LTJournal-V25N2-2015-07.pdf

(5239 KB) Pobierz
July 2015
Volume 25 Number 2
I N
T H I S
I S S U E
4-phase supply supports
120A in tiny footprint
10
one driver is all you
need for automotive LED
headlight clusters
17
extend remote sensor
battery life with thermal
energy harvesting
24
Op Amp Combines Femtoamp Bias
Current with 4GHz Gain Bandwidth
Product, Shines New Light on
Photonics Applications
Glen Brisebois
simplify small solar
systems with hysteretic
controller
27
powering a Dust
®
mote
from a piezo
29
Einstein published his seminal paper on the photoelectric effect
110 years ago, essentially inventing the discipline of photonics. One
would think that over so many years the science and engineering
surrounding photonics must have fully matured. But not so. Optical
sensors—photodiodes, avalanche photodiodes, and photomultiplier
tubes—continue to achieve astoundingly high dynamic ranges,
enabling electronics to peer ever more
deeply into the photonic world.
Photosensors typically convert photons to electron current
and are followed by a transimpedance function to transform
the current into a voltage. The transimpedance function
may be either a simple resistor or, for higher bandwidth,
the summing node of an op amp, in which case it is called
a transimpedance amplifier (
TIA)
. The traditional enemies
of the
TIA
are voltage noise, current noise, input capaci-
tance, bias current and finite bandwidth. Enter the new
LTC
®
6268-
1
0 with 4.25
n
V/
√
H
z
voltage noise, 0.005
p
A/
√
H
z
current noise, a very low 0.45
p
F
of input capacitance,
3f
A
of bias current and 4
GH
z
of gain-bandwidth.
(continued on page 4)
The LTC6268’s performance meets the demands of the latest photonics applications.
w w w. li n e ar.co m
In this issue...
COVER STORY
Op Amp Combines Femtoamp Bias Current
with 4GHz Gain Bandwidth Product, Shines
New Light on Photonics Applications
Glen Brisebois
1
Linear in the News
THE INDUSTRIAL INTERNET OF THINGS
DESIGN FEATURES
4-Phase Power Supply Delivers 120A in Tiny Footprint,
Features Ultralow DCR Sensing for High Efficiency
Yingyi Yan, Haoran Wu and Jian Li
10
3mm × 3mm Monolithic DC/DC Boost/Inverting
Converters with 65V Power Switches
Joshua Moore
14
Everywhere you turn, there is discussion around the emerging Internet of
Things (IoT
)
—the concept that the Internet of the future will create a vast net-
work of interconnected physical objects or “things” with embedded sensors,
electronics and software. IoT devices are able to exchange data with other con-
nected devices and human operators. According to industry analyst, Stifel, the
IoT market today is already sizable, with an estimated
1
9.7 billion IoT devices
deployed, and expected to grow to 95.5 billion devices deployed by 2025.
Much of the recent discussion about IoT revolves around “wear-
able technology”—Google glasses and Apple watches—but these appli-
cations are only part of a much larger IoT picture. There remains
significant opportunity in the Industrial Internet of Things; analysts
expect industrial to be one of the fastest growing IoT segments.
One major area of promise is to leverage real-time data gathered via wire-
less sensor networks (
WSN
s
) to improve efficiency and streamline processes.
Sensors can be placed in a broad range of environments, including buildings,
city streets, bridges and tunnels, industrial plants, moving vehicles, or in remote
locations such as along pipelines and weather stations. These applications
require that
WSN
s
draw extremely low power and yield wired-like reliabil-
ity, often over a range of network architectures, sizes and data rates. Wireless
mesh networks are increasingly popular because they can easily cover large
areas using low power radios that reliably relay data from node to node.
Linear has developed expertise in this area via its Dust Networks
®
WSN
product line. Linear’s wireless sensor network products are suited for
a wide range of Industrial IoT applications, including:
• Flow and process monitoring in industrial environments
• Data center energy management
• Fence line security
• Rail car preventive maintenance
• Smart street parking systems
SOFTWARE DEVELOPMENT KIT ACCELERATES INDUSTRIAL IoT
APPLICATION DEVELOPMENT
One LED Driver Is All You Need for
Automotive LED Headlight Clusters
Keith Szolusha and Kyle Lawrence
17
DESIGN IDEAS
What’s New with LTspice IV?
Gabino Alonso
22
Extend Remote Sensor Battery Life
with Thermal Energy Harvesting
Dave Salerno
24
Simplify Small Solar Systems with Hysteretic Controller
Mitchell Lee
27
Powering a Dust Mote from a Piezoelectric Transducer
Jim Drew
29
31
32
new product briefs
back page circuits
Linear Technology now offers a software development kit for the Industrial
IoT
.
Linear Technology’s SmartMesh
IP
™
wireless sensor networking products
now provide the ability to program applications directly on the embedded
ARM
Cortex-
M
3, running Micrium’s
µ
C/OS-II
real-time operating system. Application
2 | July 2015 :
LT Journal of Analog Innovation
Linear in the news
synchronous buck converter that delivers
up to 50
m
A
of continuous output current
and operates from an input voltage range
of 2.7
V
to 20
V,
ideal for a wide range of
energy harvesting and IoT battery-pow-
ered applications including “keep-alive”
sensor and industrial control power.
