isp1122 NXP Semiconductors, isp1122 Datasheet - Page 24

• Manufacturer Part Number: isp1122
• Description: Isp1122 Universal Serial Bus Stand-alone Hub
• Manufacturer: NXP Semiconductors

12. Overcurrent detection
Philips Semiconductors
9397 750 07002
Product specification
Fig 8. Typical voltage drop components in bus-powered mode (no overcurrent detection).
handbook, full pagewidth
(1) Includes PCB traces, ferrite beads, etc.
connector
upstream
port
12.1 Overcurrent circuit description
12.2 Power switch selection
SHIELD
V BUS
GND
D
D
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
For bus-powered hubs overcurrent protection is optional. It may be implemented for
all downstream ports on a global or individual basis.
The ISP1122 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
The overcurrent detection circuit can be switched off using an external EEPROM (see
Table
The integrated overcurrent detection circuit of ISP1122 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
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 m .
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 fit into this application.
4.50 V(min)
23). In this case, the overcurrent pins OCn function as logic inputs (TTL level).
8.
trip
). The overcurrent circuit detects this and switches off the power
ISP1122
Table
power
switch
Rev. 03 — 29 March 2000
voltage drop
75 mV
4).
PMOS switch
low-ohmic
voltage drop
hub board
resistance
25 mV
Figure
trip
(1)
6,
). This delay acts as a ‘debounce’
Figure 7
4.40 V(min)
and
USB stand-alone hub
Figure
V BUS
D
D
GND
SHIELD
© Philips Electronics N.V. 2000. All rights reserved.
MGR783
ISP1122
8, the power
downstream
connector
port
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