LT6109IMS-1#TRPBF Linear Technology, LT6109IMS-1#TRPBF Datasheet - Page 11

• Manufacturer Part Number: LT6109IMS-1#TRPBF
• Manufacturer: Linear Technology

Specifications of LT6109IMS-1#TRPBF

• Lead Free Status / Rohs Status: Compliant

APPLICATIONS INFORMATION
The output current can be transformed back into a voltage
by adding a resistor from OUTA to V
The output voltage is then:
where R
Table 1. Example Gain Configurations
Useful Equations
Note that V
the amplifier, however, output accuracy will degrade as
V
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 measured load current will cause power
dissipation as well as a voltage drop in R
result, the sense resistor should be as small as possible
while still providing the input dynamic range required by
the measurement. Note that the 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 sense ampli-
fier of the LT6109. To ensure the specified performance,
R
exceed V
example, an application may require the maximum sense
voltage be 100mV. If this application is expected to draw
2A at peak load, R
GAIN
SENSE
SENSE
100
20
50
V
Input Voltage: V
Current Gain:
Voltage Gain:
OUT
goes beyond V
499Ω
200Ω
100Ω
should be small enough that V
= V
OUT
R
SENSE(MAX)
IN
SENSE(MAX)
–
= R1 + R2 + R3 as shown in Figure 3.
+ I
R
10k
10k
10k
OUTA
OUT
I
V
I
SENSE
SENSE
OUTA
V
SENSE
SENSE
OUT
V
• R
under peak load conditions. As an
SENSE
can be exceeded without damaging
SENSE(MAX)
OUT
should be set to 50mΩ.
=
= I
=
FOR V
250mV
100mV
R
50mV
R
SENSE
SENSE
SENSE
R
R
OUT
IN
IN
OUT
.
, has a significant effect
= 5V
•R
–
SENSE
(typically ground).
I
SENSE
OUTA
SENSE
AT V
500µA
500µA
500µA
does not
OUT
. As a
= 5V
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 amplifier is limited by
the input offset. As an example, the LT6109 has a maximum
input offset of 350µV. If the minimum current is 20mA, a
sense resistor of 17.5mΩ will set V
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
system that has 100mV across the sense resistor at peak
load (2A), while input offset causes an error equivalent to
only 7mA of load current.
In the previous example, the peak dissipation in R
is 200mW. If a 5mΩ sense resistor is employed, then
the effective current error is 70mA, while the peak sense
voltage is reduced to 10mV at 2A, dissipating only 20mW.
The low offset and corresponding large dynamic range of
the LT6109 make it more flexible than other solutions in this
respect. The 350µV maximum offset gives 63dB of dynamic
range for a sense voltage that is limited to 500mV max.
Sense Resistor Connection
Kelvin connection of the SENSEHI and SENSELO inputs
to the sense resistor should be used in all but the lowest
power applications. Solder connections and PC board
interconnections that carry high current can cause sig-
nificant error in measurement due to their relatively large
resistances. One 10mm × 10mm square trace of 1oz 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 for a full-scale V
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 by orders of magnitude. A sense resistor with
integrated Kelvin sense terminals will give the best results.
Figure 3 illustrates the recommended method for connect-
ing the SENSEHI and SENSELO pins to the sense resistor.
SENSE
will maximize the dynamic range and provide a
LT6109-1/LT6109-2
SENSE
value is determined, the mini-
SENSE
SENSE
to 350µV. This
of 100mV.
11
SENSE
610912f