HCPL-7520-300 Datasheet, PDF (15/16 Pages) AVAGO TECHNOLOGIES LIMITED – 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. Addition-
ally, 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.iwaki-
musen.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 per-
centage 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 ca-
pacitors 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 require-
ment 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.
Avago Technologies 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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