HCPL788J Datasheet, PDF (17/20 Pages) Agilent(Hewlett-Packard) – Isolation Amplifier with Short Circuit and Overload Detection

17
Frequently Asked Questions about the HCPL-788J
1. The Basics
1.1: Why should I use the HCPL-788J for
sensing current when Hall-effect sensors
are available which don’t need an
isolated supply voltage?
1.2: What is the purpose of the V_REF input?
1.3: What is the purpose of the rectified
(ABSVAL) output on pin 13?
Historically, motor control current sense designs
have required trade-offs between signal accuracy,
response time, and the use of discrete components
to detect short circuit and overload conditions.
The HCPL-788J greatly simplifies current-sense
designs by providing an output voltage which can
connect directly to an A/D converter as well as
integrated short circuit and overload detection
(eliminating the need for external circuitry). Available
in an auto-insertable, SO-16 package, the HCPL-788J
is smaller than and has better linearity, offset vs.
temperature and Common Mode Rejection (CMR)
performance than most Hall-effect sensors.
The V_REF input establishes the full scale output
range. V_REF can be connected to the supply voltage
(V_DD2) or a voltage between 4 V and V_DD2. The nom-
inal gain of the HCPL-788J is the output full scale
range divided by 504 mV.
When 3 phases are wire-ORed together, the 3 phase
AC currents are combined to form a DC voltage with
very little ripple on it. This can be simply filtered and
used to monitor the motor load. Moderate overload
currents which don’t trip the FAULT output can thus
be detected easily.
2. Sense Resistor and Input Filter
2.1: Where do I get 10 mΩ resistors? I have
never seen one that low.
2.2: Should I connect both inputs across the
sense resistor instead of grounding V_IN-
directly to pin 8?
2.3: How can I avoid false tripping of the fault
output due to cable capacitance charging
transients?
Although less common than values above 10 Ω,
there are quite a few manufacturers of resistors
suitable for measuring currents up to 50 A when
combined with the HCPL-788J. Example product
information may be found at Dale’s web site
(http://www.vishay.com/vishay/dale) and Isotek’s
web site (http://www.isotekcorp.com).
This is not necessary, but it will work. If you do, be
sure to use an RC filter on both pin 1 (V_IN+) and
pin 2 (V_IN-) to limit the input voltage at both pads.
In PWM motor drives there are brief spikes of cur-
rent flowing in the wires leading to the motor each time
a phase voltage is switched between states. The amp-
litude and duration of these current spikes is deter-
mined by the slew rate of the power transistors and
the wiring impedances. To avoid false tripping of the
FAULT output (pin 14) the HCPL-788J includes a
blanking filter. This filter ignores over-range input
conditions shorter than 1 µs. For very long motor
wires, it may be necessary to increase the time con-
stant of the input RC anti-aliasing filter to keep the
peak value of the HCPL-788J inputs below ± 230 mV.
For example, a 39 Ω, 0.047 µF RC filter on pin 1 will
ensure that 2 µs wide 500 mV pulses across the sense
resistor do not trip the FAULT output.