Bug #1012

AP1117 voltage regulator unstable

Added by Tomasz Wlostowski on 24 Oct 2014 at 16:57. Updated on 20 Nov 2015 at 17:26.

Status:Closed Start date:2014-10-24
Priority:High Due date:
Assignee:Tomasz Wlostowski % Done:

0%

Category:- Spent time: -
Target version:EDA-02267-V6-1

Description

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.

Investigation:
- 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 oscillations).
- Does this mean AP1117's design was changed between the two revisions of the datasheet?

Solution:
- Leave only one 10 uF capacitor on each LDO's output.
- Do not mount: C32, C59, C19, C14, C61, C66, C15.

Notes:
- 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.

History

Updated by Tomasz Wlostowski on 24 Oct 2014 at 17:02

  • Subject changed from AP1117 voltage regularor unstable to AP1117 voltage regulator unstable

Updated by Tomasz Wlostowski on 02 Dec 2014 at 10:20

  • Assignee set to Tomasz Wlostowski
  • Status changed from New to Resolved

- Manufacturers informed about the issue and the way to fix it.
- CERN DEM working on official documentation update.

Updated by Erik van der Bij on 20 Jan 2015 at 13:43

  • Priority changed from Immediate to Normal
  • Status changed from Resolved to Feedback

V6-1 has been released in EDMS. Please inform all vendors.

Updated by Erik van der Bij on 20 Nov 2015 at 11:57

  • Priority changed from Normal to High

Have vendors been informed?

Yes. The new batch of the cards by INCAA is the V6-1 EDMS release.

Updated by Erik van der Bij on 20 Nov 2015 at 17:26

  • Status changed from Feedback to Closed

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