HCPL-7800-000E View Datasheet(PDF) - Avago Technologies

Part Name

Description

View to exact match

HCPL-7800-000E

Isolation Amplifer

Avago Technologies 

the sense resistor to help distribute the current between

the layers of the PC board. The PC board should use 2 or

4 oz. copper for the layers, resulting in a current carrying

capacity in excess of 20 A. Making the current carrying

traces on the PC board fairly large can also improve the

sense resistor’s power dissipation capability by acting as a

heat sink. Liberal use of vias where the load current enters

and exits the PC board is also recommended.

Note: Please refer to Avago Technologies Application Note 1078 for

additional information on using Isolation Amplifiers.

Sense Resistor Connections

The recommended method for connecting the HCPL-

7800(A) to the current sensing resistor is shown in Figure

18. VIN+ (pin 2 of the HPCL-7800(A)) is connected to the

positive terminal of the sense resistor, while VIN- (pin

3) is shorted to GND1 (pin 4), with the power-supply

return path functioning as the sense line to the negative

terminal of the current sense resistor. This allows a single

pair of wires or PC board traces to connect the HCPL-

7800(A) circuit to the sense resistor. By referencing the

input circuit to the negative side of the sense resistor,

any load current induced noise transients on the resistor

are seen as a common-mode signal and will not interfere

with the current-sense signal. This is important because

the large load currents flowing through the motor drive,

along with the parasitic inductances inherent in the

wiring of the circuit, can generate both noise spikes and

offsets that are relatively large compared to the small

voltages that are being measured across the current

sensing resistor.

Output Side

The op-amp used in the external post-amplifier circuit

should be of sufficiently high precision so that it does not

contribute a significant amount of offset or offset drift

relative to the contribution from the isolation amplifier.

Generally, op-amps with bipolar input stages exhibit

better offset performance than op-amps with JFET or

MOSFET input stages.

In addition, the op-amp should also have enough

bandwidth and slew rate so that it does not adversely

affect the response speed of the overall circuit. The post-

amplifier circuit includes a pair of capacitors (C5 and C6)

that form a single-pole low-pass filter; these capacitors

allow the bandwidth of the post-amp to be adjusted

independently of the gain and are useful for reducing

the output noise from the isola-tion amplifier. Many

different op-amps could be used in the circuit, including:

MC34082A (Motorola), TLO32A, TLO52A, and TLC277

(Texas Instruments), LF412A (National Semiconductor).

The gain-setting resistors in the post-amp should have a

tolerance of 1% or better to ensure adequate CMRR and

adequate gain toler-ance for the overall circuit. Resistor

networks can be used that have much better ratio toler-

ances than can be achieved using discrete resistors. A

resistor network also reduces the total number of compo-

nents for the circuit as well as the required board space.

If the same power supply is used both for the gate

drive circuit and for the current sensing circuit, it is very

important that the connection from GND1 of the HCPL-

7800(A) to the sense resistor be the only return path for

supply current to the gate drive power supply in order

to eliminate potential ground loop problems. The only

direct con­nection between the ­ HCPL-7800(A) circuit

and the gate drive circuit should be the positive power

supply line.

16

Share Link: