Controller IC Eliminates Remote Sense Lines for Long Cable Runs

DESIGNfeature
SAM DAVIS, Editor in Chief
Controller IC Eliminates Remote
Sense Lines for Long Cable Runs
By sensing the current in
the cable line connecting
a power supply to a distant
load, a Virtual Remote Sense
IC feeds back a control voltage that adjusts the power
supply output to maintain an
accurate load voltage.
26
L
inear Technology’s LT4180 is a Virtual Remote Sense™ power supply controller that eliminates the remote sense connections used with
power converters to compensate for the voltage drop in lines going to
the output load. To understand use of the LT4180, we first have to
look at a power supply’s role in regulating voltage applied to a load,
which means it must obtain an accurate load voltage in order to maintain tight regulation. The LT4180 addresses various applications where
a high current load is located a long distance from the power supply, such as lighting
systems, remote instrumentation, and surveillance equipment.
In a typical system, lines that connect the power supply’s output to its load have
some resistance, so load current causes an unwanted voltage drop between the power
supply and its load. Therefore, this voltage drop produces an inaccurate output voltage
to be fed back to the supply for regulating the load voltage. For the tightest regulation,
the power supply should feed back an accurate load voltage. As shown in Fig. 1, many
power supplies have a remote sense function to eliminate inaccurate feedback voltage.
Connecting two remote sense lines (plus and minus) that draw virtually no current
from the power supply
to the load, the sup+
+V
ply can get an accurate
–
–V
load voltage as feedPOWER
LOAD
SUPPLY
back for load voltage
+S
regulation. For proper
–S
operation, steps should
be taken to ensure the
SHIELD
remote sense wires do
Fig. 1. Conventional remote sense for a power supply employs two sense lines not pick up noise by
(plus and minus) to obtain an accurate load voltage as feedback for load volt- either twisting the two
sense wires together
age regulation.
and/or shielding them
from noise. Also, you
LT4180 FEATURES
must observe the correct polarities with the
tTight Load Regulation without Remote Sense Wires
+sense line connected
tCompatible with Isolated & Non-isolated Power Supplies
to the +V load and the
tWorks with any topology
–sense line connected
tVIN Range from 3V to 50V
to the –V load.
tAbility to drive opto-coupler
Virtual
Remote
tLow turn-on transients
Sense (VRS) eliminates
tUndervoltage and overvoltage protection
the need for the pair of
tSpread spectrum modulation dither
remote sense wires. The
t±1% internal reference voltage over temperature
basic VRS measures the
tSSOP-24 package
incremental change in
Power Electronics Technology
y | Mayy 2010
www.powerelectronics.com
Fig. 2. Simplified virtual remote
sensing circuit measures the
incremental change in power
supply output current that
occurs with an incremental
change in the output load.
CAT5E
A
CABLE
RSENSE
+
LINE
CL
RL
LINE
SWITCHING
REGULATOR
A
VC
VIN SENSE DIV0 DIV1 DIV2 SPREAD CHOLD1 CHOLD2 CHOLD3 CHOLD4
L
LT4180
DRAIN
ROSC
VIRTUAL REMOTE SENSE
COMP
COSC
RUN FB
OV
–
of the peaks and prevents beat notes if the
load is pulsed at a frequency close to the
interrogation cycle.
SENSING LINE CURRENT
voltage that occurs with an incremental change in current in
the output voltage wiring (Fig. 2). The VRS circuit uses this
measurement to compute and compensate the voltage drop
in the line from the power supply to the load.
In the application shown in Fig. 3, The LT4180 continuously interrogates the line impedance and corrects the
regulator’s output voltage. The IC maintains a corrected,
regulated voltage at the load regardless of current or line
impedance. Virtual Remote Sense takes over control of the
power supply via its feedback pin (FB) that drives the optocoupler. The VRS circuit maintains tight regulation of load
voltage at RL.
Virtual remote sensing relies on sampling techniques to
hold the different voltages during a correction cycle. Because
switching power supplies are commonly used or loads may
be pulsed, the LT4180 uses a variety of techniques to
minimize potential interference in the form of beat notes
that may occur. Besides several types of internal filtering,
and the option for VRS/power supply synchronization, the
LT4180 also provides spread spectrum operation on the
dither. Spread spectrum techniques dither the interrogation frequency from cycle to cycle. With spread spectrum,
low modulation index pseudo-random phasing is applied to
Virtual Remote Sense timing. This lowers the average value
VFB
–
TRACK IOUT
REGULATE IOUT LOW
TRACK ΔVOUT
RW
VOUT
VOUT
REGULATE VOUT
TRACK VOUT HIGH
TRACK VOUT LOW
IL
POWER SUPPLY
L +
The voltage drop across
RSENSE is proportional to line current, IL. Select the value of
RSENSE so that it produces a 100mV voltage drop at maximum load current. For best accuracy, the VIN and SENSE
pins should be Kelvin-connected to RSENSE.
