LT6108-1/LT6108-2
14
610812fa
APPLICATIONS INFORMATION
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
as a resistor divider which has a voltage tap going to the
comparator input to set the comparator threshold.
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 impedance
of that circuit must be considered. If the subsequent circuit
has high enough input impedance, then almost any use-
ful 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
•
R
OUT
R
IN
; 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:
Output Voltage Error, ∆V
OUT(VOS)
, Due to the Amplifier
DC Offset Voltage, V
OS
∆V
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
. As V
SENSE
is increased,
accuracy improves. This is the dominant error of the system
and it limits the available dynamic range.
Output Voltage Error, ∆V
OUT(IBIAS)
, Due to the Bias
Currents I
B
+
and I
B
–
The amplifier bias current I
B
+
flows into the SENSELO pin
while I
B
–
flows into the SENSEHI pin. The error due to I
B
is the following:
∆V
OUT(IBIAS)
= R
OUT
I
B
+
•
R
SENSE
R
IN
–I
B
–
Since I
B
+
≈ I
B
–
= I
BIAS
, if R
SENSE
<< R
IN
then,
∆V
OUT(IBIAS)
= –R
OUT
(I
BIAS
)
It is useful to refer the error to the input:
∆V
VIN(IBIAS)
= –R
IN
(I
BIAS
)
For instance, if I
BIAS
is 100nA and R
IN
is 1k, the input re-
ferred error is 100µV. This error becomes less significant
as the value of R
IN
decreases. The bias current error can
