Datasheet

17
1. THE BASICS
1.1: Why should I use the HCPL-7800(A) for sensing cur-
rent 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-7800(A) is smaller than and has better linearity,
oset vs. temperature and Common Mode Rejection
(CMR) performance than most Hall-eect sensors. Ad-
ditionally, 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-7800(A). 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 resistor 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.
FREQUENTLY ASKED QUESTIONS ABOUT
THE HCPL-7800(A)
2. Filter resistor: The equivalent input resistance for
HCPL-7800(A) is around 500 kΩ. It is therefore best
to ensure that the lter resistance is not a signicant
percentage of this value; otherwise the oset 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 to 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
HCPL-7800(A) 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-7800(A) is not de-
stroyed as a result of short circuit conditions which cause
voltage drops across the sense resistor that exceed the rat-
ings of the HCPL-7800(A)’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-7800(A) can not be damaged by sense
resistors going open-circuit.
3. ISOLATION AND INSULATION
3.1: How many volts will the HCPL-7800(A) withstand?
The momentary (1 minute) withstand voltage is 3750 V
rms per UL 1577 and CSA Component Acceptance Notice
#5.
4. ACCURACY
4.1: Can the signal to noise ratio be improved?
Yes. Some noise energy exists beyond the 100 kHz
bandwidth of the HCPL-7800(A). Additional filtering
using dierent lter R,C values in the post-amplier
application circuit can be used to improve the signal
to noise ratio. For example, by using values of R3 = R4
= 10 kΩ, C5 = C6 = 470 pF in the application circuit
the rms output noise will be cut roughly by a factor of
2. In applications needing only a few kHz bandwidth
even better noise performance can be obtained. The
noise spectral density is roughly 500 nV/š Hz below
20 kHz (input referred).