ISL70100SEH, ISL73100SEH

5. Applications Information

R34DS0009EU0101 Rev.1.01

Page 21 of 27

Jul.10.20

5.

Applications Information

5.1

Selection of the External Current-Sense Resistor

To pick the current-sense resistor value, a decision has to be made between power dissipation and measurement

accuracy. As a general rule for all applications, the sense resistor should be as small as possible while still

providing adequate input dynamic range across the operating range. The minimum accurately sensed input

voltage is primarily limited by the offset voltage of the ISL70100SEH.

The sense resistor value can be calculated once the maximum sensed load current is determined. The maximum

recommended sense voltage for the ISL70100SEH is 150mV, so dividing that by the maximum load current

provides the sense resistor value.

where:

•RSENSE is the sense resistor in mΩ.

•IOUT(MAX) is the maximum expected load current to be sensed in amps.

5.2

Gain Setting

The gain on the ISL70100SEH can be adjusted using a single resistor on the output. The ISL70100SEH outputs

2µA for every 1mV of differential across the inputs. Equation 5 can be used to calculate the output load resistance

to get a specific gain:

where:

•ROUT is the output load resistance in kΩ.

•ACSA is desired voltage gain.

•gm is the transconductance of the ISL70100SEH which is 2µA/mV (typical).

5.3

Selection of Output Resistor

The output signal is a current conducted through the output resistor to generate a voltage. With a maximum input

range of 150mV, the output current is 300µA. The voltage gain should be determined based on the input voltage

range of the downstream device. For example, if there is a 5V ADC on the output of the ISL7x100SEH, the output

resistance should be 16.6kΩ to ensure that VOUT stays under 5V. This also means that the minimum power

supply voltage has to be 7V to achieve 5V on the output. Renesas recommends using resistors that have a low

temperature coefficient on the output as it adds error to the measurement.

5.4

Output Filtering

Because the output of these amplifiers is a current source, filtering is straight forward. A capacitor can be placed

in parallel to the output resistance to create a pole based on Equation 6:

where:

•f-3dB is frequency location of the pole in hertz

•ROUT is the output resistance in ohms

•COUT is the output capacitance in parallel with ROUT in farads

RSENSE

150mV

IOUT MAX



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

=

(EQ. 4)

ROUT

ACSA

gm

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

=

(EQ. 5)

f 3dB

–

1

2





ROUT



COUT



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

=

(EQ. 6)