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Philips Semiconductors
V BUS
D+
upstream
port
D−
connector
GND
SHIELD
(1) Includes PCB traces, ferrite beads, etc.
Fig 8. Typical voltage drop components in bus-powered mode (no overcurrent detection).
12. Overcurrent detection
12.1 Overcurrent circuit description
12.2 Power switch selection
9397 750 08958
Product data
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.
voltage drop
75 mV
4.50 V(min)
ISP1122A
power
switch
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
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).
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
). The overcurrent circuit detects this and switches off the power
trip
switch control signal after a delay of 15 ms (t
period to minimize false tripping, especially during the inrush current produced by 'hot
plugging' of a USB device.
From the voltage drop analysis given in
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 mΩ.
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 mΩ fit into this application.
Rev. 03 — 30 October 2001
voltage drop
25 mV
hub board
(1)
low-ohmic
resistance
PMOS switch
4).
trip
Figure
ISP1122A
USB stand-alone hub
4.40 V(min)
V BUS
D+
downstream
D−
GND
SHIELD
04aa06
). This delay acts as a 'debounce'
6,
Figure 7
and
Figure
8, the power
© Koninklijke Philips Electronics N.V. 2001. All rights reserved.
port
connector
24 of 48
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