Datasheet

LT6110
11
6110fa
For more information www.linear.com/LT6110
applicaTions inForMaTion
INTRODUCTION
The LT6110 provides a simple and effective solution to
a common problem in power distribution. When a load
draws current through a long or thin wire, wire resistance
causes an IR drop that reduces the voltage delivered to
the load. A regulator IC cannot detect this drop without a
Kelvin sense at the load, which requires a multi-conductor
wire that is not supported in some applications.
The LT6110 detects the load current and sets a propor
-
tional current at an output that can be used to control the
output voltage of an adjustable regulator to compensate
for the drop in the wire.
The accuracy and wide output current range of the LT6110
allow it to compensate for either small or large voltage drops
to a high degree of precision. The LT6110 can sense the
load current with its internal sense resistor or an external
sense resistor can be used to improve accuracy and handle
currents greater than 3A. Resistor-programmable gain
gives substantial flexibility to the compensation circuit. A
signal bandwidth of 180kHz enables fast response time
to load changes and provides good loop characteristics
so that the power supply circuit remains stable.
The
LT6110 requires
that the resistance of the wire be
known. However, that resistance does not have to be very
accurate for the LT6110 to provide good compensation
since the regulation at the load is the product of the error
due to the wire resistance and the error in the LT6110
compensation circuit.
For example, a 5V regulator circuit has 10% regulation at
the load due to a wire resistance drop of 0.5V. Even if the
wire resistance doubled, causing an error in the LT6110
compensation circuit of 50%, the regulation at the load
is still reduced to 10% • 50% = 5%.
For systems that are better controlled, the load regula
-
tion can be improved to far exceed that possible without
the LT6110. As an example, for a known wire resistance,
and with an external 1% sense resistor, the same 10%
load regulation in the previous example can be reduced
to less than 0.5%.
The LT6110 has two output pins, IOUT and IMON. Either
pin may be used to provide a current that is proportional to
the load current. The IOUT pin provides a sinking current
to compensate regulators with a ground referred voltage-
reference, such as the LT3980. The IMON pin provides a
sourcing current to
compensate regulators with an output
referred
reference like the LT1083 and current-referenced
regulators like the LT3080. As an added feature, the output
current from either pin can be converted to a voltage via a
simple resistor, creating a voltage that is also proportional
to load current. This voltage may be used to measure
or monitor the load current. Either or both pins may be
used for regulator control, and either or both pins may
be used for monitoring, allowing substantial flexibility in
system design.
THEORY OF OPERATION
The outputs of the LT6110 are proportional to a sense
voltage, V
SENSE
, developed across an internal or external
sense resistor, R
SENSE
(see Figure 1).
A sense amplifier loop forces +IN to the same voltage as
–IN. Connecting an external resistor, R
IN
, between V
+
and
+IN forces a voltage across R
IN
equal to V
SENSE
, creating
a current into +IN, I
+IN
, equal to V
SENSE
/R
IN
. This current
is precisely mirrored to IOUT. The emitter currents of
the three transistors in the mirror are combined to form
the IMON output current. Ideally, the IOUT sink current
is equal to I
+IN
and the IMON source current is equal to
three times I
+IN
.
V
+
and V
–
The LT6110 is designed to operate with a supply voltage
(V
+
to V
–
) up to 50V. However, when using a supply volt-
age greater than 36V, additional care must be taken not
to
exceed the absolute maximum ratings. The V
+
to IOUT
voltage must be kept less than 36V to avoid the breakdown
of internal transistors.
The V
+
pin needs to be bypassed with at least a 0.1µF
capacitor placed close to the pin.