HCPL-7520 datasheet(13/16 Pages) AVAGO

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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 affect 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 first 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 amplifier. 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 m

Ω. 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 offset and noise, which are fixed, 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 specific design.

When sensing currents large enough to cause sig-

nificant heating of the sense resistor, the temperature

coefficient (tempco) of the resistor can introduce non-

linearity due to the signal dependent temperature rise

of the resistor. The effect increases as the resistor-to-

ambient thermal resistance increases. This effect 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 significant percentage of the total resistance. This

has two primary effects on resistor accuracy. First, the

effective 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 effects 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.

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