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Philips Semiconductors
12.3 Tuning the overcurrent trip voltage
handbook, halfpage
V CC
R tu
V CC
SP/BP
ISP1122
I
= 5 A
ref(nom)
I
= 0.5 A
OC(nom)
a.
Self-powered mode.
Fig 9. Tuning the overcurrent trip voltage.
12.4 Reference circuits
1.
The following PMOS power switches have been tested to work well with the ISP1122: Philips PHP109, Vishay Siliconix Si2301DS,
Fairchild FDN338P.
9397 750 07002
Product specification
The ISP1122 overcurrent detection circuit has been designed with a nominal trip
voltage ( V
) of 85 mV. This gives a typical trip current of approximately 850 mA for
trip
a power switch with an on-resistance of 100 m
The ISP1122 trip voltage can optionally be adjusted through external components to
set the desired trip current. This is done by inserting tuning resistors at pins SP/BP or
OCn (see
Figure
9). R
according to
Equation
V
=
V
trip
trip intrinsic
with I
= 5 A and I
ref(nom)
low-ohmic
PMOS switch
I ref
I OC
R td
OCn
MBL042
Some typical examples of port power switching and overcurrent detection modes are
given in
Figure 10
to
The RC circuit (47 k and 0.1 F) around the PMOS switch provides for soft turn-on.
The series resistor connecting the SP/BP pin to V
voltage slightly (see
net names for pins V
trace for V
and a narrow trace to connect pin SP/BP.
CC
Rev. 03 — 29 March 2000
tunes up the trip voltage V
tu
1.
+
I
R
–
I
R
ref
tu
OC
td
= 0.5 A.
OC(nom)
handbook, halfpage
V BUS
V CC
I
= 0.5 A
OC(nom)
b.
Bus-powered mode.
Figure
13.
Figure
9). In the schematic diagram the resistor separates the
and SP/BP. This allows an automatic router to use a wide
CC
ISP1122
USB stand-alone hub
1
.
and R
tunes it down
trip
td
low-ohmic
PMOS switch
I OC
R td
SP/BP
OCn
ISP1122
MBL043
tunes up the overcurrent trip
CC
© Philips Electronics N.V. 2000. All rights reserved.
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