LTC6101HV Linear Technology, LTC6101HV Datasheet - Page 11

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

APPLICATIONS INFORMATION
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 limit the output voltage, then R
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
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
in order to preserve the accuracy of the output. As an
example, if the input impedance of the driven circuit is
100 times R
by 1% since:
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. However, offset
voltage, bias current and fi nite gain in the amplifi er cause
additional errors:
OUT
V
V
OUT
OUT
must be chosen so that I
=I
= V
=I
OUT
OUT
OUT
SENSE
, then the accuracy of V
•
• R
R
R
OUT
OUT
•
OUT
R
R
OUT
OUT
IN
+R
•
• R
100
101
, determines how the output cur-
IN(DRIVEN)
;V
IN(DRIVEN)
SENSE
= 0.99 •I
OUT(MAX)
OUT
OUT
=R
value may be required
is simply I
OUT
SENSE
OUT
• R
OUT
OUT
• R
will be reduced
•I
OUT
OUT
OUT
is less than
SENSE
• R
must be
OUT
OUT
.
.
Output Error, E
Voltage, V
The DC offset voltage of the amplifi er 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
B
SENSE
(+) ≈ I
B
LOAD
OS
OUT(IBIAS)
(–)
V
+
= V
= R
≈ –R
= ± R
Figure 4. Second Input R Minimizes
Error Due to Input Bias Current
OUT
OUT
B
BIAS
LTC6101/LTC6101HV
R
R
(–) = I
OS
IN
IN
OUT
B
B
, Due to the Amplifi er DC Offset
, Due to the Bias Currents,
–
+
IN
(+) fl ows into the positive input of the
(–) fl ows into the negative input.
OUT
• (R
OUT
is 100nA and R
+IN
(+) = (R
V
SENSE
((I
≈ 0. In applications where R
–
BIAS
R
• I
OUT
IN
B
• I
LTC6101
(+) • (R
+
BIAS
=
OS
, if R
R
/R
+
that cancels the error due to
IN
IN
; I
–
SENSE
IN
–
OS
R
– R
)
SENSE
SENSE
SENSE
SENSE
–
= I
SENSE
. This is the dominant
OUT
B
(+) – I
<< R
/R
≈ R
is 1kΩ, the output
) is connected as
IN
–IN
V
OUT
IN
6101 F04
+
) – I
IN
, I
B
then,
(–)
B
R
B
(+) causes
OUT
(–))
11
SENSE
V
OUT
6101fg
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