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參數(shù)資料
| 型號(hào)： | APPNOTE17 |
| 英文描述： | Universal Serial Bus Power Management |
| 中文描述： | 通用串行總線電源管理 |
| 文件頁(yè)數(shù)： | 6/12頁(yè) |
| 文件大?。?/td> | 125K |
| 代理商： | APPNOTE17 |
Application Note 17
Micrel
Application Note 17
6
October 1998
Ganged Overcurrent Limiting
Figure 7 shows a ganged-output, self-powered hub using a
single MIC2505 2A switch to protect up to four outputs.
Output filters with two ferrite beads per port satisfy USB
requirements by preventing hot-attach transients at one port
from causing transient voltage droop of more than 330mV on
adjacent ports.
Bus-Powered Hub Requirements
USB specifies that bus-powered hubs have power switching
for their downstream ports, so that downstream current may
be enumerated and controlled. Port power is switched under
host software control on an individual port basis or by a single
switch for all ports (gang-mode power control). Port power is
not switched off during a reset, so that attached devices may
be monitored. The upstream cable connector must receive a
minimum of 4.75V, while each downstream port provides a
minimum of 4.40V at 100mA. Bus-powered hubs obtain all
power from an upstream self-powered hub. They may draw
up to 100mA upon power up. After configuration of internal
functions and downstream ports, a maximum of 400mA is
allocated up to a maximum of four downstream ports. Each
port must be capable of supplying a minimum of 100mA.
Transient Regulation
Bus-powered hubs have similar requirements to that of self-
powered hubs. USB requires that voltage transients at a 4.4V
peripheral port must never go below 4.07V. Layout, ferrite
beads, and both capacitor value and type affect transient
regulation.
Inrush Current
When power switches are energized, output capacitors may
draw high inrush current causing an upstream voltage droop
which could affect other circuits. USB requires that hot-attach
or switched events present one unit load maximum (44
min.
with 10
μ
F max.). High inrush current may also cause EMI or
damage tantalum capacitors. Micrel’s power switch products
eliminate these problems by providing soft start which limits
inrush current. Micrel’s MIC2526 high-side switches have a
charge pump which slows turn-on time between 1 and 2ms.
Suspend Power
When a hub is suspended, downstream ports are still pow-
ered. When downstream devices are suspended, the maxi-
mum allowed average current per port is 500
μ
A including the
data line pull-up resistors and power switch operating cur-
rent.
Voltage-Drop Regulation
A 350mV maximum total drop is allowed from an upstream
self-powered hub connector to bus-powered hub down-
stream socket connector, as shown in Figure 4. This drop
includes the cable, connectors, switch, and board. An at-
tachedcable may have 20m
per IDC header, or soldered
connection, instead of 30m
per USB cable connector con-
tact, providing a larger voltage margin for the hub compo-
nents. Standard length cables have a wire size chosen to
produce a voltage drop of less than 190mV for two conduc-
tors at 500mA current. Shorter cables, using the same gauge
wire, produce a smaller voltage drop. Figure 4 shows the
component voltage drops for a typical bus-powered hub.
ON/OFF
OVERCURRENT
MIC2505
3.3V USB Controller
V
BUS
D+
D–
GND
V
BUS
D+
D–
GND
10k
MIC5203-3.3
MIC5207-3.3
LDO Regulator
IN
OUT
GND
33μF*
Ferrite
Bead
V
BUS
D+
D–
GND
D+
D–
V+
EN
FLG
GND
GATE
OUT
IN
OUT
IN
Bold
lines indicate
0.1" wide, 1-oz. copper
high-current traces.
Downstream
USB
Port 1
4.75V min.
500mA max.
33μF*
V
BUS
D+
D–
GND
V
BUS
D+
D–
GND
33μF*
33μF*
0.01μF
0.01μF
0.01μF
0.01μF
4.7
μF
1.0
μF
GND
0.1μF
4.45V to 5.25V
3.5A max.
(–)
(+)
33μF
Downstream
USB
Port 2
4.75V min.
500mA max.
Downstream
USB
Port 2
4.75V min.
500mA max.
Downstream
USB
Port 3
4.75V min.
500mA max.
0.1μF
10k
* 33μF, 16V tantalum or 100μF, 10V electrolytic per port
Figure 7. Ganged-Switch Self-Powered Hub
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