AWARDS
Linear’s Dust Networks wireless
sensor network products fuel
applications in the Industrial Internet
of Things.
The LTC2000
1
6-bit, 2
G
sps
DAC
received the 20
1
4 Product Award
from 2
1
ic.com in China.
CONFERENCES & EVENTS
IEEE Nuclear and Space Radiation Effects
Conference, MarrIott Copley Place, Boston,
development time is accelerated with
a library of reference code and source
code examples. Based on 6
L
o
WPAN,
SmartMesh
IP
mesh networking products
include a pre-compiled networking stack
that delivers >99.999% network reliabil-
ity at ultralow power. This is particularly
important for Industrial IoT applica-
tions, where wireless sensor networks
may be deployed in harsh and remote
environments, with little to no chance
for maintenance. The On-Chip Software
Development Kit (
SDK)
provided with
the LTC5800
-IPM
(system-on-chip) and
LTP
™
590
1
/2-
IPM
(
PCB
modules) is archi-
tected to ensure that developers can stably
run both the pre-compiled SmartMesh
IP
networking stack and their applications
simultaneously. For more information,
see
www.linear.com
/
solutions/5457.
ENERGY HARVESTING AND IoT
typical operating conditions. The opera-
tion of harvesting elements over several
years makes them comparable to long-life
primary batteries, both in terms of energy
provision and the cost per energy unit.
Systems incorporating
EH
are typically
capable of recharging after depletion,
something that systems powered by pri-
mary batteries cannot do. Most implemen-
tations use an ambient energy source as
the primary power source, supplemented
by a primary battery in case the ambient
energy source goes away or is disrupted.
Linear Technology’s LTC333
1
is a complete
regulating
EH
solution that delivers up
to 50
m
A
of continuous output current
to extend battery life when harvestable
energy is available. It requires no supply
current from the battery when providing
regulated power to the load from har-
vested energy and only 950
n
A
operating
when powered from the battery under
no-load conditions. The LTC333
1
inte-
grates a high voltage
EH
power supply,
plus a synchronous buck-boost
DC/DC
converter powered from a rechargeable
primary cell battery to create a single
non-interruptible output for energy
harvesting applications such as those in
wireless sensor networks. Another device,
the LTC3388-
1
/-3, is a 20
V
input-capable
Massachusetts, July 13–17, Booths 8 & 9—
Linear is showcasing its products
for space and harsh environments.
More info at
www.nsrec.com
2nd Dust Networks Consortium, Tokyo Conference
Center, Tokyo, Japan, July 21—
Software
and device vendors, integrators and
monitoring service providers are
invited to learn about wireless sensor
network system capabilities. More
info at
www.dust-consortium.jp/
The Battery Show/Electric & Hybrid Vehicle Tech
Expo, Suburban Collection Showplace, Novi,
Michigan, September 15–17—
Presenting Linear’s
battery management system products.
More info at
www.thebatteryshow.com/
IoT World Congress, Gran Via Venue, Barcelona,
Spain, September 16–18—
Linear will high-
The proliferation of wireless sensors
supporting the Internet of Things has
increased the demand for small, compact
and efficient power converters tailored to
untethered lower power devices. State-of-
the-art and off-the-shelf energy harvest-
ing (
EH)
technologies—for example, in
vibration energy harvesting and indoor or
wearable photovoltaic cells—yield power
levels on the order of milliwatts under
light its Dust Networks wireless sen-
sor network products. More info at
www.IoTsworldcongress.com
/
en
/
home
Sensors & Instrumentation for Test, Measurement &
Control, The National Exhibition Centre, Birmingham,
UK, September 30 to October 1—
Linear will
showcase products and solutions related
to wireless sensor networks. More info at
www.sensorsandinstrumentation.co.uk/
n
July 2015 :
LT Journal of Analog Innovation
| 3
The calculated CV +
I
noise for the LTC6268-10 at 1MHz
is 0.052pA/√Hz, compared to 0.156pA/√Hz for the
OPA657; a factor of three better for the LTC6268-10.
(
LTC
6268, continued from page
1
)
UNDERSTANDING VOLTAGE
NOISE AND CURRENT NOISE
CONTRIBUTIONS IN TIAs
Figure 1. The op amp with its noise sources and input capacitance. Total op amp noise
(ignoring R
F
thermal noise) is I
NOISE
= i
n
+ 2πfC
IN
e
n
(added rms-wise).
