Overview
A buck switching regulator IC, model XL1509-ADJ, was used to step 24V down to 14V.
Actual Circuit
The actual circuit implemented is shown below:
Troubleshooting
Reviewing the datasheet revealed this note:
"For output voltages greater than approximately 10V, an additional capacitor CFF is required."
The manufacturer's datasheet was available only in English.
The note indicates that for output voltages above 10V, a compensation capacitor CFF is required. Using the datasheet formula CFF = 1/(31*1000*R2) with R2 = 11k gives an approximate value of 3 nF. A 3.3 nF capacitor was tried but did not affect the ripple. Other capacitor values were also tested without success.
As a practical workaround, the output was reduced to 12V by changing R33 to 9.1k. After this change the output ripple became acceptable:
Although this did not fully resolve the underlying cause, the adjustment mitigated the symptom.
Summary
- Read the main IC datasheet thoroughly; it contains important application notes and compensation requirements.
- For output voltages above roughly 10V, follow the datasheet guidance on adding compensation capacitor CFF and verify component values carefully.
- If a datasheet is available only in a nonpreferred language, obtain a reliable translation to ensure no design guidance is missed.
