HCPL788J Datasheet, PDF (20/20 Pages) Agilent(Hewlett-Packard) – Isolation Amplifier with Short Circuit and Overload Detection

5. Power Supplies and Start-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?
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.
6. Miscellaneous
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?
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. HP 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.
For technical assistance or the location of
your nearest Hewlett-Packard sales office,
distributor or representative call:
Americas/Canada: 1-800-235-0312 or
408-654-8675
Far East/Australasia: Call your local HP
sales office.
Japan: (81 3) 3335-8152
Europe: Call your local HP sales office.
Data subject to change.
Copyright © 1997 Hewlett-Packard Co.
Printed in U.S.A. 5966-0001E (7/97)