HCPL-7520-300 Datasheet HCPL-7520-300E, HCPL-7520-000E, HCPL-7520-060E | P13-P15

Current Sensing Resistors

The current sensing resistor should have low resistance

(to minimize power dissipation), low inductance (to

minimize di/dt induced voltage spikes which could

adversely aect operation), and reasonable tolerance

(to maintain overall circuit accuracy). Choosing a par-

ticular value for the resistor is usually a compromise

between minimizing power dissipation and maximiz-

ing accuracy. Smaller sense resistance decreases power

dissipation, while larger sense resistance can improve

circuit accuracy by utilizing the full input range of the

HCPL -7520.

The rst step in selecting a sense resistor is determining

how much current the resistor will be sensing. The graph

in Figure 18 shows the RMS current in each phase of a

three-phase induction motor as a function of average

motor output power (in horsepower, hp) and motor

drive supply voltage. The maximum value of the sense

resistor is determined by the current being measured

and the maximum recommended input voltage of the

isolation amplier. The maximum sense resistance can

be calculated by taking the maximum recommended

input voltage and dividing by the peak current that the

sense resistor should see during normal operation. For

example, if a motor will have a maximum RMS current

of 10 A and can experience up to 50% overloads during

normal operation, then the peak current is 21.1 A (=10

x 1.414 x 1.5). Assuming a maximum input voltage of

200 mV, the maximum value of sense resistance in this

case would be about 10 mΩ. The maximum average

power dissipation in the sense resistor can also be easily

calculated by multiplying the sense resistance times the

square of the maximum RMS current, which is about

1 W in the previous example. If the power dissipation

in the sense resistor is too high, the resistance can be

decreased below the maximum value to decrease power

dissipation. The minimum value of the sense resistor is

limited by precision and accuracy requirements of the

design. As the resistance value is reduced, the output

voltage across the resistor is also reduced, which means

that the oset and noise, which are xed, become a

larger percentage of the signal amplitude. The selected

value of the sense resistor will fall somewhere between

the minimum and maximum values, depending on the

particular requirements of a specic design.

When sensing currents large enough to cause sig-

nicant heating of the sense resistor, the temperature

coecient (tempco) of the resistor can introduce non-

linearity due to the signal dependent temperature rise

of the resistor. The eect increases as the resistor-to-

ambient thermal resistance increases. This eect can

be minimized by reducing the thermal resistance of

the current sensing resistor or by using a resistor with

a lower tempco. Lowering the thermal resistance can

be accomplished by repositioning the current sensing

resistor on the PC board, by using larger PC board traces

to carry away more heat, or by using a heat sink. For a

two-terminal current sensing resistor, as the value of

resistance decreases, the resistance of the leads become

a signicant percentage of the total resistance. This

has two primary eects on resistor accuracy. First, the

eective resistance of the sense resistor can become

dependent on factors such as how long the leads are,

how they are bent, how far they are inserted into the

board, and how far solder wicks up the leads during

assembly (these issues will be discussed in more detail

shortly). Second, the leads are typically made from

a material, such as copper, which has a much higher

tempco than the material from which the resistive

element itself is made, resulting in a higher tempco

overall. Both of these eects are eliminated when a

four-terminal current sensing resistor is used. A four-

terminal resistor has two additional terminals that are

Kelvin-connected directly across the resistive element

itself; these two terminals are used to monitor the

voltage across the resistive element while the other two

terminals are used to carry the load current. Because

of the Kelvin connection, any voltage drops across the

leads carrying the load current should have no impact

on the measured voltage.

When laying out a PC board for the current sensing

resistors, a couple of points should be kept in mind. The

Kelvin connections to the resistor should be brought

together under the body of the resistor and then run

very close to each other to the input of the HCPL-7520;

this minimizes the loop area of the connection and

reduces the possibility of stray magnetic elds from in-

terfering with the measured signal. If the sense resistor

is not located on the same PC board as the HCPL-7520

circuit, a tightly twisted pair of wires can accomplish

the same thing. 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 dis-

tribute 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-

7520 to the current sensing resistor is shown in Figure

17. VIN+ (pin 2 of the HPCL-7520) is connected to the

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

3) is shorted to GND1 (pin 4), with the powersupply

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-7520 circuit to the sense resistor. By referenc-

ing 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 owing

through the motor drive, along with the parasitic in-

ductances inherent in the wiring of the circuit, can

generate both noise spikes and osets that are relative-

ly 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 important that

the connection from GND1 of the HCPL-7520 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 con-

nection between the HCPL-7520 circuit and the gate

drive circuit should be the positive power supply line.

Figure 18. Motor output horsepower vs. motor phase current and supply

voltage.

15 20 25 30

MOTOR PHASE CURRENT – A (rms)

MOTOR OUTPUT POWER – HORSEPOWER

5 350

440

380

220

120

FREQUENTLY ASKED QUESTIONS ABOUT THE HCPL-7520

1. THE BASICS

1.1: Why should I use the HCPL-7520 for sensing

current 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-7520 is smaller than and has better linearity, oset

vs. temperature and Common Mode Rejection (CMR)

performance than most Hall-eect sensors. Addition-

ally, 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-7520. 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) and

Iwaki Musen Kenkyusho’s website (http://www.iwaki-

musen.co.jp) and Micron Electric’s website (http://www.

micron-e.co.jp).

2.2: Should I connect both inputs across the sense

resistor instead of grounding VIN- 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 (VIN+) and pin 3 (VIN-)

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 different filter

resistors or capacitors are:

1. (Filter resistor:) The equivalent input resistance for

HCPL-7520 is around 700 kΩ. It is therefore best to

ensure that the lter resistance is not a signicant per-

centage 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 VOS = (Vin * 1%) = 2 mV

for a maximum 200 mV input and VOS will vary with

respect to Vin.]

2. 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.

3. (Filter capacitance:) The input capacitance of

the HCPL-7520 is approximately 1.5 pF. For proper

operation the switching input-side sampling ca-

pacitors must be charged from a relatively fixed

(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-7520 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-7520’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-7520 can not be damaged by

sense resistors going open-circuit.

3. ISOLATION AND INSULATION

3.1: How many volts will the HCPL-7520 withstand?

The momentary (1 minute) withstand voltage is 3750

V rms per UL 1577 and CSA Component Acceptance

Notice #5.

4. ACCURACY

4.1: Does the gain change if the internal LED light

output degrades with time?

No. The LED is used only to transmit a digital pattern.

Avago Technologies has accounted for LED degradation

in the design of the product to ensure long life.

5. MISCELLANEOUS

5.1: How does the HCPL-7520 measure negative

signals with only a +5 V supply?

The inputs have a series resistor for protection against

large negative inputs. Normal signals are no more than

200 mV in amplitude. Such signals do not forward bias

any junctions sufficiently to interfere with accurate

operation of the switched capacitor input circuit.

P1-P3 P4-P6 P7-P9 P10-P12 P13-P15 P16-P16

HCPL-7520-300

Mfr. #:

Buy HCPL-7520-300

Manufacturer:

Broadcom / Avago

Description:

Optically Isolated Amplifiers 4.5 - 5.5 SV +/-5%

Lifecycle:

New from this manufacturer.

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HCPL-7520-300

HCPL-7520-300

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