LTC6103
7
6103f
APPLICATIONS INFORMATION
In this dual current sense device, amplifi ers A and B are
independent except for sharing the same V
–
pin. So supply
voltage and component values can be chosen independently
for each amplifi er.
Selection of External Current Sense Resistor
The external sense resistor, R
SENSE
, has a signifi cant 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 amplifi er of the LTC6103.
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
larger than 50mΩ.
R
V
I
mV
A
m
SENSE
SENSE
PEAK
===
100
2
50 Ω
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 LTC6103 has a typical
input offset of 85µV. If the minimum current is 20mA, a
sense resistor of 4.25mΩ will set V
SENSE
to 85µV. 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 1.7mA of load current.
Peak dissipation is 200mW. If instead a 5mΩ sense resis-
tor is employed, then the effective current error is 17mA,
while the peak sense voltage is reduced to 10mV at 2A,
dissipating only 20mW.
The low offset and corresponding large dynamic range of
the LTC6103 make it more fl exible than other solutions
in this respect. The 85µV typical offset gives 60dB of
dynamic range for a sense voltage that is limited to 85mV
max, and over 75dB of dynamic range for a maximum
input of 500mV.
THEORY OF OPERATION
An internal sense amplifi er 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, (I
LOAD
+ I
S
) • R
SENSE
/R
IN
, will fl ow through R
IN
.
The high impedance inputs of the sense amplifi er will not
conduct this input current, so the current will fl ow through
an internal MOSFET to the OUT pin. In most application
cases, I
S
<< I
LOAD
, so I
OUT
≈ I
LOAD
• R
SENSE
/R
IN
.
The output current can be transformed into a voltage by
adding a resistor from OUT to V
–
. The output voltage is
then V
OUT
= (V
–
) + (I
OUT
• R
OUT
).
