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| 型號： | APPNOTE17 |
| 英文描述： | Universal Serial Bus Power Management |
| 中文描述： | 通用串行總線電源管理 |
| 文件頁數(shù)： | 2/12頁 |
| 文件大小： | 125K |
| 代理商： | APPNOTE17 |
Application Note 17
Micrel
Application Note 17
2
October 1998
overcurrent condition has caused a permanent disconnect
(such as a blown fuse), the hub must report it upon completing
its reset or power-up.
Overcurrent protection may be implemented on all down-
stream ports in aggregate (ganged), or on an individual (per
port) basis. Overcurrent limiting devices may include polyfuses,
standard fuses, or a solid-state switch. A practical limit is
seven downstream ports per protection device.
Voltage Regulation Requirements
The dc output voltage, measured at the board side of the self-
powered hub downstream connector, must remain below
5.25V and above 4.75V under all legal continuous load
conditions.
The internal power supply tolerance dictates the allowable
component voltage drops within a self-powered hub. Table 1
depicts the range of resistance of the overcurrent protection
device, assuming a 30mV (or 60m
) drop across the PCB
components. For example, Table 1 shows that using a 5V,
3% power supply allows a maximum resistance of 140m
for
the overcurrent protection device. A 5.1V, 3% supply allows
for a 340m
device. If it is expected that the PCB component
voltage drops will be other than 30mV, then these values can
be scaled appropriately.
The components that make up the voltage drop include the
circuit trace, overcurrent protection device, and output filter
ferrite beads. The printed circuit board power and ground
traces, solder connections, and ferrite beads on both power
and ground output lines may total 60m
, dropping a total of
30mV at 0.5A. Additional traces, ferrite beads, and protection
devices are needed for each output port. Figure 2 represents
the component voltage drops of a typical self-powered hub.
Transient Droop Requirements
USB supports dynamic attachment (hot plug-in) of peripher-
als. A current surge is caused by the input capacitance of the
downstream device. Ferrite beads are recommended in
series with all power and ground connector pins. Ferrite
beads reduce EMI and limit the inrush current during hot-
attachment by attenuating high-frequency signals while dc
current passes freely. The simplest ferrite beads consist of a
small ferrite tube on a tinned solid copper wire. The resis-
tance of the ferrite bead wire should be as low as possible,
with a large solder-pad to minimize connection resistance.
USB requires that momentary droop of the 4.75V at a bus-
powered hub’s upstream cable connector never goes below
4.42V.
70mV
V
BUS
GND
Power Supply
4.85V to 5.25V
3.5A max.
33μF
0.1μF
0.01
5mV
5mV
(trace resistance)
0.01
500mA max.
0.14
33μF*
15mV
0.03
15mV
0.03
10mV
0.02
Ferrite Bead
(solder joint resistance)
0.02
10mV
0.01μF
USB Self-Powered Hub
1/2
MIC2526
Switch
(+)
(–)
Cable
Connector
Downstream
Peripheral Port
4.75V min.
500mA max.
* Tantalum or 100μF electrolytic, per port
Figure 2. Self-Powered Hub Per-Port Voltage Drop
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Table 1. Maximum Allowed On-Resistance
with 30mV PCB Voltage Drop*
* Shading represents USB-compliant conditions.
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