#7571 LOW Retriag: Backlight instability at dim settings (was: OFW can cause damage to backlight)

[Zarro Boogs per Child]

#7571: Backlight instability at dim settings
-------------------------+--------------------------------------------------
   Reporter:  ffm        |       Owner:  wad              
       Type:  defect     |      Status:  new              
   Priority:  low        |   Milestone:  Retriage, Please!
  Component:  hardware   |     Version:  not specified    
 Resolution:             |    Keywords:                   
Next_action:  never set  |    Verified:  0                
  Blockedby:             |    Blocking:                   
-------------------------+--------------------------------------------------
Changes (by wmb at firmworks.com):

  * owner:  wmb at firmworks.com => wad
  * priority:  blocker => low
  * component:  ofw - open firmware => hardware
  * milestone:  8.2.0 (was Update.2) => Retriage, Please!


Comment:

 My best guess is that the backlight voltage regulation circuit is unstable
 at low brightness levels.

 That circuit is pretty difficult to analyze.  If you were to disconnect
 R143, the combination of R556 and C599 is just a straightforward low-pass
 filter for the PWM signal from the DCON (DBC).  But when you consider the
 overall circuit, in which R143 and R144 load the capacitor with an
 impedance that is 1/4 of R556, and throw in the feedback loop through the
 voltage regulator (which has some time constants of its own), things get
 complicated in a hurry.  I can easily imagine that circuit "motorboating"
 under some conditions.

-- 
Ticket URL: <http://dev.laptop.org/ticket/7571#comment:1>
One Laptop Per Child <http://laptop.org/>
OLPC bug tracking system