AP1117 voltage regulator unstable
The AP1117 regulators (IC6, IC9 and IC21) used to produce clean supply voltages for the ACAM TDC behave instably due to too low ESR of the output decoupling capacitors. The voltages on VDDC_OH, VDDQ_TDC and VDDC_TDC may have as much as 400 mV of sawtooth-like ripple, which causes TDC jitter outside the spec and test failure.
- All cards from V5-2 batch (70 pieces) pass the tests. No oscillations on LDOs were spotted. Datasheet for AP1117 from July 2012, used for design of the V5-2 board says that the ESR of output capacitors should be less than 0.5 ohm. Combined ESR of all decoupling capacitors is 0.075
- 0.15 ohm.
- All cards from V6-0 pre-series fail the PTS due to LDO oscillations. The newest datasheet, from June 2013 restricts the output capacitor ESR range to 0.15 - 0.5 ohm. In this case, ESR of the decoupling capacitors (C15, etc.) is too low.
- Re-soldering IC9 from a V5-2 card to a V6-0 one fixed the issue (no
- Does this mean AP1117's design was changed between the two revisions of the datasheet?
- Leave only one 10 uF capacitor on each LDO's output.
- Do not mount: C32, C59, C19, C14, C61, C66, C15.
- Despite the claims from ACAM's datasheet and PLL regulation circuit design, recommending a huge decoupling capacitance, the modified V6-0 cards show even slightly better jitter performance than the V5-2 (1.07 bins rms vs 1.14 bins rms, 1 bin = 40.5 ps). Measurement was done for 2 V6-0 cards and 2 V5-2 cards.