Openmoko Bug #2310: charging N900 with gta02 causes the gta02 to overheat

[Openmoko Public Trac]

#2310: charging N900 with gta02 causes the gta02 to overheat
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 Reporter:  lindi            |          Owner:  joerg                                   
     Type:  defect           |         Status:  assigned                                
 Priority:  normal           |      Milestone:                                          
Component:  System Software  |        Version:                                          
 Severity:  major            |       Keywords:  usb hostmode U4904 L4901 hw heat USB_FLT
 Haspatch:  0                |      Blockedby:                                          
Estimated:                   |    Patchreview:                                          
 Blocking:                   |   Reproducible:                                          
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Changes (by joerg):

  * keywords:  battery pmu charging => usb hostmode U4904 L4901 hw heat
               USB_FLT
  * owner:  openmoko-kernel => joerg
  * version:  unspecified =>
  * status:  new => assigned
  * severity:  normal => major


Comment:

 Replying to [comment:5 lindi]:
 > Some comments from IRC:
 >
 > < DocScrutinizer> lindi-: could you check USB_FLT aka GPM5 GPIO

 oops, that's GPG10 of course, and /USB_FLT, so low/0 should indicate a usb
 current of >500mA (the limit set by R4918=14k).

 From AAT1275 datasheet:
 >>
 Operation in Current Limit
 When a heavy load is applied to OUT2 of the
 AAT1275, the load current is limited to the value of
 ILIM (determined by RSET) causing a drop in the
 output voltage. This increases the AAT1275 power
 dissipation and die temperature. When the die tem-
 perature exceeds the over-temperature limit, the
 AAT1275 shuts down until it has cooled sufficiently,
 at which point it will start up again. The AAT1275
 will continue to cycle on and off until the load is
 removed, power is removed, or until a logic low
 level is applied to the EN pin.
 A fault flag indicates when the OUT2 pin load cur-
 rent has exceeded the current limit level set by
 RSET. The fault flag is an active low, open-drain pin
 <<

 So that suggests the normal way for U4904 to handle overload on USB output
 is to heat up to a temp that easily might make the device emit smell.

 For sake of component stress kernel should disable EN_USBHOST on detection
 of /USB_FLT, as well as raise some warning signal, both in syslog and
 events/sysfs


 Nevertheless for L4901 the BOM says:
 >> INDUCTOR 2.2uH 10% 7.96MHZ 800mA SMT0805 PLL201212F2R2K SUBARU FOR GSM
 LR
 it seems this maybe doesn't exactly match the considerations for inductor
 component selection as found in "Selecting the Boost Inductor"
 (AAT1275.pdf, p.11 + p.15) regarding Ipp and saturation.
 Alas I wasn't able to find any datasheet for this inductor :-/
 So the jury is still out on that one. If the inductor gets saturated on
 high loads, this might easily result in massive increase of input current
 and heating up of the inductor itself.


 A final note on current_now: this is the total of current taken from the
 battery cell, including CPU, backlight, and GSM modem. The function of
 AAT1275 is to take a low voltage/high current on input, and convert this
 to a higher voltage at lower current at output. So even the consumption of
 AAT1275 is far beyond the 500mA it can provide for output ->  Iin = Iout *
 Vout / Vin / Const(efficiency) [0.5 * 5 / 3.6 / 0.8(guess) = 0.87A]

-- 
Ticket URL: <https://docs.openmoko.org/trac/ticket/2310#comment:6>
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