ISP1122ABD datasheet(24/48 Pages) PHILIPS

24 / 48 page

Philips Semiconductors

ISP1122A

USB stand-alone hub

Preliminary speciﬁcation

Rev. 01 — 27 March 2000

24 of 48

9397 750 06986

The PCB resistance may cause a drop of 25 mV, which leaves 75 mV for the power

switch and overcurrent sense device. The voltage drop components are shown in

Figure 8.

For bus-powered hubs overcurrent protection is optional. It may be implemented for

all downstream ports on a global or individual basis.

12. Overcurrent detection

The ISP1122A has an analog overcurrent detection circuit for monitoring downstream

port lines. This circuit automatically reports an overcurrent condition to the host and

turns off the power to the faulty port. The host must reset the condition ﬂag.

Pins OC1 to OC5/GOC are used for individual port overcurrent detection. Pin

OC5/GOC can also be used for global overcurrent detection. This is controlled by

input INDV (see Table 4).

The overcurrent detection circuit can be switched off using an external EEPROM (see

Table 23). In this case, the overcurrent pins OCn function as logic inputs (TTL level).

12.1 Overcurrent circuit description

The integrated overcurrent detection circuit of ISP1122A senses the voltage drop

across the power switch or an extra low-ohmic sense resistor. When the port draws

too much current, the voltage drop across the power switch exceeds the trip voltage

threshold (

∆V

trip). The overcurrent circuit detects this and switches off the power

switch control signal after a delay of 15 ms (ttrip). This delay acts as a ‘debounce’

period to minimize false tripping, especially during the inrush current produced by ‘hot

plugging’ of a USB device.

12.2 Power switch selection

From the voltage drop analysis given in Figure 6, Figure 7 and Figure 8, the power

switch has a voltage drop budget of 75 mV. For individual self-powered mode, the

current drawn per port can be up to 500 mA. Thus the power switch should have

maximum on-resistance of 150 m

Ω.

If the voltage drop due to the hub board resistance can be minimized, the power

switch can have more voltage drop budget and therefore a higher on-resistance.

Power switches with a typical on-resistance of around 100 m

Ω ﬁt into this application.

(1) Includes PCB traces, ferrite beads, etc.

Fig 8.

Typical voltage drop components in bus-powered mode (no overcurrent detection).

handbook, full pagewidth

MGR783

low-ohmic

PMOS switch

ISP1122

power

switch

VBUS

−

GND

SHIELD

VBUS

−

GND

SHIELD

4.40 V(min)

4.50 V(min)

downstream

port

connector

upstream

port

connector

hub board

resistance

voltage drop

25 mV

voltage drop

75 mV

(1)

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