OMO ElectronicOMO Electronic
Expand menu
Hello, Sign inMy Account
0 Cart

LT6106HS5#TRMPBF

P1-P3P4-P6P7-P9P10-P12P13-P14
LT6106
7
6106fb
For more information www.linear.com/LT6106
APPLICATIONS INFORMATION
Introduction
The LT6106 high side current sense amplifier (Figure 1) pro
-
vides 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 sup-
ply 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 –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 flow through R
IN
.
The high impedance inputs of the sense amplifier will not
conduct this current, so it will flow 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 Configurations
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 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
and voltage loss in R
SENSE
. As a result, the sense resis-
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
measured signal, and is limited primarily by input DC offset
of the internal amplifier of the LT6106. In addition, R
SENSE
must be small enough that V
SENSE
does not exceed the
maximum input voltage specified by the LT6106, 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 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 amplifier 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 flexible 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 ap
-
plications. Solder connections and PC board interconnec-
tions that carry high current can cause significant 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 flowing through this small interconnect.
This will cause a 1% error in a 100mV signal. 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
LT6106
8
6106fb
For more information www.linear.com/LT6106
APPLICATIONS INFORMATION
by orders of magnitude. A sense resistor with integrated
Kelvin sense terminals will give the best results. Figure 2
illustrates the recommended method.
This approach can be helpful in cases where occasional
bursts of high currents can be ignored.
Care should be taken when designing the board layout for
R
IN
, especially for small R
IN
values. All trace and inter-
connect resistances will increase the effective R
IN
value,
causing a gain error.
Selection of External Output Resistor, R
OUT
The output resistor, R
OUT
, determines how the output cur-
rent is converted to voltage. V
OUT
is simply I
OUT
• R
OUT
.
In choosing an output resistor, the maximum output volt
-
age must first be considered. If the following circuit is a
buffer or ADC with limited input range, then R
OUT
must be
chosen so that I
OUT(MAX)
• R
OUT
is less than the allowed
maximum input range of this circuit.
In addition, the output impedance is determined by R
OUT
. If
the circuit to be driven has high enough input impedance,
then almost any useful output impedance will be accept
-
able. However, if the driven circuit has relatively low input
impedance, or draws spikes of current such as an ADC might
do, then a lower R
OUT
value may be required in order to
preserve the accuracy of the output. As an example, if the
input impedance of the driven circuit is 100 times R
OUT
,
then the accuracy of V
OUT
will be reduced by 1% since:
V
OUT
=I
OUT
•
R
OUT
•R
IN(DRIVEN)
R
OUT
+R
IN(DRIVEN)
=I
OUT
•R
OUT
•
100
101
= 0.99 •I
OUT
•R
OUT
Error Sources
The current sense system uses an amplifier and resistors
to apply gain and level shift the result. The output is then
dependent on the characteristics of the amplifier, such as
gain and input offset, as well as resistor matching.
Ideally, the circuit output is:
V
OUT
= V
SENSE
•
R
OUT
R
IN
; V
SENSE
=R
SENSE
•I
SENSE
In this case, the only error is due to resistor mismatch,
which provides an error in gain only. However, offset volt
-
age and bias current cause additional errors.
Figure 3. Shunt Diode Limits Maximum Input Voltage to Allow
Better Low Input Resolution Without Overranging
Figure 2. Kelvin Input Connection Preserves Accuracy with
Large Load Currents
Selection of External Input Resistor, R
IN
R
IN
should be chosen to allow the required resolution
while limiting the output current to 1mA. In addition, the
maximum value for R
IN
is 500Ω. By setting R
IN
such that
the largest expected sense voltage gives I
OUT
= 1mA, then
the maximum output dynamic range is available. Output
dynamic range is limited by both the maximum allowed
