ISL70100SEH, ISL73100SEH

4. Functional Description

R34DS0009EU0101 Rev.1.01

Page 20 of 27

Jul.10.20

4.4

Sources of Error

4.4.1

Input Impedance of Downstream Devices

The value of the output resistance is not critical if the circuit that is being driven has a high input impedance. If the

driven circuit has a low input impedance, the accuracy of VOUT is reduced. Using an example where the input

impedance is 100 times larger than the output resistance. It can be seen that the accuracy drops by 1%.

where:

•VOUT is the output voltage in volts

•IOUT is the output current in amps

•ROUT is output resistance loading the output of the ISL7x100SEH in ohms

•RIN is the input impedance of the downstream device in ohms

4.4.2

Input Offset Voltage

The dynamic range of the ISL7x100SEH is inversely proportional to the input offset voltage. The dynamic range

can be thought of as the maximum sense voltage divided by VOS. These amplifiers have an offset voltage of 10µV

typically. The electrical specification table shows how the offset voltage can vary across various operating points.

4.4.3

Input Bias and Offset Current

A typical current sense amplifier has the ability to change its transconductance by changing the input resistors. In

this case, the input bias currents and offset currents can induce more error on top of the offset voltage. The

ISL7x100SEH gets around this by internalizing these input resistors and trimming out the error associated with

them during production.

4.4.4

Sensed Current Error

The effective current error can be determined with Equation 3. It shows the amount of current that the

current-sense amplifier is blind to.

where:

•VOS is the offset voltage in µV.

•RSENSE is the sense resistor in mΩ.

•IERROR is the current error in mA.

The RSENSE can be increased to lower the current error, but this reduces the maximum current that can be

sensed and increases power dissipation. Careful consideration must be made to balance power dissipation and

accuracy.

VOUT

IOUT

ROUT RIN



ROUT RIN

+

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



IOUT

100

101

----------



IOUT ROUT



0.99



==

=

(EQ. 2)

IERROR

VOS

RSENSE

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

=

(EQ. 3)