Datasheet

18
FREQUENTLY ASKED QUESTIONS ABOUT THE HCPL-7840
1. THE BASICS
1.1: Why should I use the HCPL-7840 for sensing current when Hall-eect sensors are available which don’t need an
isolated supply voltage?
Available in an auto-insertable, 8-pin DIP package, the HCPL-7840 is smaller than and has better linearity, oset vs.
temperature and Common Mode Rejection (CMR) performance than most Hall-eect 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 commo n than values above 10 Ω, there are quite a few manufactur-
ers of resistors suitable for measuring currents up to 50 A when combined with the HCPL- 7840.
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).
2.2: Should I connect both inputs across the sense resistor instead of grounding V
IN-
directly
to pin 4?
This is not necessary, but it will work. If you do, be sure to use an RC lter on both pin 2 (V
IN+
) and pin 3 (V
IN-
) to limit
the input voltage at both pads.
2.3: Do I really need an RC lter on the input? What is it for? Are other values of R and C okay?
The input anti-aliasing lter (R=39 Ω, C=0.01 µF) shown in the typical application circuit is recommended for ltering
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 oset voltage.)
Some issues to keep in mind using dierent lter resistors or capacitors are:
1. (Filter resistor:) Input bias current for pins 2 and 3: This is on the order of 500 nA. If you are using a single lter resis-
tor in series with pin 2 but not pin 3 the IxR drop across this resistor will add to the oset error of the device. As long
as this IR drop is small compared to the input oset voltage there should not be a problem. If larger-valued resistors
are used in series, it is better to put half of the resistance in series with pin 2 and half the resistance in series with pin
3. In this case, the oset voltage is due mainly to resistor mismatch (typically less than 1% of the resistance design
value) multiplied by the input bias.
2. (Filter resistor:) The equivalent input resistance for -7840 is around 500 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 eect. [As an example, if R
lt
= 5.5 kΩ, then V
OS
= (Vin * 1%) =
2 mV for a maximum 200 mV input and V
OS
will vary with respect with Vin.]
3. The input bandwidth is changed as a result of this dierent R-C lter conguration. In fact this is one of the main
reasons for changing the input-lter R-C time constant.
4. (Filter capacitance:) The input capacitance of the -78XX is approximately 1.5 pF. For proper operation the switching
input-side sampling capacitors must be charged from a relatively xed (low impedance) voltage source. Therefore, if
a lter capacitor is used it is best for this capacitor to be a few orders of magnitude greater than the C
INPUT
(A value of
at least 100 pF works well.)
2.4: How do I ensure that the HCPL-7840 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-7840’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-7840 can not be damaged by sense resistors going open-circuit.