1. Linear Technology Corporation
LT4180 Backgrounder
Tony Armstrong Director of Product Marketing

2. LT4180 Virtual Remote Sense Controller
Background
Limitations of Remote Voltage Sensing w/ Wires
Advantages of Virtual Remote Sensing w/o Wires
LT4180
Definition
Features & Benefits
How Does it Work
How Well Does it Work
Application Circuits
Target Markets
Target Applications

3. Remote Sense Background
Why Remote Sense?
Voltage drop (IR Losses) in cables and wiring
IR losses can cause the regulation at the load to exceed its limit
Remote sense with 2 wires works but has limitations
Remote Sense Wire Limitations
Extra wires can be impractical
Less reliable
Higher cost
Not available in 2-wire cables
Voltage drops in wiring and cables can cause considerable load regulation errors in electrical systems. As the load current increases, the voltage drop (IR losses) in the wiring increases (due to the line impedance) and the voltage delivered to the system drops. The traditional approach to solving this problem is to sense the output voltage with a pair of wires that run from the regulator’s feedback network to the load. While this approach works well, the additional pair of wires may not be practical or reliable, adds cost and is not possible in 2-wire cables.

4. LT4180: Virtual Remote Sense – What is it?
The LT4180 virtual remote sense (VRS) controller is a new revolutionary way of eliminating the need for remote sense wires to compensate for the IR losses in cables and wiring. This device continuously interrogates the line impedance and corrects the power supply output voltage to maintain a steady voltage at the load regardless of current changes. VRS works with a small modulation on the output of the regulator to compute the correction. This modulation is filtered by an output capacitor at the load. The LT4180 works with any conversion topology, isolated or non-isolated power supplies, DC/DC converters including bricks, modules and adjustable linear regulators. The Drain pin from the LT4180 controls the feedback loop of the regulator.

5. LT4180: VRS Block Diagram and Waveform
This simplified block diagram is shown with waveforms of how the LT4180 reacts to a 500mA step-load over a cable with a 2.4Ω line impedance with an output voltage of 5V. Without VRS, the voltage at the load changes 1.2V (25% of the desired output voltage). With VRS, the voltage change at the load is very minimal.
NOTE: Voltage at the load is 5V

6. LT4180 Virtual Remote Sense Features
Tight Load Regulation with Highly Resistive Cables without Requiring Remote Sense Wires
Compatible with all Conversion Topology’s including Isolated and Non-Isolated Power Supplies
VIN Range: 3V to 50V
Wide VOUT Range: 1.21V to 50V
5mA Sink Current Capability to Drive Opto-Coupler
Accurate ±1% Internal Reference Voltage
Soft-Correct Reduces Turn-On Transients
Undervoltage and Overvoltage Protection
Programmable or Synchronizable Dither Frequency
Optional Spread Spectrum Dither Frequency
The LT4180 maintains a tight regulation at the load over highly resistive cables. It is compatible with any conversion topology and can operate over an input voltage ranging from 3V to 50V and can produce an output voltage from 1.21V to 50V. There is a 5mA sink output that is powerful enough to drive an opto-coupler. Additional features include a programmable fixed dither frequency or spread spectrum frequency, along with soft-start.

7. LT4180: How Does it Work?
LT4180 forces a current delta of ±5% of the output current
Square wave current at a programmable dither frequency
A decoupling capacitor filters out dither current at the load and is sized for low impedance (AC short) at the programmed dither frequency
This results in a voltage equal to 1/10 the drop in the cable
The LT4180 adjusts the regulation voltage locally at the regulator output to compensate for the cable drop. This assures tight regulation at the other end of the cable, where the load is.
The LT4180 forces the output current to change from 95% and 105% of the nominal output current at a pre-determined dither frequency. In other words, the LT4180 forces a square wave current with a peak-to-peak amplitude of ±5% on top of the regulators output current. A decoupling capacitor filters out the dither current from the VRS square wave at the load. The decoupling capacitor is sized to produce an “AC short” at the square wave frequency, resulting in a voltage square waveform produced at the power supply with a peak-to-peak amplitude equal to 1/10 of the line impedance. This direct measurement of the voltage drop across the wiring is monitored and the LT4180 corrects the feedback loop of the regulator to provide accurate load regulation.
Ohm’s law is represented by Voltage = Current x Resistance (V=IR)
And ∆I is ±5%
Then ∆ V = ILOAD X ZL

8. LT4180 – How Well Does it Work?
This curve shows the static load regulation of the LT4180. The load current is increased from zero until it produces a 2.5V drop in a cable. The load voltage dropped only 73mV at the maximum current with the LT4180 continuously interrogating the line impedance and correcting the regulators output voltage to less than ±1% (With a 5V Output). Less dramatic wiring drops produce even better results.

9. LT4180: Typical Buck Application Schematic
This schematic shows the LT4180 controlling a monolithic buck converter.

10. LT4180: Typical Isolated Application Schematic
This circuit shows the LT41480 driving an Opto-coupler in an isolated application.

11. LT4180: Typical Boost (5V12V) Applications Schematic
The LT4180 is shown in a boost application.

12. LT4180: Package
The LT4180 is available in a SSOP-24 package.

13. LT4180 Typical Applications
CAT5 & CAT6 Cable Systems
Wall Adaptors
Notebook Computer Power
Remote Surveillance Equipment
Halogen Lighting
Well-Logging
Remote Instrumentation
Battery Charging
PDA’s
The LT4180 opens up opportunities previously unavailable in battery charging, notebook computers, PDA’s, portable entertainment devices, industrial, and well-logging applications. VRS allows low voltage devices (with a high operating current) to operate over a CAT5 and CAT6 cables. High-efficiency lighting benefit from VRS because the power consumption, light output, and lifetime are severely affected by the applied voltage. VRS can be used in remote instrument applications where the instrumentation is often connected by cables several thousand feet away.

14. LT4180: Conclusion
LT4180 is a revolutionary new way to compensate for wiring and cable voltage drops
Limitless applications
Works with any conversion topology
Numerous features & benefits
The LT4180 is the first device to use virtual remote sensing to provide accurate load voltage regulation over highly resistive wires and cabling without the use of remote sense wires. Virtual remote sense provides alternatives previously unavailable to design engineers for simplifying or improving designs. The LT4180 works with any power supply or regulator: switching or linear, isolated or non-isolated making the possible applications limitless.