ISP1122ABD datasheet(23/48 Pages) PHILIPS

23 / 48 page

Philips Semiconductors

ISP1122A

USB stand-alone hub

Preliminary speciﬁcation

Rev. 01 — 27 March 2000

23 of 48

9397 750 06986

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

−

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

−

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)

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