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| 型號： | APPNOTE36 |
| 英文描述： | Electroluminescent Display Drivers |
| 中文描述： | 電致發(fā)光顯示驅(qū)動器 |
| 文件頁數(shù)： | 7/12頁 |
| 文件大?。?/td> | 125K |
| 代理商： | APPNOTE36 |
October 1998
7
Application Note 17
Application Note 17
Micrel
USB Cables
USB specifies the construction and wire gauge of cables
used to connect host to hub to peripherals. USB also speci-
fies a maximum cable power wire resistance of 190m
and
a maximum cable length of 5m (3m for a low-speed attached
subchannel cable) to minimize voltage drop and transmission
time.
Connector Resistance
The standard mated USB connector is assumed to have
30m
maximum contact resistance per pin after 1500 inser-
tions, including the solder connections to the PC board and
cable wires. Since the voltage cannot be measured at the
contact point, USB voltages are usually measured at the
circuit board side of a connector, between the power pin and
ground pin (not power supply ground).
Detachable Cables
Detachable cables have different style connectors on each
end to prevent incorrect connections (for example: the down-
stream port of one hub to the downstream port of a second
hub).
A standard USB cable has a Series A connector on the
upstream end and a Series B connector on the downstream
end, with 28AWG data wires and 90
±
15% impedance.
Cables are not allowed to have the same series (A or B)
connector on both ends to ensure proper data and power
flow.
The maximum voltage drop allowed across a bus-powered
hub and its cable is 350mV. A bus-powered hub may draw
500mA, producing a 250mV drop in a standard 190m
maximum cable and its connectors, leaving 100mV margin
for a bus-powered hub circuit board, switch, and connectors.
ON/OFF
OVERCURRENT
MIC2525
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
Upstream
V
BUS
500mA max.
V
BUS
D+
D–
GND
D+
D–
V+
EN
FLG
GND
NC
OUT
IN
OUT
NC
Bold
lines indicate
0.1" wide, 1-oz. copper
high-current traces.
4.45V
Downstream
USB
Port 1
4.4V min.
100mA max.
Downstream
USB
Port 1
4.4V min.
100mA max.
33μF*
V
BUS
D+
D–
GND
V
BUS
D+
D–
GND
33μF
Downstream
USB
Port 1
4.4V min.
100mA max.
Downstream
USB
Port 1
4.4V min.
100mA max.
33μF*
0.01μF
0.01μF
0.01μF
0.01μF
4.7
μF
1.0
μF
GND
0.1μF
10k
0.1μF
* 33μF, 16V tantalum or 100μF, 10V electrolytic capacitor per port.
Figure 8. Ganged-Output Bus-Powered Hub
Specifying an attached bus-powered hub cable simplifies the
voltage drop equation by reducing cable length and the
downstream connector resistance. A keyboard with a 1m
cable would be a typical example of an attached cable bus-
powered hub. Since the keyboard would plug into the monitor
or self-powered hub near the user, it does not need a longer
cable.
Lower Cable Resistance
Table 6.2 in the USB specification shows resistance per
meter for various AWG wire sizes. A keyboard-based bus-
powered hub, with a short 2m, 22AWG attached cable, as
opposed to the standard 3.1m, 22AWG detachable cable,
has a cable resistance of 114m
per conductor.
Four-Port Bus-Powered Hub Voltage Drop
See Figure 4 for the various voltage drops for a bus-powered
hub. There are two components of voltage drop:
High Current in Upstream Cable
Use the 500mA maximum to compute the voltage drops in the
upstream cable connector, cable wires, and downstream
cable connector. Good layout should produce less than
35mV drop in the 500mA section of the hub printed circuit
board. A ganged-output (MIC2526) has a typical on-resis-
tance of 145m
, including lead solder joints, for a 400mA
voltage drop of 58mV. Subtracting the above voltage drop
(93mV) from the 100mV maximum, leaves 7mV for each
100mV output filter and connector.
V
CON
= 0.5A
×
30m
×
4 pins = 60mV
V
WIRE
= 0.5A
×
R
WIRE
×
2
×
wire length
V
0.5
= V
CON
+ V
WIRE
= 50mV + V
WIRE
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