LTC6101HV Linear Technology, LTC6101HV Datasheet - Page 11

• Manufacturer Part Number: LTC6101HV
• Description: High-Side Current Sense Amplifier
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
Selection of External Output Resistor, R
The output resistor, R
current is converted to voltage. V
In choosing an output resistor, the max output voltage
must first be considered. If the circuit that is driven by the
output does not limit the output voltage, then R
be chosen such that the max output voltage does not
exceed the LTC6101 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
If the circuit to be driven has high enough input imped-
ance, then almost any useful output impedance will be
acceptable. 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
order to preserve the accuracy of the output. As an
example, if the input impedance of the driven circuit is 100
times R
1% since:
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:
In this case, the only error is due to resistor mismatch,
which provides an error in gain only. However, offset
voltage, bias current and finite gain in the amplifier cause
additional errors:
V
V
OUT
OUT
OUT
OUT
=
=
=
must be chosen so that I
I
I
V
, then the accuracy of V
OUT
OUT
SENSE
•
•
R
R
R
OUT
•
OUT
OUT
U
R
R
OUT
IN
OUT
•
+
•
100
101
R
R
U
;
IN DRIVEN
, determines how the output
V
IN DRIVEN
SENSE
(
OUT
(
=
0 99
OUT
value may be required in
.
OUT(MAX)
OUT
=
W
)
is simply I
)
•
R
I
OUT
SENSE SENSE
will be reduced by
OUT
•
• R
R
•
OUT
OUT
I
U
OUT
OUT
• R
is less
must
OUT
OUT
.
.
Output Error, E
Voltage, V
The DC offset voltage of the amplifier adds directly to the
value of the sense voltage, V
error of the system and it limits the available dynamic
range. The paragraph “Selection of External Current Sense
Resistor” provides details.
Output Error, E
I
The bias current I
internal op amp. I
Since I
For instance if I
error is 0.1mV.
Note that in applications where R
a voltage offset in R
I
R
external resistor R
shown in Figure 4 below. Under both conditions:
B
B
IN
(+) and I
(–) and E
E
E
E
E
, the bias current error can be similarly reduced if an
OUT(VOS)
OUT(IBIAS)
OUT(IBIAS)
OUT(IBIAS)
R
SENSE
B
(+) ≈ I
LOAD
B
OS
OUT(IBIAS)
V
(–)
+
Figure 4. Second Input R Minimizes
Error Due to Input Bias Current
= V
= R
≈ –R
= ± R
LTC6101/LTC6101HV
B
BIAS
OUT
OUT
R
R
(–) = I
OS
IN
IN
OUT
B
B
–
+
IN
(+) flows into the positive input of the
, Due to the Amplifier DC Offset
, Due to the Bias Currents,
(–) flows into the negative input.
OUT
• (R
OUT
is 100nA and R
(+) = (R
4
2
SENSE
((I
≈ 0. In applications where R
R
BIAS
IN
LTC6101
• I
OUT
B
• I
+
=
(+) • (R
BIAS
R
OS
+
IN
, if R
/R
that cancels the error due to
–
IN
; I
–
SENSE
IN
R
OS
SENSE
– R
)
SENSE
SENSE
SENSE
–
= I
SENSE
. This is the dominant
OUT
B
(+) – I
<< R
/R
≈ R
is 1kΩ, the output
6101 F04
) is connected as
IN
3
5
1
IN
) – I
IN
B
, I
R
(–)
then,
OUT
B
B
(+) causes
(–))
V
OUT
11
SENSE
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