Four track/hold circuits capture and hold input voltages obtained from RSENSE (due to line current) at times
determined by division ratio of the spread spectrum clock.
The division ratio depends on the 3-bit code set by DIV2,
DIV1 and DIV0. The track/hold circuits include a switch
and a storage capacitor. In the track mode, the switch closes
thereby coupling the input signal to the specific CHOLD(1,
2, 3, or 4) capacitor, allowing the storage capacitor to track
the RSENSE input signal. In the hold mode, the switch
opens, isolating the storage capacitor from the input signal
and allowing it to hold the amplitude value of the input
signal constant.
+
POWER WIRING
P
VL
SYSTEM
–
Fig. 4. Timing diagram for virtual remote sensing begins when the power supply
and VRS close the loop around VOUT.
REGULATOR
VIRTUAL REMOTE
SENSE
+
–
ITH OR
VC
LT4180
DRAIN
Fig. 5. Working with a nonisolated power supply, the
LT4180’s drain pin controls
virtual remote sensing.
Fig. 3. Typical application of the LT4180 provides virtual remote sense for a
flyback converter connected to a distant load (RL).
www.powerelectronics.com
Mayy 2010 | Power Electronics Technology
27
CONTROLLERICs
REGULATOR
+
VC
INTVCC
O
OPTOCOUPLER
LT4180
–
DRAIN
Fig. 7. If the power supply’s control pin, VC,
exceeds 5V, a cascode
circuit can be used to
keep the LT4180’s DRAIN
pin below 5V.
TO VC > 5V
COMP
LT 4180
INTVCC
DRAIN
Fig. 6. The LT4180 employs an opto-coupler when working with an isolated power
supply.
Fig. 4 shows the timing diagram for virtual remote sensing
(VRS). A new cycle begins when the power supply and VRS
close the loop around VOUT (regulate VOUT = H). Both VOUT
and IOUT slew and settle to a new value, and these values are
stored in the track-and-hold capacitor (track VOUT high =
L and track IOUT = L). The VOUT feedback loop is opened
and a new feedback loop is set up commanding the power
supply to deliver 90% of the previously measured current
(0.9IOUT). VOUT drops to a new value as the power supply
reaches a new steady state, and this information is also stored
For 40 years, AAC has provided fast
response to requests for compact,
mission-critical current, voltage,
power, and frequency transducers.
Let us deliver the same timely, highly reliable
performance in designing and manufacturing
standard or custom transducers for you,
for a variety of applications:
#5
+51
t Space, Aircraft, Land VVehicles, Shipboard
t Rail Transit
t Industrial Controls
t Military and
Commercial C Grade
#OGTKECP#GTQURCEG%QPVTQNU
570 Smith Street Farmingdale, NY 11735
Toll Free: (888) 873-8559
Tel: (631) 694-5100 T
T
Fax: (631) 694-6739 / (631) 845-0766
Email: [email protected] www.a-a-c.com
28
Power Electronics Technology
y | Mayy 2010
in the track/hold capacitor. At this point, the change in output voltage (ΔVOUT) for a –10% change in output current
has been measured and is stored in the track/hold capacitor.
It is amplified by a gain of 10 to obtain the total correction to
the output. The four track/hold circuits store the amplitude
values in sequence to capacitors at CHOLD1, CHOLD2,
CHOLD3, and CHOLD4. During the next VRS cycle, the
total voltage is fed back to the power supply, which adjusts
the voltage at the load to compensate for voltage drops due
to line resistance.
To insure orderly start-up, the LT4180 has a soft-correct function. When the RUN pin rising threshold is first
exceeded (indicating VIN has crossed its undervoltage lockout threshold), power supply output voltage is set to a value
corresponding to zero wiring voltage drop (no correction for
wiring). Over a period of time (determined by CHOLD4),
the power supply output voltage ramps up to account for
wiring voltage drops, optimizing load-end voltage regulation. A new soft-correct cycle is also initiated whenever an
overvoltage condition occurs.
The LT4180 will work with either isolated (with an opto)
or non-isolated power supplies. A variety of power supplies
and regulators having either an external feedback or control
pin can be used with the LT4180. Tying the supply’s existing inverting input to ground disables its error amplifier (Fig.
5). This converts the error amplifier into a simple constantcurrent source with the output voltage then controlled by
the LT4180’s drain pin. This method eliminates the regulator error amplifier from the control loop so the control loop
with VRS in the LT4180 can control the output.
Isolated power supplies and regulators may also be used
by adding an opto-coupler (Fig. 6
6). LT4180 internal regulator (INTVCC) supplies power to the opto-coupler LED. In
situations where the control pin VC of the regulator may
exceed 5V, a cascode may be added to keep the DRAIN pin
of the LT4180 below 5V (Fig. 7).
7
Three temperature grades are available. They include an
extended grade version from -40 to 85°C, an industrial grade
version from -40°C to 125°C and a military grade option
from -55°C to 125°C. The LT4180 comes in an SSOP-24
package.
www.powerelectronics.com
Similar pages