December 2009 - Robust, Quiet, Stable Power Supply for Active Antenna Systems with Built-In Protection and Diagnostic Capabilities

L DESIGN FEATURES
Robust, Quiet, Stable Power Supply
for Active Antenna Systems with
Built-In Protection and Diagnostic
Capabilities
by Sam Rankin and Steve Knoth
Introduction
The days of the simple car radio, like
the Mercedes dashboard shown in Figure 1, are over, supplanted by the era
of the automobile infotainment system
(see Figure 2). The venerable AM/FM
radio still holds a place at the infotainment table, but it now shares space
with digital audio broadcasting (DAB),
digital and high-definition television
(HDTV), satellite radio, integrated cell
phones, CD/DVD/MP3 players, global
positioning system (GPS) navigation
and video game systems.
Behind the dashboard, fueling
this wealth of information, are active
antenna systems. As infotainment
centers have expanded in complexity, the number of active antennas
needed to feed music and data into the
infotainment center has multiplied. It
is now common for an automobile to
have on average, three to five active
antenna systems, from a combination of AM, FM, DAB, HDTV, satellite
radio, traffic alerts, cellular, WiMax
and GPS—sometimes with multiple
antennas per band to improve reception quality. The sensitive circuits in
these active antenna systems require
protection and isolation from the harsh
automotive environment and a way to
provide antenna status and diagnostic
feedback to a host system.
The LT3050 is an innovative regulator with precision current limit and
diagnostic functions. It combines the
robust performance, reliability and
durability common to Linear Technology linear regulators with an enhanced
feature set geared towards active
antenna systems, including:
qProgrammable current limit
qSoft-start
qOpen-circuit detection
qOutput current monitor
26
Figure 1. Old school dashboard; one passive antenna for AM/FM reception
qOpen-collector fault signal
The LT3050 is a single IC solution
that replaces a complex arrangement
of current sense amplifiers, operational amplifiers and discrete components
and other ICs that would otherwise be
needed to meet the demanding combination of protection requirements
and diagnostic features required in
automobile antenna systems.
The LT3050 delivers up to 100mA
continuous output current with a
340mV typical dropout voltage at full
load. The IC features a wide input
voltage range of 2V to 45V, delivering adjustable output voltages down
to 0.6V. A single capacitor provides
both ultralow noise operation—only
30µVRMS across a wide bandwidth
of 10Hz to 100kHz—and reference
soft-start functionality, eliminating
large inrush currents and output
voltage overshoot at turn-on. The
LT3050’s output voltage tolerance is
highly accurate at ±2% over line, load
and temperature. The LT3050’s low
operating quiescent current of 50µA
allows it to idle continuously with
Figure 2. New school dashboard; high performance, active multi-antenna system
Linear Technology Magazine • December 2009
DESIGN FEATURES L
45V
VIN
10V/DIV
VOUT = 5V
IOUT = 50mA
COUT = 2.2µF
12V
1ms/DIV
Figure 3. LT3050 transient response to load
dump condition (AC coupled)
minimal battery drain and drops to
<1µA in shutdown. The IC is housed
in a 12-lead 2mm × 3mm DFN and
12-lead thermally enhanced MSOP
packages, respectively, offering a
compact footprint.
Single IC Antenna Power
Supply with Protection and
Diagnostic Features
The 12V car battery, the starting
point for many active antenna voltage
supplies, is far from the quiet, stable
supply required by these systems. In
addition to noise, this 12V “supply” can
be subjected to reverse battery conditions or load dumps where the voltage
can range or spike anywhere from –36V
to 80V. The LT3050 protects both itself
and the antenna in this demanding
electrical environment, while providing
a stable, low noise output voltage. The
LT3050 also protects active antenna
supply circuitry in the event of a short
circuit within the antenna supply itself
with an accurate and programmable
current limit. Thermal conditions in
automotive environments are equally
challenging, requiring the supply to be
stable over a –40 to 125°C temperature
range with robust overtemperature
protection.
In addition to these difficult protection requirements, the LT3050
simplifies the gathering of diagnostic
information required to report antenna
status. Programmable open circuit
detection monitors the antenna supply current in case it should drop
below a specified minimum operating
condition. Programmable short circuit
detection monitors the antenna supply current in case it should exceed
a defined maximum and protects the
antenna, and its supply, by limiting
Linear Technology Magazine • December 2009
Protection Features in an Accurate,
Stable and Quiet Power Supply
The LT3050 generates a stable and
low noise supply for active antenna
systems, isolating and protecting the
antenna system from the car’s noisy
and volatile 12V supply. The IC can
withstand input voltages of ±50V and
reverse battery conditions potentially
generated from the 12V supply as
well as output reversals to ±50V (see
Figure 3).
The LT3050 provides excellent
power supply noise rejection, effectively isolating the antenna supply
from noise on the 12V supply line or
from an intermediate step-down regulator (see Figure 4). A single capacitor
provides both reference soft-start and
noise bypass, enabling a programmable start-up time and ultralow
noise operation.
