LT6109IMS-1#TRPBF Linear Technology, LT6109IMS-1#TRPBF Datasheet - Page 13

• Manufacturer Part Number: LT6109IMS-1#TRPBF
• Manufacturer: Linear Technology

Specifications of LT6109IMS-1#TRPBF

• Lead Free Status / Rohs Status: Compliant

APPLICATIONS INFORMATION
to place R
thermal gradient is minimized.
Selection of External Output Resistor, R
The output resistor, R
rent is converted to voltage. V
Typically, R
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
chosen so that I
maximum input range of this circuit.
In addition, the output impedance is determined by R
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
accuracy of V
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
V
OUT
OUT
= I
= I
= V
IN
OUTA
OUTA
OUT
SENSE
and R
OUT
is a combination of resistors configured
OUTA(MAX)
•R
•
will be reduced by 1% since:
R
OUT
R
•
IN(DRIVEN)
OUT
R
OUT
OUT
R
OUT
OUT
IN
such that the gain error due to the
•
+R
•R
, determines how the output cur-
100
101
; V
• R
IN(DRIVEN)
IN(DRIVEN)
SENSE
, is 100 times R
OUT
= 0.99 •I
OUT
is less than the allowed
is simply I
= R
OUTA
SENSE
OUT
OUT
•R
OUTA
OUT
•I
SENSE
OUT
, then the
must be
• R
OUT
OUT
.
.
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
DC Offset Voltage, V
The DC offset voltage of the amplifier adds directly to the
value of the sense voltage, V
accuracy also improves. This is the dominant error of the
system and it limits the available dynamic range.
Output Voltage Error, ∆V
Currents I
The amplifier bias current I
while I
is the following:
Since I
It is useful to refer the error to the input:
For instance, if I
ferred error is 100µV. This error becomes less significant
as the value of R
be reduced if an external resistor, R
shown in Figure 5, the error is then reduced to:
Adding R
of the circuit as shown in Figure 5.
∆V
∆V
V
∆V
∆V
OUT(IBIAS)
OUT(IBIAS)
VIN(IBIAS)
OUT(VOS)
OUT(IBIAS)
B
B
–
+
IN
flows into the SENSEHI pin. The error due to I
≈ I
B
+
+
as described will maximize the dynamic range
B
and I
–
= ±R
= I
= –R
= V
= –R
= R
BIAS
IN
BIAS
B
OS
OUT
decreases. The bias current error can
–
OUT
LT6109-1/LT6109-2
IN
OUT
is 100nA and R
OS
•
, if R
(I
• I
R

 
BIAS
R
(I
OUT
I
OUT(VOS)
OUT(IBIAS)
OS
B
IN
BIAS
+
B
SENSE
SENSE
•
+
)
; I
R
flows into the SENSELO pin
OS
)
SENSE
R
IN
<< R
= I
. As V
, Due to the Amplifier
, Due to the Bias
B
IN
IN
+
–I
IN
SENSE
+
– I
is 1k, the input re-
, is connected as
B
then,
–

 
B
–
is increased,
13
610912f
B