Basically I want to offload the current load from a PC's USB-port and instead feed it via the TPS2540 whilst keeping an active USB-data-connection.
I believe that the way to do that is by using the TPS2540 to convert the PC's SDP to a CDP with an external power supply.
The reason is that I have an iPad in an information booth. The iPad should be powered on continuously with the display on, while having an active USB-port for data transmission.
This is my setup:
(PC with standard USB 500mA 2.0 USB Port) <-- D+ / D- / GND --> (TPS2540 with external power supply +5V and >4000mA) <-- VBUS / D+ / D- / GND --> (iPad)
TPS2540 configuration is
IN: +5V from power supply
DM_OUT: D- to PC USB port
DP_OUT: D+ to PC USB port
ILIM_SEL: GND to select ILIM0
EN/DSC: +5V
CTL1: +5V
CTL2: +5V
CTL3: +5V
NC: not connected
DP_IN: D+ to iPad
DM_IN: D- to iPad
OUT: VBUS to iPad
FAULT: via 10k ohm to +5v
GND: gnd
ILIM1: not connected
ILIM0: 20k ohm to gnd
What happens is that the iPad only uses ~500mA (measured) which is the limit for the PC's USB port, and not the 1500mA that is the limit for a CDP, that the TPS2540 can provide. It seems like the iPad gets the max current setting from the PC's USB port instead of the TPS2540... The Dedicated charging mode works fine and I get 1800mA (measured) current draw, but then I can't use the data lines... When I look in the Control Truth Table on page 25 in the documentation, I find that (1, 1, 1) (CTL1, CTL2, CTL3) should be the right setting.
If I replace the PC with a MAC the max current increases to ~1000mA.
If I connect a USB-hub between the MAC and the TPS2540 the max current is ~500mA
Does anyone know if this is the expected behavior and/or how to enable the functionality that i'm describing?
