Application Note 17
Micrel
Application Note 17
Universal Serial Bus Power Management
by Kevin Lynn
Introduction
Power management and distribution is a major factor in
correctly designing USB (Universal Serial Bus) peripherals.
Proper methods of designing USB peripheral power distribu-
tion are crucial to ensure full compliance with the USB
specification, including compliance with electromagnetic in-
terference (EMI) and voltage regulation requirements.
Functions
Functions are endpoint low- or high-power devices and do not
repeat data or provide downstream power.
High-power functions
may initially draw 100mA at 4.4V
from an upstream self-powered port, increasing after enu-
meration up to 500mA. If, during enumeration, there is
insufficient power available from upstream, the remainder of
the function is not powered and a power limit warning mes-
sage is sent. Self-powered functions may draw 100mA from
upstream with the remainder from a local power supply.
Low-power functions
may draw up to 100mA from an
upstream cable.
Device Classes
USB defines several device classes, differentiated by their
power requirements or capabilities. The most important de-
vice classes are
hubs
and
functions.
Hubs
Hubs provide for distribution of data and power to down-
stream (away from the host) devices and communicate with
the host. Hubs may be either locally powered (self-powered),
bus-powered from an upstream cable (toward the host), or a
combination of the two.
Self-powered hubs
have a local power supply. Examples
are PCs (host), stand-alone hubs, monitors, printers, scan-
ners, or docking stations, which may draw up to 100mA (one
unit load) from an upstream port. Self-powered hubs are
required to limit and report overcurrent conditions and may
supply up to 500mA to each downstream port.
Bus-powered hubs
obtain all power from the bus (a cable to
an upstream self-powered hub) and may supply 100mA or
more to each downstream port if the input power budget is not
exceeded. Bus-powered hubs may draw 100mA at start-up
from an upstream hub, increasing up to 500mA after enu-
meration, apportioning the power in increments of 100mA per
downstream port plus 100mA internally. Bus-powered hubs
must provide power switching for downstream ports on a per
port basis or have a single switch for all of the ports (gang-
mode power control).
USB Power Distribution
Connection between devices are made using cables which
are either detachable (such as a USB modem) or perma-
nently attached (such as a keyboard). High speed (>1.5Mb/
s) peripherals require shielded cable, while low-speed pe-
ripherals may operate with unshielded cable to minimize
cost. The upstream port of a hub is, by definition, a high-
speed port and requires a shielded cable. Figure 1 shows the
voltage regulation requirements of a USB system based on
current USB requirements.
Self-Powered Hub Requirements
USB self-powered hubs are required to provide a minimum of
4.75V at downstream ports under all legal load conditions.
Self-powered hubs may draw up to 100mA operating current
from an upstream hub.
To comply with UL defined safety requirements, current from
any port must be limited to less than 25VA (5A for 5V ports).
If such an overcurrent condition occurs, even if it is only
momentary, it must be reported to the hub controller. Detec-
tion of overcurrent must disable all affected ports. If the
4.4V min.
at 100mA
4.75V min.
at 500mA
USB Self-
Powered Hub
Power
Supply
4.85V to 5.25V
Hub
USB
Bus-Powered
Hub
Low-
Power
Device
High-
Power
Device
Figure 1. Typical USB Voltage Distribution System
Micrel, Inc. 鈥?1849 Fortune Drive 鈥?San Jose, CA 95131 鈥?USA 鈥?tel + 1 (408) 944-0800 鈥?fax + 1 (408) 944-0970 鈥?http://www.micrel.com
October 1998
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