Datasheet

LT4180
7
4180fb
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operaTion
Voltage drops in wiring can produce considerable load
regulation errors in electrical systems (Figure 1). As
load current, I
L
, increases the voltage drop in the wiring
(I
L
• RW) increases and the voltage delivered to the sys-
tem (V
L
) drops. The traditional approach to solving this
problem, remote sensing, regulates the voltage at the load,
increasing the power supply voltage (V
OUT
) to compensate
for voltage drops in the wiring. While remote sensing
works well, it does require an additional pair of wires to
measure at the load, which may not always be practical.
The LT4180 eliminates the need for a pair of remote sense
wires by creating a Virtual Remote Sense. Virtual remote
sensing is achieved by measuring the incremental change
in voltage that occurs with an incremental change in current
in the wiring (Figure 2). This measurement can then be
used to infer the total DC voltage drop in the wiring, which
can then be compensated for. The Virtual Remote Sense
takes over control of the power supply via the feedback
pin (V
FB
) of the power supply maintaining tight regulation
of load voltage, V
L
.
The LT4180 operates by modulating the output current of
the regulator and looking at the resulting voltage change.
A
large output capacitor is placed across the load so the
AC impedance at the load is low. [Normally, a capacitor
appears across the load in remote sensing situations to
keep the impedance low at that point]. This capacitor is
large enough that the AC impedance at the load is very low
compared to the line resistance. When the output current
is modulated, any voltage change that appears across the
terminals of the LT4180 is due to the resistance in the line
since the AC resistance at the load is very low.
There are four sample-and-hold capacitors in the LT4180.
The operation cycles through several stages to obtain the
correction voltage. First, the output voltage is regulated
and the control point is sampled and held. The control
loop is then switched to a current regulating control loop
and the output current is changed by 10%. Tw o sample-
and-hold currents store the voltage at the high current and
low current level of the modulation. This voltage change
is the result of a 10% change in current, making the volt-
age change 10% of the total drop in the line. The voltage
change
is amplified by a factor of 10.
The amplified voltage change that occurs with the current
is again sampled and held and is used as the correction
voltage. The correction voltage is summed into the output
and this corrects for the line drop. Since this correction
is actually open-loop, the actual voltage at the load is not
measured. The ability of the LT4180 to correct for line
drops is dependent upon the accuracy of the computations.
The LT4180 can correct better than 50 to 1 for line drops.
For example, a 10V drop in the line becomes a 200mV
change at the load.
The frequency of the correction cycle can be set from over
32kHz down to less than 250Hz, depending on the size of
the capacitors in the system. For very large capacitors in
high current systems, the dither correction clock would be
run more slowly. In simpler systems with smaller output
capacitors, the dither can be run at a higher frequency. If
the load contains frequencies similar to the dither, beat
notes can result between the load and the LT4180. A
spread spectrum option on the LT4180 allows the device
to change phasing during the correction cycle
so that it
will not inter
fere with load pulses.
Finally, the LT4180 takes into account all resistances
between the LT4180 and the load capacitor. It can correct
for cable connections, line resistances and varying contact
resistances. By measuring the peak change at the output of
the LT4180 one can monitor the impedance between the
LT4180 and the load, and detect increasing impedances
Figure 1. Traditional Remote Sensing Figure 2. Virtual Remote Sensing
4180 F02
POWER WIRING
I
L
VIRTUAL REMOTE
SENSE
POWER SUPPLY
+
–
RW
V
OUT
V
FB
SYSTEM
+
–
V
L
4180 F01
POWER WIRING
I
L
REMOTE SENSE WIRING
POWER SUPPLY
+
–
RW
V
OUT
SYSTEM
+
–
V
L