Download Chip Detects Shorts and Overloads, Measures Current

Chip Detects Shorts and Overloads, Measures
Current
Jon Titus - May 24, 2010
Avago Technologies has produced a new device that can help designers of motor control circuits
measure current and detect short circuits and overloads. The HCPL-788J chip galvanically isolates
the motor-side and microcontroller/DSP side so you don't have to include an isolation amplifier or
opto-isolator or any sensor signal-conditioning components. The 16-pin IC (SO-16 package) costs
about $9.50 for a single device.
Find more information at www.avagotech.com (search for "HCPL-788J"). The data sheet includes a
helpful 2.5-page Q&A section that covers questions such as why not just use a Hall-effect sensor,
and do I need an RC filter on the input. The chip provides three outputs, FAULT/, Vout, and ABSVAL.
The FAULT/ output goes to a logic 0 within 6 microseconds of finding a voltage across the Vin+ and
Vin- pins that exceeds a detection-threshold voltage. This output provides an open-source--in the
FET sense--output that you can wire-AND to an interrupt input on a microcontroller, DSP chip, or
other device. (See wired-AND note below.)
The Vout signal indicates the instantaneous voltage across an external shunt resistor and the
ABSVAL output provides the rectified value of the Vout voltage that you can wire-AND to provide a
signal that indicates an overload condition. The chip requires an external shunt resistor.
The February issue of the Avnet Electronics Marketing flyer, "Technology Review," shows a photo of
an Avago eval board for the HCPL-788J "isolation amplifier." A Web link in the description notes
engineers can register to "win" a free eval board for this interesting device. I registered with Avnet
previously, so I logged in and answered a few questions. Maybe I'll get a free eval board, or maybe
not.
Photo courtesy of Avnet.
Unfortunately, if you want to get your hands on an eval board NOW, you might be out of luck. A
search for HCPL-788J on the Avnet site brings up four listings for the device but no eval kit. I
couldn't find the board on the Avago Technologies Web site either. You want to pay for a kit, but you
can't find a way to purchase it. According to the Avnet site, "Offers end July 31, 2009, or while
supplies last."
I requested purchasing instructions from my contacts at Avago and Avnet both of whom told me you
should send a request to the general email box at [email protected] for purchase
information. Let me know how that works out.
Note: The HCPL-788J data sheet notes the use of a wired-OR output on the device. Actually, it's a
wired-AND circuit. The FAULT/ output normally provides a logic-1 output and you can connect
several FAULT/ outputs to generate a single interrupt signal. A logic-0 on any input pulls the
combined FAULT/ signal low. That action defines an AND gate. A wired-OR circuit normally
produces a logic-0 output. A logic-1 on any input would produce a logic 1 on the combined output.
I'm being picky here, but you wouldn't call a NAND gate a NOR gate, so let's use the proper
nomenclature.