Tighten Supply Regulation for 2A USB Devices by Dynamically
Compensating for Voltage Drops in Wiring and Connectors
Design Note 1029
Tom Hack
Introduction
These days, the Universal Serial Bus (USB) is commonly
used to power tablet computers and high rate cell phone
battery chargers—applications never envisioned at the
inception of the USB standard in the mid-1990s. The
USB standard has changed significantly over this time.
For instance, USB 3.0 requires up to 900mA (six, 150mA
unit loads) during high bandwidth communication. A
dedicated charging port can supply as much as 1.8A.
of 12V. To simplify the schematic, the protection circuitry
for load dump, reverse battery, 2-battery jump, spikes
and noise are not shown (consult Linear Technology’s
“Automotive Electronics Solutions” brochure for further
information.) The nominal switching frequency is set
above 455kHz to avoid interference in the IF of various
RF devices. It can be raised to 2MHz to avoid interfering
with AM and Travelers Information Station broadcasts,
but at the expense of some power supply efficiency.
Such high, and highly variable, load currents can
produce significant and unpredictable voltage drops
in wiring and connectors, lowering the performance
of the device. Virtual remote sensing senses losses in
the line in real time, automatically adapting to changes
in load current, line resistance, connector aging and
temperature variation. The result is improved voltage
regulation and increased device reliability.
This design corrects for total wiring and connector resistances of 0.1Ω to 0.4Ω and load currents from zero
to 2A. Thirty randomly selected LT4180 virtual remote
sense devices were tested in this design with wiring and
connector resistances of 0.1Ω, 0.2Ω, and 0.4Ω, and zero
to 2A load currents. With 0.1Ω USB cable and connector
resistance, none of the thirty devices exhibited more than
±3% variation from nominal output voltage. For total
resistances of 0.2Ω and 0.4Ω, the worst-case variation
in output voltage for all load current and devices never
exceeded ±3.4% and ±4.6%, respectively.
Virtual Remote Sensing (VRS) Power Supply
Figure 1 shows a 2A USB power supply for automotive
applications using a buck switching regulator and the
LT4180 virtual remote sense controller. The power supply
produces a 5V, 2A output from a nominal input voltage
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of their respective owners.
USB CABLE AND CONNECTORS
VIN
12V
+
4.7µF
50V
22µF
50V
GND
100k
RUN
ON
JP1
OFF
0.47µF
3
2
1
0.1µF
50V
INTVCC
30.1k
1%
10k
1%
0.033Ω
1%
VIN BD BOOST
SW
RUN/SD
PG
FB
RT
68.1k
1%
47µF
10V
UI
LT3693EDD
SYNC
6.8µH
47µF
10V
INTVCC
VC
CMDSH-3
100k
RUN VIN SENSE
2.15k
1%
DIV2
DIV1
1µF
100k
VPP
DIV0
INTVCC
SPREAD
47pF
TP1
OSC
OV
OSC
DRAIN
COMP GND CHOLD1 GUARD2 CHOLD2 GUARD3 CHOLD3 GUARD4 CHOLD4 COSC ROSC
47nF
470pF
330pF
470pF
4.7nF
Figure 1. A 2A USB Automotive Power Supply Using Virtual Remote Sensing
02/13/1029
470µF USB
10V POWERED
DEVICE
LT4180EGN
1k
23.2k
1%
+
INTVCC
1.87k
1%
5.36k
1%
470µF
10V
1µF
21.5k
1%
FB
MBRA340T3G
+
RWIRE = 0.1Ω
TO 0.4Ω
22.1k
1%
47nF
dn F01
RCOMP improves load regulation to approximately ±3.2%.
2
Results vary depending on how well RCOMP matches
the resistance of the cable between the USB power
device and the decoupling network (RCOMP/CLOAD).
Any capacitance internal to the USB powered device
(if it becomes a significant fraction of CLOAD) may also
degrade the results.
1
0
–1
RANGE OF REGULATION
OVER 30 PARTS
–2
–3
0.5
0
1.0
1.5
LOAD CURRENT (A)
2.0
dn F02
Figure 2. Worst-Case Load Regulation with
RWIRE = 0.1Ω
% DEVIATION (VOUT)
4
2
0
RANGE OF REGULATION
OVER 30 PARTS
–2
–4
One final note: adding RCOMP reduces the filtering effectiveness of CLOAD, resulting in increased power supply ripple.
Conclusions
Virtual remote sensing significantly improves load
regulation in USB products where unknown wiring
resistances would otherwise degrade regulation at the
device. By dynamically adapting to changes in load current, line resistance, connector aging and temperature
variation, voltage tolerances are improved, ensuring
consistent and reliable operation.
VRS
POWER
SUPPLY
0
0.5
1.0
1.5
LOAD CURRENT (A)
USB A
USB A
RECEPTACLE PLUG
CONNECTOR
DECOUPLING
CAPACITOR
IN USB
RECEPTACLE
2.0
dn F03
Figure 3. Worst-Case Load Regulation with
RWIRE = 0.2Ω
Adding VRS to Existing Devices and Designs
Virtual remote sensing requires an AC short at the
regulation point for best results, which may not be
feasible in some existing designs. For example, in
Figure 4, a typical USB device is connected directly
to the power supply shown in Figure 1. In this case,
regulation is maintained up to the USB A receptacle,
but the supply cannot correct for additional voltage
drops beyond this point.
Fortunately, the simple trick shown in Figure 5 removes
most of this error. By adding a resistor, RCOMP, in series
with the decoupling capacitor, CLOAD, the voltage at the
USB A receptacle rises with increasing load current,
thus compensating for any additional voltage drop in
the USB device caused by increasing load current.
Figure 6 shows typical results with 0.2Ω USB cable
resistance, and 0.1Ω USB device cable resistance. The
connector uses two 470µF capacitors (for a total CLOAD
equal to 940µF) in series with RCOMP = 0.1Ω. Without
RCOMP, load regulation would be about ±5.2%. Adding
Data Sheet Download
www.linear.com/4180
Linear Technology Corporation
USB POWERED DEVICE
USB CABLE
dn F04
CANNOT CORRECT
THIS VOLTAGE DROP
Figure 4. Incomplete Wiring Drop Correction
CONNECTOR WITH
USB
USB CABLE BUILT-IN DECOUPLING DEVICE CABLE
POWER
SUPPLY
+
RCOMP
USB
POWERED
DEVICE
CLOAD
dn F05
Figure 5. Correcting for Downstream Wiring
Voltage Drops
4
% DEVIATION (VOUT)
% DEVIATION (VOUT)
3
2
0
–2
–4
0
0.5
1.0
1.5
LOAD CURRENT (A)
2.0
dn F06
Figure 6. Typical Load Regulation at the USB
Powered Device Depicted in Figure 5
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