i've got an enquiry out with Linear, however it looks like there's a
serious design flaw with the LTC4155 - not one that's safety-related
but one that makes it functionally completely useless.
the clue is in looking at page 15 of the datasheet - a diagram which
shows the internal operation. in the bottom left corner is a
power-selection arrangement, where there are two AND gates. note that
they're not symmetrically arranged. the difference between the two is
that WALLSNS output is connected to the *input* (inverted) of the
USBGT enabler AND gate.
what that means is that it is ABSOLUTELY CAST-IRON GUARANTEED - i.e.
hard-wired in an inviolate way - that if the 5VDC input is plugged
into a wall charger, USB OTG Host Power is ABSOLUTELY 100% WITHOUT
FAIL ABSOLUTE guaranteed to be terminated without warning.
which:
(a) violates the USB-OTG Power Specification
(b) is in direct contravention of the statement on page 1 of the
LTC4155 datasheet which *clearly* states that the LTC4155 is compliant
with USB-OTG Host Power Mode
(c) is in clear violation of various laws like "Trade Descriptions
Act" of the UK due to the qualifier statement in paragraph 2 which
reads "....without needing additional components".
all of which is a frickin nuisance, as i now have to go find
alternative dual-supply OTG-compliant [boost-mode] battery charger
ICs. i've found two already - bq24192i ($7 in low volume and it would
require a 2A 5V step-up converter IC as well....], or the MAX77818
which is a 0.4mm pitch BGA - perfect because it even has battery
status monitoring built-in, but the 0.4mm pitch BGA _wow_ that's gonna
be tough for small china pcb assembly factories to deal with when
they're used to QFNs and QFPs.
argh!
i'll let people know how this goes: i'm expecting to hear from Linear,
they *might* confirm that the I2C "REQUEST_OTG" mode does in fact
honour the USB OTG Host Power mode properly. if they do, then all is
well - otherwise i will need to do a redesign of PCB3.
l.
