LTC6102HV Linear Technology, LTC6102HV Datasheet - Page 10

• Manufacturer Part Number: LTC6102HV
• Description: Zero Drift High Side Current Sense Amplifier
• Manufacturer: Linear Technology

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APPLICATIONS INFORMATION
LTC6102/LTC6102HV
I
the largest expected sense voltage gives I
the maximum output dynamic range is available. Output
dynamic range is limited by both the maximum allowed
output current (Note 1) and the maximum allowed output
voltage, as well as the minimum practical output signal. If
less dynamic range is required, then R
accordingly, reducing the output current and power dis-
sipation. If small sense currents must be resolved ac-
curately in a system that has very wide dynamic range, a
smaller R
another way, such as with a Schottky diode across R
(Figure 3). This will reduce the high current measurement
accuracy by limiting the result, while increasing the low
current measurement resolution. This approach can be
helpful in cases where occasional large burst currents
may be ignored.
Care should be taken when designing the PC board lay-
out for R
interconnect impedances will increase the effective R
value, causing a gain error. It is important to note that the
large temperature drift of copper resistance will cause a
change in gain over temperature if proper care is not taken
to reduce this effect.
To further limit the effect of trace resistance on gain,
maximizing the accuracy of these circuits, the LTC6102 has
been designed with a Kelvin input. The inverting terminal
(–INS) is separate from the feedback path (–INF). During
operation, these two pins must be connected together.
The design of the LTC6102 is such that current into –INS
is input bias current only, which is typically 60pA at 25°C.
Almost all of the current from R
the LTC6102, and into R
minimize gain error, –INS should be routed in a separate
path from –INF to a point as close to R
10
OUT
Figure 3. Shunt Diode Limits Maximum Input Voltage to Allow
Better Low Input Resolution Without Overranging
may be as much as 1mA. By setting R
IN
IN
, especially for small R
may be used if the max current is limited in
R
SENSE
LOAD
V
OUT
+
6102 F03
via the OUT pin. In order to
IN
fl ows into –INF, through
D
IN
SENSE
values. All trace and
IN
IN
can be increased
OUT
as possible. In
IN
= 1mA, then
such that
SENSE
IN
addition, the higher potential terminal of R
connected directly to the positive terminal of R
any input voltage source). For the highest accuracy, R
should be a four-terminal resistor if it is less than 10Ω.
Selection of External Output Resistor, R
The output resistor, R
rent is converted to voltage. V
In choosing an output resistor, the max output voltage
must fi rst be considered. If the circuit that is driven by the
output does not have a limited input voltage, then R
must be chosen such that the max output voltage does
not exceed the LTC6102 max output voltage rating. If the
following circuit is a buffer or ADC with limited input range,
then R
than the allowed maximum input range of this circuit.
In addition, the output impedance is determined by R
the circuit to be driven has high enough input impedance,
then almost any output impedance will be acceptable.
However, if the driven circuit has relatively low input imped-
ance, or draws spikes of current, such as an ADC might
do, then a lower R
preserve the accuracy of the output. As an example, if the
input impedance of the driven circuit is 100 times R
then the accuracy of V
Error Sources
The current sense system uses an amplifi er and resistors
to apply gain and level shift the result. The output is then
dependent on the characteristics of the amplifi er, such as
gain and input offset, as well as resistor matching.
Ideally, the circuit output is:
In this case, the only error is due to resistor mismatch,
which provides an error in gain only.
V
V
OUT
OUT
OUT
=
=
=
must be chosen so that I
I
I
V
OUT
OUT
SENSE
•
•
R
R
R
OUT
OUT
•
OUT
OUT
R
R
OUT
OUT
OUT
IN
value may be required in order to
•
+
•
100
, determines how the output cur-
101
R
R
;
IN DRIVEN
will be reduced by 1% since:
V
IN DRIVEN
SENSE
(
(
=
OUT
0 99
.
is simply I
OUT(MAX)
=
)
)
•
R
I
OUT
SENSE SENSE
OUT
•
• R
IN
R
•
OUT
OUT
I
OUT
should be
SENSE
• R
is less
OUT
OUT
OUT
OUT
6102f
(or
. If
IN
.
,