Datasheet

LT6108-1/LT6108-2
13
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APPLICATIONS INFORMATION
The low offset and corresponding large dynamic range of
the LT6108 make it more flexible than other solutions in this
respect. The 125µV maximum offset gives 72dB of dynamic
range for a sense voltage that is limited to 500mV max.
Sense Resistor Connection
Kelvin connection of the SENSEHI and SENSELO inputs
to the sense resistor should be used in all but the lowest
power applications. Solder connections and PC board
interconnections that carry high currents can cause sig-
nificant error in measurement due to their relatively large
resistances. One 10mm × 10mm square trace of 1oz copper
is approximately 0.5mΩ. A 1mV error can be caused by as
little as 2A flowing through this small interconnect. This
will cause a 1% error for a full-scale V
SENSE
of 100mV.
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 3 illustrates the recommended method for connect-
ing the SENSEHI and SENSELO pins to the sense resistor.
Selection of External Input Gain Resistor, R
IN
R
IN
should be chosen to allow the required speed and
resolution while limiting the output current to 1mA. The
maximum value for R
IN
is 1k to maintain good loop sta-
bility. For a given V
SENSE
, larger values of R
IN
will lower
power dissipation in the LT6108 due to the reduction
in I
OUT
while smaller values of R
IN
will result in faster
response time due to the increase in I
OUT
. If low sense
currents must be resolved accurately in a system that has
a very wide dynamic range, a smaller R
IN
may be used
if the maximum I
OUTA
current is limited in another way,
such as with a Schottky diode across R
SENSE
(Figure 4).
This will reduce the high current measurement accuracy
by limiting the result, while increasing the low current
measurement resolution.
Figure 3. LT6108-1 Typical Connection
Figure 4. Shunt Diode Limits Maximum Input Voltage to Allow
Better Low Input Resolution Without Overranging
OUTA
I
OUTA
–
+
V
+
C1
SENSEHI
INC
6
7
8
5
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V
+
V
+
V
–
LT6108-1
SENSELO
EN/RST
OUTC
1
2
3
4
V
RESET
R
C
V
PULLUP
LOAD
V
SUPPLY
V
SENSE
R
SENSE
OVERCURRENT
FLAG
R
IN
+
–
*R
OUT
= R1 + R2
V
–
I
SENSE
=
V
SENSE
R
SENSE
R1*
R2* C
L
V
OUT
400mV
REFERENCE
C
LC
–
+
D
SENSE
R
SENSE
V
+
LOAD
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