HCPL7520 Datasheet, PDF (14/15 Pages) Agilent(Hewlett-Packard) – Isolated Linear Sensing IC

FREQUENTLY ASKED QUESTIONS ABOUT THE HCPL-7520
1. THE BASICS
1.1: Why should I use the HCPL-7520 for sensing current when Hall-effect sensors are available which
don’t need an isolated supply voltage?
Available in an auto-insertable, 8-pin DIP package, the HCPL-7520 is smaller than and has better linearity,
offset vs. temperature and Common Mode Rejection (CMR) performance than most Hall-effect sensors.
Additionally, often the required input-side power supply can be derived from the same supply that powers the
gate-drive optocoupler.
2. SENSE RESISTOR AND INPUT FILTER
2.1: Where do I get 10 mΩ resistors? I have never seen one that low.
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-7520. 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)
and Iwaki Musen Kenkyusho’s website (http://www.iwakimusen.co.jp) and Micron Electric’s website
(http://www.micron-e.co.jp).
2.2: Should I connect both inputs across the sense resistor instead of grounding VIN- directly to pin 4?
This is not necessary, but it will work. If you do, be sure to use an RC filter on both pin 2 (VIN+) and pin 3
(VIN-) to limit the input voltage at both pads.
2.3: Do I really need an RC filter on the input? What is it for? Are other values of R and C okay?
The input anti-aliasing filter (R=39 Ω, C=0.01 µF) shown in the typical application circuit is recommended
for filtering fast switching voltage transients from the input signal. (This helps to attenuate higher signal
frequencies which could otherwise alias with the input sampling rate and cause higher input offset voltage.)
Some issues to keep in mind using different filter resistors or capacitors are:
1. (Filter resistor:) The equivalent input resistance for HCPL-7520 is around 700 kΩ. It is therefore best to
ensure that the filter resistance is not a significant percentage of this value; otherwise the offset voltage will
be increased through the resistor divider effect. [As an example, if Rfilt = 5.5 kΩ, then VOS = (Vin * 1%)
= 2 mV for a maximum 200 mV input and VOS will vary with respect to Vin.]
2. The input bandwidth is changed as a result of this different R-C filter configuration. In fact this is one of
the main reasons for changing the input-filter R-C time constant.
3. (Filter capacitance:) The input capacitance of the HCPL-7520 is approximately 1.5 pF. For proper
operation the switching input-side sampling capacitors must be charged from a relatively fixed (low
impedance) voltage source. Therefore, if a filter capacitor is used it is best for this capacitor to be a few
orders of magnitude greater than the CINPUT (A value of at least 100 pF works well.)
2.4: How do I ensure that the HCPL-7520 is not destroyed as a result of short circuit conditions which
cause voltage drops across the sense resistor that exceed the ratings of the HCPL-7520’s inputs?
Select the sense resistor so that it will have less than 5 V drop when short circuits occur. The only other
requirement is to shut down the drive before the sense resistor is damaged or its solder joints melt. This
ensures that the input of the HCPL-7520 can not be damaged by sense resistors going open-circuit.
3. ISOLATION AND INSULATION
3.1: How many volts will the HCPL-7520 withstand?
The momentary (1 minute) withstand voltage is 3750 V rms per UL 1577 and CSA Component Acceptance
Notice #5.
4. ACCURACY
4.1: Does the gain change if the internal LED light output degrades with time?
No. The LED is used only to transmit a digital pattern. Agilent has accounted for LED degradation in the
design of the product to ensure long life.
5. MISCELLANEOUS
5.1: How does the HCPL-7520 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.
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