Datasheet

LT6106
7
6106fa
APPLICATIONS INFORMATION
Introduction
The LT6106 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
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, V
SENSE
/R
IN
, will fl ow through R
IN
.
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
OUT
.
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
.
Table 1. Useful Gain Confi gurations
GAIN R
IN
R
OUT
V
SENSE
at V
OUT
= 5V I
OUT
at V
OUT
= 5V
20 499Ω 10k 250mV 500μA
50 200Ω 10k 100mV 500μA
100 100Ω 10k 50mV 500μA
GAIN R
IN
R
OUT
V
SENSE
at V
OUT
= 2.5V I
OUT
at V
OUT
= 2.5V
20 249Ω 5k 125mV 500μA
50 100Ω 5k 50mV 500μA
100 50Ω 5k 25mV 500μA
Selection of External Current Sense Resistor
The external sense resistor, R
SENSE
, has a signifi cant ef-
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
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 mea-
sured signal, and is limited primarily by input DC offset of
the internal amplifi er of the LT6106. In addition, R
SENSE
must be small enough that V
SENSE
does not exceed the
maximum input voltage specifi ed by the LT6106, 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
SENSE
should be no more than 50mΩ.
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 amplifi er is limited by
the input offset. As an example, the LT6106 has a typical
input offset of 150μV. If the minimum current is 20mA, a
sense resistor of 7.5mΩ will set V
SENSE
to 150μV. This 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
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 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 LT6106 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 by orders of