF.
5) Press calculate. The gain/phase plot in the bottom of the window should update.
I don't know the specifics of your design so I can only make general
comments. If your design has a lot of small capacitors that sum up to
1400uF, the equivalent ESR will probably be small and your originial
design may not be stable. I would recommand that you measure (in the
lab) your equivalent capacitance and ESR, create a new component with
these values, place it in your design as C2 (with all other components
unlocked), and allow SwitcherPro to pick a new compensation.
Regards,
Michael
-- Michael Krasnicki Analog eLab Texas Instruments
If I generate a dummy capacitor from these values and use it as C2 and use the new compensation and feedback components on my design, the output oscillates with about 150kHz +-20mVpp
I've fixed it. There was a problem with the PCB, so not all bulk capacitors were correctely working. After deleting these in SwitcherPro the design works stable.
