HCPL788J datasheet(18/20 Pages) HP

18 / 20 page

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 m

Ω

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 ony 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 can not

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 HP recommends?

4. Accuracy

For zero input, the output should ideally be 1/2 of

VREF. The nominal slope of the input/output relation-

ship is VREF divided by 0.504 V. Offset errors change

only the DC input voltage needed to make the output

equal to 1/2 of VREF. Gain errors change only the

slope of the input/output relationship. For example,

if VREF 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?

HCPL788J Datasheet(PDF) 18 Page - Agilent(Hewlett-Packard)

HCPL788J Datasheet(HTML) 18 Page - Agilent(Hewlett-Packard)

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