Datasheet

LT6110
13
6110fa
For more information www.linear.com/LT6110
applicaTions inForMaTion
Step 2: Determine the resistor on the +IN pin, R
IN,
required
to cancel V
DROP
.
The regulator output voltage will increase as current is
pulled from the IOUT pin through the feedback resistor,
R
F
, creating a compensation voltage.
V
COMP
= I
IOUT
• R
F
To cancel the voltage drop at the load, set V
COMP
equal
to V
DROP
.
V
COMP
= I
IOUT
• R
F
= V
DROP
Since the IOUT current is equal to the current going into
the +IN pin and the current in the +IN pin is equal to the
sense voltage divided by R
IN
, R
IN
can be determined by
the following equations:
I
IOUT
=I
+IN
=
V
SENSE
R
IN
where V
SENSE
= I
LOADMAX
• R
SENSE
Combining the above equations,
R
IN
= I
LOADMAX
• R
SENSE
( )
•
R
F
V
DROP
R
IN
= 2A • 0.02Ω
( )
•
3.65k
0.54V
= 270
Ω
Step 3: The final step is to consider the errors in the
compensation circuit to determine if the resulting voltage
error at the load meets the desired performance.
For example, the internal R
SENSE
of the LT6110 has a typical
tolerance of ±7.5%. If the other errors in the compensation
circuit such as V
OS
, IOUT current error and the resistor
tolerances of R
F
and R
IN
add an additional ±2.5% error,
then the total error in the compensation circuit would be
±10% resulting in a voltage error at the load of the following:
V
LOADERROR
= V
COMP
• Compensation Error
V
LOADERROR
= 0.54V • (±10%) = ±0.054V
A 10× improvement.
If this is not adequate for the given application, steps can
be taken to reduce the sources of error, such as using an
external sense resistor with a tighter tolerance. See the
section on External Current Sense Resistors for more
information.
In most cases, the internal sense resistor, wire resistance
tolerances and temperature mismatch of the R
SENSE
and
R
WIRE
resistances will contribute the largest portion of the
overall compensation circuit error. See the sections on
Error Sources, Copper Wire Information and Temperature
Errors for a comprehensive discussion.
ADDITIONAL DESIGN CONSIDERATIONS
IOUT Current
The recommended range of IOUT current
is 30µA ≤ I
IOUT
≤ 300µA for the best precision. For performance outside
of this range, see the Typical Performance Curves to
determine typical errors.
If the IOUT current is less than 30µA, the feedback resis
-
tor may
need to be adjusted to reduce the error in the
compensation cir
cuit.
In the previous example,
I
IOUT
=
V
SENSE
R
IN
=
0.04
270
= 148µA
Since this is within the recommended range no further
adjustment is needed.
See the section on Compensating a Low Quiescent Current
Design for IOUT current less than 30µA.
Load Regulation
Load regulation is often specified as an error in output
voltage at a given load current, as in the previous example,
but it is also specified as a percentage of the regulator
output voltage. If the output voltage of the regulator circuit
in Figure 2 is 5V, the resulting compensated load regula
-
tion, in percent, would be the following:
LoadReg
COMP
%
( )
=
V
LOADERROR
V
REG
• 100
LoadReg
COMP
%
( )
=
±0.054V
5V
• 100 = ±1.1
%