LTC6103 Linear Integrated Systems, LTC6103 Datasheet - Page 7

• Manufacturer Part Number: LTC6103
• Description: High Side Current Sense Amplifier
• Manufacturer: Linear Integrated Systems

THEORY OF OPERATION
An internal sense amplifi er loop forces –IN to have the
same potential as +IN. Connecting an external resistor, R
between –IN and V
same as the sense voltage across R
current, (I
The high impedance inputs of the sense amplifi er will not
conduct this input current, so the current will fl ow through
APPLICATIONS INFORMATION
In this dual current sense device, amplifi ers A and B are
independent except for sharing the same V
voltage and component values can be chosen independently
for each amplifi er.
Selection of External Current Sense Resistor
The external sense resistor, R
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
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
larger than 50mΩ.
R
SENSE
LOAD
=
V
I
SENSE
PEAK
+ I
+
S
) • R
forces a potential across R
SENSE
=
SENSE
100
2
. As a result, the sense resis-
A
mV
SENSE
/R
IN
=
, will fl ow through R
SENSE
, has a signifi cant effect
50 Ω
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m
SENSE
. A corresponding
–
pin. So supply
should be no
IN
that is the
IN
IN
,
.
an internal MOSFET to the OUT pin. In most application
cases, I
The output current can be transformed into a voltage by
adding a resistor from OUT to V
then V
Once the maximum R
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
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
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.
OUT
S
<< I
= (V
LOAD
–
) + (I
, so I
SENSE
OUT
SENSE
OUT
• R
will maximize the dynamic range
≈ I
value is determined, the mini-
OUT
LOAD
).
–
. The output voltage is
• R
SENSE
SENSE
LTC6103
to 85µV. This is
/R
IN
.
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