LT6119-1/LT6119-2
14
611912f
For more information www.linear.com/LT6119-1
applicaTions inForMaTion
Selection of External Input Gain Resistor, R
IN
R
IN
should be chosen to allow the required speed and
resolution while limiting the output current to 1mA. The
maximum value for R
IN
is 1k to maintain good loop sta-
bility. For a given V
SENSE
, larger values of R
IN
will lower
power dissipation in the LT6119 due to the reduction
in I
OUT
while smaller values of R
IN
will result in faster
response time due to the increase in I
OUT
. If low sense
currents must be resolved accurately in a system that has
a very wide dynamic range, a smaller R
IN
may be used
if the maximum I
OUTA
current is limited in another way,
such as with a Schottky diode across R
SENSE
(Figure 4).
This will reduce the high current measurement accuracy
by limiting the result, while increasing the low current
measurement resolution.
as a resistor divider which has voltage taps going to the
comparator inputs to set the comparator thresholds.
In choosing an output resistor, the maximum output volt
-
age must first be considered. If the subsequent circuit is a
buffer
or ADC with limited input range, then R
OUT
must be
chosen so that I
OUTA(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 another circuit is being driven, then the input imped-
ance of
that circuit must be considered. If the subsequent
cir
cuit has high enough input impedance, then almost any
useful output impedance will be acceptable. However, if
the subsequent circuit has relatively low input impedance,
or draws spikes of current such as an ADC load, then a
lower output impedance may be required to preserve the
accuracy of the output. More information can be found
in the Output Filtering section. As an example, if the input
impedance of the driven circuit, R
IN(DRIVEN)
, is 100 times
R
OUT
, then the accuracy of V
OUT
will be reduced by 1%
since:
V
OUT
= I
OUTA
•
OUT
IN(DRIVEN)
R
OUT
+ R
IN(DRIVEN)
= I
OUTA
• R
OUT
•
100
101
= 0.99 •I
OUTA
• R
OUT
Amplifier Error Sources
The current sense system uses an amplifier and resistors
to apply gain and level-shift the result. Consequently, the
output is dependent on the characteristics of the amplifier,
such as gain error and input offset, as well as the matching
of the external resistors.
Ideally, the circuit output is:
V
OUT
= V
SENSE
•
OUT
R
; V
SENSE
= R
SENSE
• I
SENSE
In this case, the only error is due to external resistor
mismatch, which provides an error in gain only. However,
offset voltage, input bias current and finite gain in the
amplifier can cause additional errors:
D
SENSE
R
SENSE
V
LOAD 611912 F04
Figure 4. Shunt Diode Limits Maximum Input Voltage to Allow
Better Low Input Resolution Without Overranging
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.
The power dissipated in the sense resistor can create a
thermal gradient across a printed circuit board and con
-
sequently a gain error if R
IN
and R
OUT
are placed such
that they operate at different temperatures. If significant
power is being dissipated in the sense resistor then care
should be taken to place R
IN
and R
OUT
such that the gain
error due to the thermal gradient is minimized.
Selection of External Output Gain Resistor, R
OUT
The output resistor, R
OUT
, determines how the output cur-
rent is converted to voltage. V
OUT
is simply I
OUTA
• R
OUT
.
Typically, R
OUT
is a combination of resistors configured
