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HCPL-7840-500E Datasheet(PDF) 17 Page - AVAGO TECHNOLOGIES LIMITED |
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HCPL-7840-500E Datasheet(HTML) 17 Page - AVAGO TECHNOLOGIES LIMITED |
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17 / 19 page  17 Also, multiple layers of the PC board can be used to increase current carrying capacity. Numerous plated- through vias should surround each non-Kelvin terminal of 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. Sense Resistor Connections The recommended method for connecting the HCPL-7840 to the current sensing resistor is shown in Figure 18. V IN+ (pin 2 of the HPCL-7840) is connected to the positive terminal of the sense resistor resistor, while V IN- (pin 3) is shorted to GND1 (pin 4), with the power-supply return path functioning as the sense line to the negative termi- nal of the current sense resistor. This allows a single pair of wires or PC board traces to connect the HCPL-7840 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. If the same power supply is used both for the gate drive circuit and for the current sensing circuit, it is very impor- tant that the connection from GND1 of the HCPL-7840 to the sense resistor be the only return path for supply current to the gate drive power supply in order to elimi- nate potential ground loop problems. The only direct connection between the HCPL-7840 circuit and the gate drive circuit should be the positive power supply line. 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 Semicon- ductor). 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 com- ponents for the circuit as well as the required board space. Please refer to Avago Applications Note 1078 for addi- tional information on using Isolation Amplifiers. |
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