output current and the maximum allowed output voltage,
as well as the minimum practical output signal. If less
dynamic range is required, then R
IN
can be increased
accordingly, reducing the maximum output current and
power dissipation. If low sense currents must be resolved
accurately in a system that has a very wide dynamic range,
a smaller R
IN
than the maximum current spec allows may
be used if the maximum current is limited in another way,
such as with a Schottky diode across R
SENSE
(Figure 3).
This will reduce the high current measurement accuracy
by limiting the result, while increasing the low current
measurement resolution.
LT6106
R
OUT
V
OUT
6106 F02
R
IN
V
+
LOAD
R
SENSE
–
+
V
+
V
–
OUT
–IN+IN
V
+
LOAD
D
SENSE
6106 F03
R
SENSE
LT6106
9
6106fb
For more information www.linear.com/LT6106
APPLICATIONS INFORMATION
Output Error Due to the Amplifier DC Offset
Voltage, V
OS
E
OUT(VOS)
= V
OS
•
R
OUT
R
IN
The DC offset voltage of the amplifier adds directly to the
value of the sense voltage, V
SENSE
. This is the dominant
error of the system and it limits the low end of the dynamic
range. The paragraph “Selection of External Current Sense
Resistor” provides details.
Output Error Due to the Bias Currents, I
B
+
and I
B
–
The bias current I
B
+
flows into the positive input of the
internal op amp. I
B
–
flows into the negative input.
E
OUT(IBIAS)
= R
OUT
I
B
+
•
R
SENSE
R
IN
– I
B
–
Assuming I
B
+
@ I
B
–
= I
BIAS
, and R
SENSE
<< R
IN
then:
E
OUT(IBIAS)
@ –R
OUT
• I
BIAS
It is convenient to refer the error to the input:
E
IN(IBIAS)
@ –R
IN
• I
BIAS
For instance if I
BIAS
is 60nA and R
IN
is 1k, the input referred
error is 60µV. Note that in applications where R
SENSE
@
R
IN
, I
B
+
causes a voltage offset in R
SENSE
that cancels the
error due to I
B
–
and E
OUT(IBIAS)
@ 0mV. In most applica-
tions, R
SENSE
<< R
IN
, the bias current error can be similarly
reduced if an external resistor R
IN
+
= (R
IN
– R
SENSE
) is
connected as shown in Figure 4. Under both conditions:
E
IN(IBIAS)
= ±R
IN
• I
OS
; where I
OS
= I
B
+
– I
B
–
If the offset current, I
OS
, of the LT6106 amplifier is 6nA,
the 60µV error above is reduced to 6µV.
Adding R
IN
+
as described will maximize the dynamic
range of the circuit. For less sensitive designs, R
IN
+
is
not necessary.
Output Error Due to Gain Error
The LT6106 exhibits a typical gain error of –0.25% at 1mA
output current. The primary source of gain error is due to
the finite gain to the PNP output transistor, which results in
a small percentage of the current in R
IN
not appearing in the
output load R
OUT
.
Minimum Output Voltage
The curves of the Output Voltage vs Input Sense Voltage
show the behavior of the LT6106 with low input sense
voltages. When V
SENSE
= 0V, the output voltage will always
be slightly positive, the result of input offset voltages and
of a small amount of quiescent current (0.7µA to 1.2µA)
flowing through the output device. The minimum output
voltage in the Electrical Characteristics table include both
these effects.
Power Dissipation Considerations
The power dissipated by the LT6106 will cause a small
increase in the die temperature. This rise in junction tem
-
perature can be calculated if the output current and the
supply current are known.
The power dissipated in the LT6106 due to the output
signal is:
P
OUT
= (V
IN
–
– V
OUT
) • I
OUT
Since V
IN
–
@ V
+
, P
OUT
@ (V
+
– V
OUT
) • I
OUT
The power dissipated due to the quiescent supply current is:
P
Q
= I
S
• (V
+
– V
–
)
The total power dissipated is the output dissipation plus
the quiescent dissipation:
P
TOTAL
= P
OUT
+ P
Q
The junction temperature is given by:
T
J
= T
A
+ θ
JA
• P
TOTAL
At the maximum operating supply voltage of 36V and the
maximum guaranteed output current of 1mA, the total
Figure 4. Second Input R Minimizes Error Due to Input Bias Current
LT6106
R
OUT
V
OUT
6106 F04
R
IN
–
R
IN
+
V
+
LOAD
R
SENSE
–
+
V
+
V
–
OUT
R
IN
+
= R
IN
–
– R
SENSE
–IN+IN
P1-P3P4-P6P7-P9P10-P12P13-P14

LT6106HS5#TRMPBF

Mfr. #:
Buy LT6106HS5#TRMPBF
Manufacturer:
Analog Devices Inc.
Description:
Current Sense Amplifiers L Cost, 36V Hi Side C Sense Amp
Lifecycle:
New from this manufacturer.
Delivery:
DHL FedEx Ups TNT EMS
Payment:
T/T Paypal Visa MoneyGram Western Union

Products related to this Datasheet