lt6107mps5-trpbf Linear Technology Corporation, lt6107mps5-trpbf Datasheet - Page 7

• Manufacturer Part Number: lt6107mps5-trpbf
• Description: High Temperature High Side Current Sense Amp In Sot-23
• Manufacturer: Linear Technology Corporation

APPLICATIONS INFORMATION
Introduction
The LT6107 high side current sense amplifi er (Figure 1) pro-
vides accurate monitoring of current through a user-selected
sense resistor. The sense voltage is amplifi ed 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 fi lter.
Theory of Operation
An internal sense amplifi er loop forces –IN to have the
same potential as +IN. Connecting an external resistor,
R
that is the same as the sense voltage across R
corresponding current, V
The high impedance inputs of the sense amplifi er will not
conduct this current, so it will fl ow through an internal
PNP to the output pin as I
The output current can be transformed into a voltage by
adding a resistor from OUT to V
then V
Table 1. Useful Gain Confi gurations
Selection of External Current Sense Resistor
The external sense resistor, R
fect 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
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 amplifi er of the LT6107. In addition, R
IN
GAIN
GAIN
100
100
20
50
20
50
, between –IN and V
O
499Ω
200Ω
100Ω
249Ω
100Ω
= V
50Ω
R
R
IN
IN
–
+ I
R
R
10k
10k
10k
OUT
5k
5k
5k
OUT
OUT
SENSE
• R
V
OUT
SENSE
. As a result, the sense resistor
V
SENSE
+
SENSE
OUT
forces a potential across R
.
250mV
100mV
125mV
50mV
50mV
25mV
at V
at V
.
SENSE
/R
OUT
OUT
–
IN
. The output voltage is
= 5V
, will fl ow through R
, has a signifi cant ef-
=
I
OUT
I
OUT
at V
at V
500μA
500μA
500μA
500μA
500μA
500μA
OUT
SENSE
OUT
= 2.5V
SENSE
= 5V
. A
IN
IN
.
must be small enough that V
maximum input voltage specifi ed by the LT6107, even un-
der 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
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 amplifi er is limited by
the input offset. As an example, the LT6107 has a typical
input offset of 150μV. If the minimum current is 20mA, a
sense resistor of 7.5mΩ will set V
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 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 fl exible than other solutions in
this respect. The 150μV typical offset gives 60dB of dy-
namic 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 signifi cant 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 fl owing through this small interconnect. This
will cause a 1% error in a 100mV signal. A 10A load cur-
rent 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
SENSE
SENSE
should be no more than 50mΩ.
will maximize the dynamic range and provide a
SENSE
value is determined, the mini-
SENSE
SENSE
does not exceed the
to 150μV. This is
LT6107
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