Philips Universal Serial Bus ISP1122 - Manual (Page 23)

Philips Semiconductors
ISP1122
USB stand-alone hub
Product specification Rev. 03 — 29 March 2000 23 of 48
9397 750 07002 © Philips Electronics N.V. 2000. All rights reserved.
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 .
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
±
3%, as shown in Figure 7 .
11.1.2 Bus-powered hubs
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.
(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 V
POWER SUPPLY
± 3% regulated
−
+
4.85 V(min)
MGR781
low-ohmic
PMOS switch
ISP1122
power
switch
VBUS
D
+
D −
GND
SHIELD
4.75 V(min)
downstream
port
connector
hub board
resistance
voltage drop
25 mV
voltage drop
75 mV
(1)
(1) Includes PCB traces, ferrite beads, etc.
Fig 7. Typical voltage drop components in self-powered mode using global overcurrent detection.
handbook, full pagewidth
5.1 V KICK-UP
POWER SUPPLY
± 3% regulated
−
+
4.95 V(min)
MGR782
low-ohmic
PMOS switch
ISP1122
power
switch
VBUS
D
+
D −
GND
SHIELD
4.75 V(min)
downstream
port
connector
hub board
resistance
voltage drop
25 mV
voltage drop
75 mV
low-ohmic
sense resistor
for overcurrent
detection
voltage drop
100 mV
(1)