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Philips Semiconductors
handbook, full pagewidth
5 V
POWER SUPPLY
3% regulated
(1) Includes PCB traces, ferrite beads, etc.
Fig 6. Typical voltage drop components in self-powered mode using individual overcurrent detection.
handbook, full pagewidth
5.1 V KICK-UP
POWER SUPPLY
3% regulated
(1) Includes PCB traces, ferrite beads, etc.
Fig 7. Typical voltage drop components in self-powered mode using global overcurrent detection.
11.1.2 Bus-powered hubs
9397 750 07002
Product specification
Assuming a 5 V
3% power supply the worst case supply voltage is 4.85 V. This only
allows a voltage drop of 100 mV across the hub printed-circuit board (PCB) to each
downstream connector. This includes a voltage drop across:
•
Power supply connector
•
Hub PCB (power and ground traces, ferrite beads)
•
Power switch (FET on-resistance)
•
Overcurrent sense device.
PCB resistance and power supply connector resistance may cause a drop of 25 mV,
leaving only 75 mV as the voltage drop allowed across the power switch and
overcurrent sense device. The individual voltage drop components are shown in
Figure
6.
voltage drop
75 mV
4.85 V(min)
PMOS switch
ISP1122
power
switch
In case of global overcurrent detection an increased voltage drop is needed for the
overcurrent sense device (in this case a low-ohmic resistor). This can be realized by
using a special power supply of 5.1 V
voltage drop
100 mV
4.95 V(min)
low-ohmic
sense resistor
for overcurrent
detection
ISP1122
power
switch
Bus-powered hubs are guaranteed to receive a supply voltage of 4.5 V at the
upstream port connector and must provide a minimum of 4.4 V to the downstream
port connectors. The voltage drop of 100 mV across bus-powered hubs includes:
•
Hub PCB (power and ground traces, ferrite beads)
•
Power switch (FET on-resistance)
•
Overcurrent sense device.
Rev. 03 — 29 March 2000
voltage drop
25 mV
4.75 V(min)
hub board
(1)
low-ohmic
resistance
3%, as shown in
voltage drop
voltage drop
75 mV
25 mV
hub board
low-ohmic
resistance
PMOS switch
ISP1122
USB stand-alone hub
V BUS
D
downstream
port
D
connector
GND
SHIELD
MGR781
Figure
7.
4.75 V(min)
V BUS
D
(1)
downstream
D
port
connector
GND
SHIELD
MGR782
© Philips Electronics N.V. 2000. All rights reserved.
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