LTC6102-1_15 Datasheet, PDF (12/26 Pages) Linear Technology – Precision Zero Drift Current Sense Amplifier

LTC6102
LTC6102-1/LTC6102HV
Applications Information
The LTC6102 high side current sense amplifier (Figure 1)
provides 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 supply 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 –INS to have the
same potential as +IN. Connecting an external resistor,
R_IN, between –INS 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 input current, so it will flow through the –INF
pin and an internal MOSFET to the output pin.
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.
Useful Gain Configurations
GAIN
R_IN
R_OUT
200
49.9Ω
10k
500
20Ω
10k
1000
10Ω
10k
4990
1Ω
4.99k
V_SENSE AT V_OUT = 5V
25mV
10mV
5mV
1mV
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
considered. The system load current will cause both heat
dissipation 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
reproduced signal, and is limited primarily by input DC
offset of the internal amplifier of the LTC6102. In addition,
R_SENSE must be small enough that V_SENSE does not exceed
the maximum sense voltage specified by the LTC6102 or
the sense resistor, 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 20A at peak load, R_SENSE should be no more
than 5mΩ.
Once the maximum R_SENSE value is determined, the
minimum sense resistor value will be set by the resolu-
tion 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 LTC6102
has a typical input offset of 3µV. If the minimum current
is 1mA, a sense resistor of 3mΩ will set V_SENSE to 3µV.
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.
For this example, choosing a 5mΩ R_SENSE will maximize
the dynamic range and provide a system that has 100mV
across the sense resistor at peak load (20A), while input
offset causes an error equivalent to only 0.6mA of load
current.
Peak dissipation is 2W. If a 0.5mΩ sense resistor is em-
ployed, then the effective current error is 6mA (0.03%
of full-scale), while the peak sense voltage is reduced to
10mV at 20A, dissipating only 200mW.
The low offset and corresponding large dynamic range of
the LTC6102 make it more flexible than other solutions
in this respect. The 3µV typical offset gives 100dB of dy-
namic range for a sense voltage that is limited to 300mV
max, and over 116dB of dynamic range if a maximum of
2V is allowed.
The previous example assumes that a large output dynamic
range is required. For circuits that do not require large
dynamic range, the wide input range of the LTC6102 may
be used to reduce the size of the sense resistor, reducing
power loss and increasing reliability. For example, in a
100A circuit requiring 60dB of dynamic range, the input
offset and drift of most current-sense solutions will require
that the shunt be chosen so that the sense voltage is at
least 100mV at full scale so that the minimum input is
greater than 100µV. This will cause power dissipation in
excess of 10W at full scale! That much power loss can put
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