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ISP1520 Datasheet PDF : 51 Pages
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Philips Semiconductors
ISP1520
Hi-Speed USB hub controller
9.1.2 Power switching
Power switching of downstream ports can be done individually or ganged, where all
ports are simultaneously switched with one power switch. The ISP1520 supports both
modes, which can be selected using input PSWn_N; see Table 5.
Voltage drop requirements: Self-powered hubs are required to provide a minimum
of 4.75 V to its output port connectors at all legal load conditions. To comply with
Underwriters Laboratory Inc. (UL) safety requirements, the power from any port must
be limited to 25 W (5 A at 5 V). Overcurrent protection may be implemented on a
global or individual basis.
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 the:
• Power supply connector
• Hub PCB (power and ground traces, ferrite beads)
• Power switch (FET on-resistance)
• Overcurrent sense device.
The 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 5.
For 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 6.
5V
+
POWER SUPPLY
± 3 % regulated −
4.85 V (min)
voltage drop
75 mV
ISP1520
power switch
(PSWn_N)
low-ohmic
PMOS switch
voltage drop
25 mV
4.75 V (min)
hub board
resistance
(1)
VBUS
D+
D−
GND
SHIELD
downstream
port
connector
004aaa261
(1) Includes PCB traces, ferrite beads, and so on.
Fig 5. Typical voltage drop components in the self-powered mode using individual overcurrent detection.
5.1 V KICK-UP +
POWER SUPPLY
± 3 % regulated −
voltage drop
4.95 V (min) 100 mV
low-ohmic
sense resistor
for overcurrent
detection
voltage drop
75 mV
ISP1520
power switch
(PSWn_N)
low-ohmic
PMOS switch
voltage drop
25 mV 4.75 V (min)
hub board (1)
resistance
VBUS
D+
D−
GND
SHIELD
004aaa262
downstream
port
connector
(1) Includes PCB traces, ferrite beads, and so on.
Fig 6. Typical voltage drop components in the self-powered mode using global overcurrent detection.
9397 750 11689
Product data
Rev. 02 — 04 May 2004
© Koninklijke Philips Electronics N.V. 2004. All rights reserved.
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