Datasheet

20
This filter prevents damage from input spikes which
may go beyond the absolute maximum ratings of the
HCPL-788J inputs during ESD and other transient
events. The filter also prevents aliasing of high fre-
quency (above 3 MHz) noise at the sampled input.
Other RC values are certainly OK, but should be
chosen to prevent the input voltage (pin 1) from ex-
ceeding ±5 V for any conceivable current waveform
in the sense resistor. Remember to account for in-
ductance of the sense resistor since it is possible to
momentarily have tens of volts across even a 1 mΩ
resistor if di/dt is quite large.
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-788J
cannot be damaged by sense resistors going
open-circuit.
2.4: Do I really need an RC filter on the input?
What is it for? Are other values of R and
C okay?
2.5: How do I ensure that the HCPL-788J 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-788J’s inputs?
3. Isolation and Insulation
The momentary (1 minute) withstand voltage is
3500 V rms per UL1577 and CSA Component
Acceptance Notice #5.
These capacitors are to reduce the narrow output
spikes caused by high common mode slew rates. If
your application does not have rapid common mode
voltage changes, these capacitors are not needed.
3.1: How many volts will the HCPL-788J
withstand?
3.2: What happens if I don’t use the 470 pF
output capacitors Avago recommends?
4. Accuracy
For zero input, the output should ideally be
1
/
2
of
V
REF
. The nominal slope of the input/output
relationship is V
REF
divided by 0.504 V. Offset errors
change only the DC input voltage needed to make
the output equal to
1
/
2
of V
REF
. Gain errors change
only the slope of the input/output relationship. For
example, if V
REF
is 4.0 V, the gain should be
7.937 V/V. For zero input, the output should be
2.000 V. Input offset voltage of ±3 mV means the
output voltage will be 2.000 V ±0.003*7.937 or
2.000 ±23.8 mV when the input is zero. Gain
tolerance of ±5% means that the slope will be
7.937 ±0.397. Over the full range of ±3 mV input
offset error and ±5% gain error, the output voltage
will be 2.000 ±25.0 mV when the input is zero.
4.1: What is the meaning of the offset errors
and gain errors in terms of the output?