Datasheet

LT6110
19
6110fa
For more information www.linear.com/LT6110
applicaTions inForMaTion
To create this current at full load requires an R
IN
value of
V
SENSE
/I
IOUT
, 40mV/80µA, or 500Ω. Using the nearest
standard 1% tolerance value of 499Ω will be sufficient.
Without considering any error terms other than this slight
change in value for R
IN
results in nearly perfect cable drop
compensation. The theoretical load regulation would be
improved from 15% to less than 0.01%.
The single largest source of compensation error comes
from any change in the connecting wire resistance from the
design assumptions. This could be caused by temperature,
aging and possibly corrosion. In the compensator circuit,
component tolerances and errors terms will combine to
deviate from the near perfect designed amount of com
-
pensation. Figure 7 shows this simple example design and
indicates the various error sources within the LT6110. All
of the error terms can be determined from the Electrical
Characteristics Table. The error terms for any compensa
-
tor design include:
•
R
SENSE
tolerance
• R
IN
tolerance
• R
F
tolerance
• V
OS
, the offset voltage in µV of the internal current
sense amplifier
• ∆V
OS
/∆I
+IN
is an error term caused by the finite gain
of the current sense amplifier.
This is the change in the offset voltage
as the sense
voltage
and resulting input current varies from 0 to the
maximum value. It is a factor specified in mV/mA which
is ohms and is accounted for as a small resistance in
series with R
IN
. The voltage across this small resistance
is included in the total offset voltage term. The change
in I
+IN
current is relative to 100µA where the LT6110
is trimmed for accuracy.
• ∆V
OS
/∆V
IOUT
is a change in the offset voltage caused
by a change in the voltage applied to the IOUT pin
specified in mV/V. The change in V
IOUT
is relative to
1.2V DC where the LT6110 is trimmed for accuracy.
• ∆V
OS
/∆V
IMON
is a change in the offset voltage caused
by a change in the voltage applied to the IMON pin
specified in mV/V.
• IOUT current error is the accuracy of the internal current
mirror. This is a percent deviation from I
+IN
.
• IMON current error is the accuracy of the total internal
mirror current sourced to the IMON output. This is a
percent deviation from 3 • I
+IN
.
• Temperature Related Errors (see Temperature Errors
section)
Table 1 is an example of the
stack-up of all error terms in
the design of Figure 7. This table uses typical variances to
be seen at 25°C. It is not a rigorous worst case analysis
over all possible operating conditions, but instead serves
to illustrate what to expect for load regulation improvement
under nominal conditions.
In this example, including all typical error terms, the LT6110
still provides a factor of 10 improvement in voltage regula
-
tion at
the remote load. To
obtain the same level of load
voltage stability without using the LT6110 would require
reducing the amount of cable drop loss. The easiest way
to do so would be to increase the wire gauge used to
connect to the load. For a 76mV change in load voltage
at 2A full load current would require a wire resistance
of only 38mΩ and a 6 foot length 18AWG gauge wire is
required. A larger wire gauge can be significantly more
costly and is less flexible in routing to the load. These are
two significant design compromises to be considered.