LT6105
12
6105fa
The LT6105 extended input range current sense am-
plifi er (see Figure 1) provides accurate unidirectional
monitoring of current through a user-selected sense resis-
tor. The LT6105 is fully specifi ed over a –0.3V to 44V input
common mode range. A high PSRR V
+
supply (2.85V to
36V) powers the current sense amplifi er. The input sense
voltage is level shifted from the sensed power supply to
the ground reference and amplifi ed by a user-selected gain
to the output. The output voltage is directly proportional
to the current fl owing through the sense resistor.
THEORY OF OPERATION
(Refer to Figure 1)
Case 1: High Input Voltage (1.6V < V
–IN
< 44V)
Current from the source at V
S
+
fl ows through R
SENSE
to
the load at V
S
–
, creating a sense voltage, V
SENSE
. Inputs
V
S
+
and V
S
–
apply the sense voltage to R
IN2
. The opposite
ends of resistors R
IN1
and R
IN2
are forced to be at equal
potentials by the voltage gain of amplifi er A2. Thus, the
current through R
IN2
is V
SENSE
/R
IN2
. The current through
R
IN2
is forced to fl ow through transistor Q1 and into
R
OUT
, creating an output voltage, V
OUT
. Under this input
operation range, amplifi er A1 is kept off. The base current
of Q1 has been compensated for and will not contribute
to output error. The current from R
IN2
fl owing through
resistor R
OUT
gives an output voltage of V
OUT
= V
SENSE
•
R
OUT
/R
IN2
, producing a gain voltage of A
V
= V
OUT
/V
SENSE
= R
OUT
/R
IN2
.
Case 2: Low Input Voltage (0V < V
–IN
< 1.6V)
Current from the source at V
S
+
fl ows through R
SENSE
to
the load at V
S
–
, creating a sense voltage, V
SENSE
. Inputs
V
S
+
and V
S
–
apply the sense voltage to R
IN1
. The opposite
ends of resistors R
IN1
and R
IN2
are forced to be at equal
potentials by the voltage gain of amplifi er A1. Thus, the
collector current of Q3 will fl ow out of the –IN pin through
R
IN1
. Q2 mirrors this current V
SENSE
/R
IN1
to R
OUT
, creat-
ing an output voltage, V
OUT
. Under this input operation
range, amplifi er A2 is kept off. This current V
SENSE
/R
IN1
fl owing through resistor R
OUT
gives an output voltage of
V
OUT
= V
SENSE
• R
OUT
/R
IN1
, producing a gain voltage of
A
V
= V
OUT
/V
SENSE
= R
OUT
/R
IN1
.
APPLICATIONS INFORMATION
Selection of External Current Sense Resistor
External R
SENSE
resistor selection is a delicate trade-off
between power dissipation in the resistor and current
measurement accuracy. For high current applications, the
user may want to minimize the sense voltage to minimize
the power dissipation in the sense resistor.
The system load current will cause both heat and voltage
loss in R
SENSE
. As a result, the sense resistor should be as
small as possible while still providing the input dynamic
range required by the measurement. Note that input dy-
namic range is the difference between the maximum input
signal and the minimum accurately reproduced signal,
and is limited primarily by input DC offset voltage of the
internal amplifi er of the LT6105.
The sense resistor value will be set from the minimum
signal current that can be accurately resolved by this sense
amp. As an example, the LT6105 has a typical input offset
of 100μV. If the minimum current is 20mA, a sense resistor
of 5mΩ will set V
SENSE
to 100μV, which 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, but it will limit the maximum peak
current for a given application.
For a peak current of 2A and a maximum V
SENSE
of 80mV,
R
SENSE
should not be more than 40mΩ. The input offset
causes an error equivalent to only 2.5mA of load current.
Peak dissipation is 160mW. If a 20mΩ sense resistor is
employed, then the effective current error is 5mA, while
the peak sense voltage is reduced to 40mV at 2A, dis-
sipating only 80mW.
The LT6105’s low input offset voltage of 100μV allows for
high resolution while limiting the maximum sense voltages.
Coupled with full scale sense voltage as large as 1V for
R
IN
= 1k, it can achieve 80dB of dynamic range.
Sense Resistor Connection
Kelvin connection of the LT6105’s input resistors to the
sense resistor should be implemented to provide the high-
est accuracy in high current applications. Solder connec-
tions and PC board interconnect resistance (approximately
0.5mΩ per square for 1oz copper) can be a large error
in high current systems. A 5A application might choose