A precision programmable current
limit provides additional protection by
allowing the user to set current limit
as low as 110% of maximum load,
without affecting load regulation during normal operation. In addition, the
combination of a backup current limit,
current limit fold-back, and robust
thermal shutdown with hysteresis
allow for indefinite output shorts from
a 50V input supply without damaging
the IC. The output can be pulled 50V
above the input with minimal cur90
80
RIPPLE REJECTION (dB)
VOUT
20mV/DIV
current flow. In addition, an analog
current monitor creates a signal
proportional to the antenna supply
current. This is handy as a diagnostic
input, or for signaling the system as to
which antenna type is mounted.
70
CREF/BYP = 10nF
CREF/BYP = 100pF
CREF/BYP = 0
60
50
40
30
20 IL = 100mA
COUT = 10µF
10 VOUT = 5V
VIN = 5.8V + 50mVRMS RIPPLE
0
10
100
1k
10k
100k
FREQUENCY (Hz)
1M
10M
Figure 4. LT3050 ripple rejection
rent flow into the input and without
damaging the IC.
Diagnostic Capabilities
The LT3050 provides diagnostic information to the car’s control systems.
An open-collector fault indicator,
capable of sinking 100µA, asserts if
open-circuit or short-circuit conditions are detected, or if the IC enters
thermal shutdown. The LT3050 also
features an integrated current monitor
that sources (via the IMON pin) about
1/100 of the output current for use
in antenna system monitoring and
protection. See the block diagram in
Figure 5. Simply connecting a resistor
from IMON to GND creates a groundreferenced voltage proportional to
output current.
Programmable short-circuit detection and current limit is provided at
the IMAX pin and once set, varies less
than 5% over line and temperature.
The IMAX pin is the collector of a specially designed current mirror device
that sources about 1/200 of output
current. This pin is also the input to
the precision current limit amplifier.
Connecting a resistor (RI(MAX)) between
IMAX and GND sets the short-circuit
detection and programmable current limit thresholds. The current
limit amplifier circuitry performs two
functions. First, it asserts the opencollector FAULT pin logic if the IMAX
pin voltage reaches 600mV. Second,
it regulates the output drive current
such that the IMAX pin voltage does not
exceed 600mV, thus limiting the output current to 0.6V • 200/RI(MAX).
The programmable open-circuit
detection threshold is provided at the
IMIN pin. The IMIN pin is the collector
of a specially designed current mirror
device that sources about 1/200 of
output current. This pin is also the
input to the open-circuit detection
comparator, referenced to the 600mV
internal reference. Connecting a resistor between IMIN and GND sets the
open-circuit detection threshold. If
the IMIN pin voltage falls below 600mV,
the comparator trips and the FAULT
pin asserts. The comparator uses a
small amount of hysteresis to prevent
FAULT pin glitches.
27
L DESIGN FEATURES
IN
5, 6
R1
D1
QIMON
1/100
QIMIN
1/200
9
ADJ
30k
R4
–
+
Q2
IDEAL DIODE
D3
THERMAL/
CURRENT LIMITS
Q3
ERROR
AMPLIFIER
D2
QIMAX
1/200
CURRENT
LIMIT
AMPLIFIER
+
–
QPOWER
1
OUT
7, 8
100k
R3
IMAX
IMON
11
12
IMIN
COMPARATOR
4
SHDN
+
–
100k
R2
+
–
600mV
REFERENCE
IMIN
FAULT
2
3
U1
QFAULT
1
GND
10, 13
REF/BYP
Figure 5. LT3050 block diagram
Figure 6 shows a typical LT3050
application circuit set up as an active
antenna supply. Current limit, opencircuit fault threshold values, output
voltage, etc. were chosen arbitrarily for
illustrative purposes. In this example,
the open circuit detection threshold
is set by the 11.3k IMIN resistor to
10mA. The 1.15k IMAX resistor sets
the short circuit fault threshold and
current limit to 100mA (a 10nF IMAX
capacitor is required for current limit
amplifier stability). The 3k IMON resistor provides a full-scale 3V signal when
output current equals 100mA. The
10nF REF/BYP capacitor provides a
5.5ms soft-start time and low noise
operation.
Conclusion
As car infotainment components have
grown more complex, the number of
active antenna systems has grown as
well. Sensitive circuitry in these active
antenna systems requires protection
and isolation from the harsh automotive environment as well as diagnostic
feedback to report antenna status.
The LT3050 active antenna supply
and diagnostics linear regulator addresses active antenna design issues
with features such as programmable
IN
12V
VIN
1µF
120k
OUT
SHDN
ADJ
FAULT
0.1µF
1.15k
(THRESHOLD = 100mA)
11.3k
(THRESHOLD = 10mA)
IMAX
IMIN
5V
1%
442k
2.2µF
1%
60.4k
LT3050
10nF
current limit, soft-start, open-circuit
detection, output current monitor,
and an open-collector fault signal.
The LT3050 also features a wide input
voltage range, low quiescent current,
low output noise over a wide bandwidth, high output voltage accuracy,
low dropout voltage and ability to
withstand input and output voltage
reversal.
The LT3050’s stability and low
noise output benefits the end user as
well, with clearer and more reliable
antenna transmission/receive signals
to enhance the modern automotive
infotainment experience. L
IMON
3k
(ADC FULL SCALE = 3V)
TO µP ADC
0.1µF
REF/BYP
GND
10nF
Figure 6. LT3050 active antenna supply circuit
28
Linear Technology Magazine • December 2009
Similar pages