Datasheet

LT6106
8
6106fa
APPLICATIONS INFORMATION
magnitude. A sense resistor with integrated Kelvin sense
terminals will give the best results. Figure 2 illustrates the
recommended method.
This approach can be helpful in cases where occasional
bursts of high currents can be ignored.
Care should be taken when designing the board layout for
R
IN
, especially for small R
IN
values. All trace and inter-
connect resistances will increase the effective R
IN
value,
causing a gain error.
Selection of External Output Resistor, R
OUT
The output resistor, R
OUT
, determines how the output cur-
rent is converted to voltage. V
OUT
is simply I
OUT
• R
OUT
.
In choosing an output resistor, the maximum output volt-
age must fi rst be considered. If the following circuit is a
buffer or ADC with limited input range, then R
OUT
must be
chosen so that I
OUT(MAX)
• R
OUT
is less than the allowed
maximum input range of this circuit.
In addition, the output impedance is determined by R
OUT
. If
the circuit to be driven has high enough input impedance,
then almost any useful output impedance will be accept-
able. However, if the driven circuit has relatively low input
impedance, or draws spikes of current such as an ADC
might do, then a lower R
OUT
value may be required in order
to preserve the accuracy of the output. As an example, if
the input impedance of the driven circuit is 100 times R
OUT
,
then the accuracy of V
OUT
will be reduced by 1% since:
VI
RR
RR
OUT OUT
OUT IN DRIVEN
OUT IN DRIVEN
=
+
=
•
•
()
()
IIR IR
OUT OUT OUT OUT
•• .••
100
101
099=
Error Sources
The current sense system uses an amplifi er and resistors
to apply gain and level shift the result. The output is then
dependent on the characteristics of the amplifi er, such as
gain and input offset, as well as resistor matching.
Ideally, the circuit output is:
VV
R
R
VRI
OUT SENSE
OUT
IN
SENSE SENSE SENSE
==•; •
In this case, the only error is due to resistor mismatch,
which provides an error in gain only. However, offset volt-
age and bias current cause additional errors.
Figure 3. Shunt Diode Limits Maximum Input Voltage to Allow
Better Low Input Resolution Without Overranging
Figure 2. Kelvin Input Connection Preserves Accuracy with
Large Load Currents
Selection of External Input Resistor, R
IN
R
IN
should be chosen to allow the required resolution
while limiting the output current to 1mA. In addition, the
maximum value for R
IN
is 500Ω. By setting R
IN
such that
the largest expected sense voltage gives I
OUT
= 1mA, then
the maximum output dynamic range is available. Output
dynamic range is limited by both the maximum allowed
output current and the maximum allowed output voltage,
as well as the minimum practical output signal. If less
dynamic range is required, then R
IN
can be increased
accordingly, reducing the maximum output current and
power dissipation. If low sense currents must be resolved
accurately in a system that has a very wide dynamic range,
a smaller R
IN
than the maximum current spec allows may
be used if the maximum current is limited in another way,
such as with a Schottky diode across R
SENSE
(Figure 3).
This will reduce the high current measurement accuracy
by limiting the result, while increasing the low current
measurement resolution.
LT6106
R
OUT
V
OUT
6106 F02
R
IN
V
+
LOAD
R
SENSE
–
+
V
+
V
–
OUT
–IN+IN
V
+
LOAD
D
SENSE
6106 F03
R
SENSE