LTC6101/LTC6101HV
10
Rev I
For more information www.analog.com
APPLICATIONS INFORMATION
Figure 2. Kelvin Input Connection Preserves
Accuracy Despite Large Load Current
LTC6101
R
OUT
V
OUT
6101 F02
R
IN
V
+
LOAD
R
SENSE
–
+
V
+
V
–
OUT
–IN+IN
Useful Gain Configurations
Gain R
IN
R
OUT
V
SENSE
at V
OUT
= 5V I
OUT
at V
OUT
= 5V
20 499 10k 250mV 500µA
50 200 10k 100mV 500µA
100 100 10k 50mV 500µA
Selection of External Current Sense Resistor
The external sense resistor, R
SENSE
, has a significant effect
on the function of a current sensing system and must be
chosen with care.
First, the power dissipation in the resistor should be
considered. The system load current will cause both heat
and voltage loss in R
SENSE
. As a result, the sense resis-
tor should be as small as possible while still providing
the input dynamic range required by the measurement.
Note that input dynamic range is the difference between
the maximum
input signal and the minimum accurately
reproduced signal, and is limited primarily by input DC
offset of the internal amplifier of the LTC6101. In addition,
R
SENSE
must be small enough that V
SENSE
does not exceed
the maximum input voltage specified by the LTC6101, even
under peak load conditions. As an example, an application
may require that the maximum sense voltage be 100mV.
If this application is expected to draw 2A at peak load,
R
SENSE
should be no more than 50mΩ.
Once the maximum R
SENSE
value is determined, the mini-
mum sense resistor value will be set by the resolution or
dynamic range required. The minimum signal that can be
accurately represented by this sense amp is limited by the
input offset. As an example, the LTC6101B has a typical
input offset of 150µ
V. If the minimum current is 20mA, a
sense resistor of 7.5mΩ will set V
SENSE
to 150µV. This is
the same value as the input offset. A larger sense resistor
will reduce the error due to offset by increasing the sense
voltage for a given load current.
Choosing a 50mΩ R
SENSE
will maximize the dynamic range
and provide a system that has 100mV across the sense
resistor at peak load (2A), while input offset causes an
error equivalent to only 3mA of load current.
Peak dissipation is 200mW. If a 5mΩ sense resistor is
employed, then the effective current error is 30mA, while
the peak sense voltage is reduced to 10mV at 2A, dis
-
sipating only 20mW.
The low offset and corresponding large
dynamic range of
the LTC6101 make it more flexible than other solutions in
this respect. The 150µV typical offset gives 60dB of dy
-
namic range for a sense voltage that is limited to 150mV
max, and over 70dB of dynamic range if the rated input
maximum of 500mV is allowed.
Sense Resistor Connection
Kelvin connection of the IN
–
and IN
+
inputs to the sense
resistor should be used in all but the lowest power ap-
plications. Solder connections and PC board interconnec-
tions that carry high current can cause significant error
in measurement due to their relatively large resistances.
One 10mm x 10mm square trace of one-ounce copper
is approximately 0.5mΩ. A 1mV error can be caused by
as little as 2A flowing through this small inter
connect.
This will cause a 1% error in a 100mV signal. A 10A load
current in the same interconnect will cause a 5% error
for the same 100mV signal. By isolating the sense traces
from the high-current paths, this error can be reduced
by orders of magnitude. A sense resistor with integrated
Kelvin sense terminals will give the best results. Figure 2
illustrates the recommended method.
