Digital Current Sensing
Circuit
As shown in Figure 1, using the
Isolated 2-chip A/D converter to
sense current can be as simple as
connecting a current-sensing
resistor, or shunt, to the input
and reading output data through
the 3-wire serial output interface.
By choosing the appropriate
shunt resistance, any range of
current can be monitored, from
less than 1 A to more than 100 A.
Even better performance can be
achieved by fully utilizing the
more advanced features of the
Isolated A/D converter, such as
the pre-trigger circuit which can
reduce conversion time to less
than 1
µs,
the fast over-range
detector for quickly detecting
short circuits, different conversion
modes giving various resolution/
speed trade-offs, offset calibra-
tion mode to eliminate initial
offset from measurements, and
an adjustable threshold detector
for detecting non-short circuit
overload conditions.
NON-ISOLATED
+5V
ISOLATED
+5V
+
INPUT
CURRENT R
SHUNT
0.02
V
DD1
V
IN+
V
DD2
MCLK
MDAT
GND2
CCLK
CLAT
CDAT
MCLK1
MDAT1
MCLK2
MDAT2
GND
V
DD
CHAN
SCLK
SDAT
CS
THR1
OVR1
RESET
3-WIRE
SERIAL
INTERFACE
C1
0.1 µF
V
IN-
GND1
C2
0.1 µF
+
C3
10 µF
HCPL-7860
HCPL-x870
Figure 1: Typical Application Circuit.
Product Overview
Description
The HCPL-7860 Isolated Modu-
lator and the HCPL-x870 Digital
Interface IC together form an
isolated programmable two-chip
analog-to-digital converter. The
isolated modulator allows direct
measurement of motor phase
currents in power inverters while
the digital interface IC can be
programmed to optimize the
conversion speed and resolution
trade-off.
In operation, the HCPL-7860
Isolated Modulator (optocoupler
with 3750 V
RMS
dielectric with-
stand voltage rating) converts a
low-bandwidth analog input into
a high-speed one-bit data stream
by means of a sigma-delta (
∑
∆)
oversampling modulator. This
modulation provides for high
noise margins and excellent
immunity against isolation-mode
transients. The modulator data
and on-chip sampling clock are
encoded and transmitted across
the isolation boundary where they
are recovered and decoded into
separate high-speed clock and
data channels.
The Digital Interface IC converts
the single-bit data stream from
the Isolated Modulator into
fifteen-bit output words and
provides a serial output interface
that is compatible with SPI
®
,
QSPI
®
, and Microwire
®
proto-
cols, allowing direct connection
to a microcontroller. The Digital
Interface IC is available in two
package styles: the HCPL-7870 is
in a 16-pin DIP package and the
HCPL-0870 is in a 300-mil wide
SO-16 surface-mount package.
Features of the Digital Interface
IC include five different conver-
sion modes, three different pre-
trigger modes, offset calibration,
fast over-range detection, and
adjustable threshold detection.
Programmable features are con-
figured via the Serial Configura-
tion port. A second multiplexed
input is available to allow
measurements with a second
1-261
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