AN3222

Selecting the components

Doc ID 17551 Rev 1

5/18

4

Selecting the components

Depending on the type of application, various component values can be selected, such as

Rshunt or resistors for the amplifier gain.

To select the correct component values:

1.

Find the maximum current.

This is the maximum current that goes through the sensing resistor (the maximum

current to sense in your system).

For example:

2.

Find the correct shunt resistor. This value must be limited to avoid a significant

voltage drop (such as 1%) and to limit power dissipation. It must, however, be high

enough to obtain better accuracy.

For example: Vsense_max = 1% voltage, with voltage = 5 V and Imax = 1 A

Rshunt

× Imax ≤Vsense_max

So Rshunt must be lower than or equal to 50 m

Ω.. In the current example, Rshunt has

been set to 30 m

Ω.

3.

Calculate the maximum power dissipation in the shunt resistor. To avoid

damaging the sensing resistor, the shunt resistor has to sustain a suitable wattage.

For example:

In this case, a 1 W shunt resistor is sufficient.

4.

Choose the appropriate configuration gain.

To avoid saturation:

In the current configuration, Rg = 100

Ω and Voh = 4.985 V (TS507 at 25°C, Vcc = 5 V).

Rf must therefore be lower than 16.6 k

Ω to avoid saturation of the TS507 at maximum

currents. It is recommended to choose the highest possible Rf to benefit from the

output voltage capability of the amplifier. Selecting Rf in the E192 series leads to

Rf = 16.2 k

Ω.

Imax

Power_max

voltage

⁄

5 W

5 V

⁄

1 A

==

=

=> Rshunt

1%

5 V

⋅

1 A

---------------------------

≤

Pmax

Rshunt

Imax

2

⋅

0.03

1

2

⋅

0.03 W

==

=

Vout

Rf

Rg

⁄

() V

⋅

Rf

Rg

⁄

() Rshunt I

⋅

⋅

==

Vout

Voh => Rf

Voh

Rg

⋅

Rshunt

Imax

⋅

------------------------------------------

<

≤

Rf max

4.985

100

⋅

0.03

1

⋅

--------------------------------

16.6k

Ω

==