FREQUENTLY ASKED QUESTIONS ABOUT THE HCPL-7510
1. THE BASICS
1.1: Why should I use the HCPL-7510 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-7510 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-7510. 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-7510 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 * 1ꢀ) = 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-7510 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-7510 is not destroyed as a result of short circuit conditions which
voltage drops across the sense resistor that exceed the ratings of the HCPL-7510’s inputs?
cause
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-7510 can not be damaged by sense
resistors going open-circuit.
3. ISOLATION AND INSULATION
3.1: How many volts will the HCPL-7510 withstand?
The momentary (1 minute) withstand voltage is 3750 V rms per UL 1577 and CSA Component
Acceptance Notice #5.
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