HCPL-788J Datasheet, PDF (20/20 Pages) AVAGO TECHNOLOGIES LIMITED – Isolation Amplifier with Short Circuit and Overload Detection SO-16 Package

5. Power Supplies and Start-Up
5.1: What are the output voltages before
the input side power supply is turned
on?
V (pin 12) is close to zero volts, ABSVAL (pin 13) is close to V and FAULT (pin 14) is
OUT
REF
in the high (inactive) state when power to the input side is off. In fact, a self test can
be performed using this information. 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.
5.2: How long does the HCPL-788J take
to begin working properly after
power-up?
About 50 μs after a V power-up and 100 μs after a V power-up.
DD2
DD1
6. Miscellaneous
6.1: How does the HCPL-788J measure
negative signals with only a +5 V
supply?
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.
6.2: What load capacitance can the HCPL-
788J drive?
Typically, noticeable ringing and overshoot begins for C above 0.02 μF. Avago
LOAD
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.
6.3: Can I use the HCPL-788J with a
bipolar input A/D converter?
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/ of V from V .
2
REF
OUT
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Data subject to change. Copyright © 2005-2010 Avago Technologies. All rights reserved. Obsoletes AV01-0570EN
AV02-1546EN - February 8, 2010