Power [1, G, H, J, M & Q] - XMOS XS1-L2 Hardware Manual

USB Audio 2.0 Reference Design, XS1-L2 Edition Hardware Manual (1.6) 17/32
14 Power [1, G, H, J, M & Q]
The board is powered from the onboard +5V DC power connector or from the USB
bus. Note that the board has not been designed as a bus-powered device due to
pre-enumeration current limits.
The two different power supplies are "ORd" together using Schottky diodes with a
maximum voltage drop of 0.34V. A 750mA resettable polyfuse and reverse polarity
protection, via a bidirectional zener diode, is provided. Ferrite beads are used on the
+5V VBus and +5V DC power input to prevent switching noise propagating down the
USB and power cables.
A latching push-button power switch is fitted to the board that activates a p-channel
MOSFET (which has an Rds(on) of approximately 80m ). A soft start circuit is
included to limit the inrush current.
When powered from the USB bus all the power used by the board is derived from the
nominally +5V VBus supply from the USB connector. The board will use approximately
300mA when fully configured and operating.
The required core and IO voltages for the XS1-L2 are derived from 5V as follows:
A low cost 1.5A buck switching regulator is used to generate the 1.0V core
supply for the XS1-L2.
A low cost 600mA buck switching regulator is used to generate the global 3.3V
supply.
Switching regulators are used on these power supplies due to their high efficiency.
The power supplies are sequenced using a 3.0V voltage supervisor on the 3.3V
supply output, to drive the enable input on the 1.0V supply. This makes sure that
the 3.3V supply is up and stable, before the 1.0V supply comes up and also provides
predictable behaviour under brownout conditions, by causing a reset if the supplies
droop significantly.
A simple low drop out (LDO) linear regulator is used to generate the 1.8V supply
required by the USB3318 USB transceiver.
A low noise LDO regulator is used to generate the analogue supply for the Audio
CODEC. The CODEC offers higher audio performance at higher supply voltages so the
voltage for this supply is set at 4.1V. This allows some headroom between the 4.5V
minimum VBus voltage and the approx 350mV dropout of the LDO + RC pre-filter.
When the board is correctly connected to a USB source the USB VBUS Power LED is
illuminated.
When the board is powered on the Power LED is illuminated.
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