Datasheet

19
1. The Basics
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.
Frequently Asked Questions about the HCPL-788J
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?
2. Sense Resistor and Input Filter
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
amplitude and duration of these current spikes is
determined 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 constant 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.
2.1: Where do I get 10 m Ω 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?