ISP1130DL Philips Semiconductors, ISP1130DL Datasheet - Page 53

• Manufacturer Part Number: ISP1130DL
• Description: Universal Serial Bus compound hub with integrated keyboard controller
• Manufacturer: Philips Semiconductors

13. Overcurrent detection
1.
Philips Semiconductors
9397 750 06895
Objective specification
The following PMOS power switches have been tested to work well with the ISP1130: Philips PHP109, Vishay Siliconix Si2301DS,
Fairchild FDN338P.
13.1 Overcurrent circuit description
13.2 Power switch selection
13.3 Tuning the overcurrent trip voltage
The ISP1130 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/DPGL1 and OC2/DPGL2 can be used for individual port overcurrent
detection or GoodLink indication. The pin functionality is selected via bit
EnableOverCurrent in the USBCON register, see
The integrated overcurrent detection circuit of ISP1130 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
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.
The ISP1130 overcurrent detection circuit has been designed with a nominal trip
voltage ( V
a power switch with an on-resistance of 100 m
The ISP1130 trip voltage can optionally be adjusted through external components to
set the desired trip current. This is done by inserting tuning series resistors at pins
OCn/DPGLn (see
Equation
with I
V
trip
OC(nom)
=
1.
V
trip
trip intrinsic
= 0.5 A.
trip
) of 85 mV. This gives a typical trip current of approximately 850 mA for
). The overcurrent circuit detects this and switches off the power
Figure
Rev. 01 — 23 March 2000
–
10). R
I
OC
R
td
td
tunes down the trip voltage V
USB compound hub with keyboard controller
Figure
trip
9, the power switch has a voltage drop
). This delay acts as a ‘debounce’
1
.
Table
53.
© Philips Electronics N.V. 2000. All rights reserved.
trip
according to
ISP1130
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