?2007 Fairchild Semiconductor Corporation
www.fairchildsemi.com
FAN4010 " Rev. 1.0.6
7
Application Information
Detailed Description
The FAN4010 measures the voltage drop (V
SENSE
)
across an external sense resistor in the high-voltage
side of the circuit. V
SENSE
is converted to a linear current
via an internal operational amplifier and precision 100 ?
resistor. The value of this current is V
SENSE
/100 ?
(internal). Output current flows from the I
OUT
pin to an
external resistor R
OUT
to generate an output voltage
proportional to the current flowing to the load.
Use the following equations to scale a load current to an
output voltage:
V
SENSE
= I
LOAD
" R
SENSE
(1)
V
OUT
= 0.01 ?V
SENSE
?R
OUT
(2)
Figure 13.    Functional Circuit
Selecting R
SENSE
Selection of R
SENSE
is a balance between desired
accuracy and allowable voltage loss. Although the
FAN4010 is optimized for high accuracy with low V
SENSE
values,   a   larger   R
SENSE
  value   provides   additional
accuracy. However, larger values of R
SENSE
create a
larger   voltage   drop,   reducing   the   effective   voltage
available to the load. This can be troublesome in low-
voltage applications. Because of this, the maximum
expected load current and allowable load voltage should
be well understood. Although higher values of V
SENSE
can be used, R
SENSE
should be chosen to satisfy the
following condition:
10mV < 5I
SENSE
< 2005Z5I
(3)
For low-cost applications where accuracy is not as
important, a portion of the printed circuit board (PCB)
trace can be used as an RSENSE resistor. Figure 14
shows an example of this configuration. The resistivity
of a 0.1-inch wide trace of two-ounce copper is about
30 m?ft.   Unfortunately,   the   resistance   temperature
coefficient is relatively large (approximately 0.4%/癈), so
systems with a wide temperature range may need to
compensate for this effect. Additionally, self heating due
to load currents introduces a nonlinearity error. Care
must be taken not to exceed the maximum power
dissipation of the copper trace.
Figure 14.    Using PCB Trace for R
SENSE
Selecting R
OUT
R
OUT
can be chosen to obtain the output voltage range
required for the particular downstream application. For
example, if the output of the FAN4010 is intended to
drive an analog-to-digital convertor (ADC), R
OUT
should
be chosen such that the expected full-scale output
current produces an input voltage that matches the input
range of the ADC. For instance, if expected loading
current ranges from 0 to 1 A, an R
SENSE
resistor of 1 ?
produces an output current that ranges from 0 to 10 mA.
If the input voltage range of the ADC is 0 to 2 V, an R
OUT
value of 200 ?should be used. The input voltage and
full-scale output current (I
OUT_FS
) needs to be taken into
account when setting up the output range. To ensure
sufficient operating headroom, choose:
R
OUT
I
OU T
FS
such that
V
IN
V
SENSE
R
OUT
I
OU T
FS
> 1.65I
(4)
Output current accuracy for the recommended V
SENSE
between 10 mV and 200 mV are typically better than
1%. As a result, the absolute output voltage accuracy is
dependent on the precision of the output resistor.
Make sure the input impedance of the circuit connected
to V
OUT
is much higher than R
OUT
to ensure accurate
V
OUT
values.
Since   the   FAN4010   provides   a   trans-impedance
function, it is suitable for applications involving current
rather than voltage sensing.
R
OU T
3
6
1
I
O U T
L o a d
V
IN
V
s e n s e
O U T
1 0 0
R
s e n s e
+
-
V
IN
R
L o a d
R
S E N S E
IN P U T
LO A D
0.3in C O PPER
0.3in C O PPER
0.1in C O PPER
R
O U T
2
3
4
I
O U T
Load
V
IN
1
G N D
N C
O U T
6
5
N C