LT6107
7
6107fc
For more information www.linear.com/LT6107
APPLICATIONS INFORMATION
Introduction
The LT6107 high side current sense amplifier (Figure 1) pro-
vides accurate monitoring of current through a user-selected
sense resistor. The sense voltage is amplified by a user-
selected gain and level shifted from the positive power sup-
ply to a ground-referred output. The output signal is analog
and may be used as is, or processed with an output filter.
Theory of Operation
An internal sense amplifier loop forces –IN to have the
same potential as +IN. Connecting an external resistor,
R
IN
, between –IN and V
+
forces a potential across R
IN
that is the same as the sense voltage across R
SENSE
. A
corresponding current, V
SENSE
/R
IN
, will flow through R
IN
.
The high impedance inputs of the sense amplifier will not
conduct this current, so it will flow through an internal
PNP to the output pin as I
OUT
.
The output current can be transformed into a voltage by
adding a resistor from OUT to V
–
. The output voltage is
then V
O
= V
–
+ I
OUT
• R
OUT
.
Table 1. Useful Gain Configurations
GAIN R
IN
R
OUT
V
SENSE
at V
OUT
= 5V I
OUT
at V
OUT
= 5V
20 499Ω 10k 250mV 500µA
50 200Ω 10k 100mV 500µA
100 100Ω 10k 50mV 500µA
GAIN R
IN
R
OUT
V
SENSE
at V
OUT
= 2.5V I
OUT
at V
OUT
= 2.5V
20 249Ω 5k 125mV 500µA
50 100Ω 5k 50mV 500µA
100 50Ω 5k 25mV 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 con-
sidered. 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 dynamic range is the difference between the
maximum input signal and the minimum accurately mea-
sured signal, and is limited primarily by input DC offset of
the internal amplifier of the LT6107. In addition, R
SENSE
must be small enough that V
SENSE
does not exceed the
maximum input voltage specified by the LT6107, 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 min-
imum sense resistor value will be set by the resolution or
dynamic range required. The minimum signal that can be
accurately represented by this sense amplifier is limited
by the input offset. As an example, the LT6107 has a typi-
cal input offset of 150µV. 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 resis-
tor 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, dissipating only 20mW.
The low offset and corresponding large dynamic range
of the LT6107 make it more flexible than other solutions
in this respect. The 150µV typical offset gives 60dB
of dynamic range for a sense voltage that is limited to
150mV maximum, and over 70dB of dynamic range if the
rated input maximum of 0.5V 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 appli
-
cations. 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 × 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 interconnect.
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