Datasheet

22
5. Power Supplies and Start-Up
V
OUT
(pin 12) is close to zero volts, ABSVAL (pin 13)
is close to V
REF
and FAULT (pin 14) is in the high
(inactive) state when power to the input side is off.
In fact, a self test can be performed using this infor-
mation. In a motor drive, it is possible to turn off
all the power transistors and thus cause all the
sense resistor voltages to be zero. In this case,
finding V
OUT
less than
1
/
4
of V
REF
, ABSVAL more
than
3
/
4
of V
REF
and FAULT in the high state
indicates that power to the input side is not on.
About 50 µs after a V
DD2
power-up and 100 µs after
a V
DD1
power-up.
5.1: What are the output voltages before the
input side power supply is turned on?
5.2: How long does the HCPL-788J take to
begin working properly after power-up?
6. Miscellaneous
The inputs have a series resistor for protection
against large negative inputs. Normal signals are
no more than 200 mV in amplitude. Such signals
do not forward bias any junctions sufficiently to
interfere with accurate operation of the switched
capacitor input circuit.
Typically, noticeable ringing and overshoot begins
for C
LOAD
above 0.02 µF. Avago recommends
keeping the load capacitance under 5000 pF
(at pin 12). ABSVAL (pin 13) typically exhibits no
instability at any load capacitance, but speed of
response gradually slows above 470 pF load.
Yes, with a compromise on offset accuracy. One way
to do this is by connecting +2.5 V to pins 10, 11, and
15 and connecting -2.5 V to pins 9 and 16 with
0.1 µF bypass capacitors from +2.5 V to -2.5 V and
from -2.5 V to ground. Note that FAULT cannot
swing above 2.5 V in this case, so a level shifter may
be needed. Alternately, a single 5 V supply could be
power the HCPL-788J which could drive an op amp
configured to subtract
1
/
2
of V
REF
from V
OUT
.
6.1: How does the HCPL-788J measure
negative signals with only a +5 V supply?
6.2: What load capacitance can the
HCPL-788J drive?
6.3: Can I use the HCPL-788J with a bipolar
input A/D converter?
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Data subject to change. Copyright © 2007 Avago Technologies Limited. All rights reserved. Obsoletes 5989-2156EN
AV01-0570EN July 7, 2007