I
PD
+ I
NOISE
I
PD
I
PD
= PHOTODIODE CURRENT
e
n
= OP AMP VOLTAGE NOISE
i
n
=
OP AMP CURRENT NOISE
C
IN
C
IN
=
OP AMP INPUT CAPACITANCE
I
NOISE
=
EFFECTIVE COMBINED CURRENT NOISE
R
F
=
FEEDBACK RESISTANCE, OR TIA GAIN
R
F
Output noise in
TIA
s
is a result of com-
bined input voltage noise and input cur-
rent noise. This combined effect is often
specified as a current noise referred to the
input—essentially the output voltage noise
divided by the gain in ohms—but it actu-
ally arises from both input noise sources.
In fact, the dominant cause of output noise
is usually input voltage noise (Figure
1
).
By virtue of feedback, the minus input is
fixed at virtual ground so the current noise
i
n
passes directly through
R
F
and contrib-
utes to total current noise with a factor of
1
. Also by virtue of feedback, the voltage
noise e
n
is placed in parallel with the input
capacitance
C
IN
and induces a current
noise of e
n
/Z(C
IN
)
. The impedance of a
capacitor is
1
/2πf
C,
so the effective cur-
rent noise due to input voltage noise and
capacitance is 2πf
C
IN
e
n
. So the total op
amp noise (ignoring
R
F
thermal noise) is
I
NOISE
=
e
n
i
n
–
V
OUT
= R
F
• (I
PD
+ I
NOISE
)
+
SAMPLE CALCULATION AND
COMPARISON BETWEEN LTC6268-10
AND COMPETITIVE OPA657
The
CV
+
I
noise is a useful figure of merit
for comparing op amps, but it does have
a dependency on frequency. An insightful
comparison can be made by initially com-
paring them at a specific frequency and
then observing the differences in the plots
of
CV
+
I
noise versus frequency that inevi-
tably arise. For example, let’s compare
the LTC6268-
1
0 and competitive
OPA
657
by starting with a calculation at
1
MH
z
.
The LTC6268-
1
0 data sheet gives plots
of current noise versus frequency
showing 0.05
p
A/
√
H
z
at
1
MH
z
, and of
voltage noise versus frequency show-
ing 4
n
V/
√
H
z
at
1
MH
z
. Using the input
capacitance of 0.55
p
F
(0.45
p
F
for
CCM,
plus 0.
1
p
F
for
CDM)
, the total
CV
noise at
1
MH
z
can be calculated as
CV NOISE
=
2
•1MHz •0.55pF •
4nV
=
0.014pA
Hz
Hz
Summing this rms-wise with the native
I
noise of 0.05
p
A/
√
H
z
, we get 0.052
p
A/
√
H
z
of total
CV
+
I
noise at
1
MH
z
.
The same calculation for the competi-
tive
OPA
657 can also be performed. It
specifies 4.8
n
V/
√
H
z
voltage noise, 5.2
p
F
input capacitance (4.5
p
F
for
C
CM
plus
0.7
p
F
for
C
DM
)
, and
1
.3f
A/
√
H
z
current
noise. Calculating total
CV
+
I
noise gives
0.
1
56
p
A/
√
H
z
at
1
MH
z
for the
OPA
657,
about three times worse than LTC6268-
1
0.
Figure 2 shows a plot of
CV
+
I
noise for
LTC6268-
1
0 and
OPA
657 versus frequency.
The reason the LTC6268-
1
0 outperforms
the
OPA
657 is its lower voltage noise and
its much lower input capacitance. And
(
2
fC
IN
e
n
)
+
(
i
n
)
2
2
Figure 2. CV + I current noise versus frequency for
the LTC6268-10 and OPA657. The LTC6268-10 is
considerably quieter.
1.8
“CV + I” CURRENT NOISE (pA/√Hz)
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0
1
2
3 4 5 6 7
FREQUENCY (MHz)
8
9
10
OPA657
LTC6268-10
This is sometimes referred to as
CV
+
I
noise and makes an excellent figure of
merit for an op amp, because it incor-
porates only op amp characteristics,
neglecting external aspects of the cir-
cuit such as photosensor capacitance
and
R
F
thermal noise. It is essentially
the best the op amp can do.
4 | July 2015 :
LT Journal of Analog Innovation
design features
A powerful method to reduce feedback capacitance is to shield the E field paths that
give rise to the capacitance. In this particular case, the method is to place a ground trace
between the resistor pads. Such a ground trace shields the output field from getting to
the summing node end of the resistor, effectively shunting the field to ground instead.
because the LTC6268-
1
0 has lower volt-
age noise, it continues to outperform
the
OPA
657 as the sensor capacitance is
added and increased. Furthermore, the
LTC6268-
1
0 features a rail-to-rail output
and can operate on a single 5
V
supply,
burning half the power of
OPA
657.
GAIN BANDWIDTH, AND ACHIEVING
HIGH BANDWIDTH AT HIGH
IMPEDANCE
RISE TIME = 88ns
PARASITIC
FEEDBACK C
+2.5
K
CASE
I
PD
PD
A
402k
+2.5
V
OUT
OUTPUT
(500MV/DIV)
LASER DRIVE
(2mA/DIV)
–
+
LTC6268-10
–2.5
Another advantage of the LTC6268-
1
0 is
its serious 4
GH
z
gain bandwidth product.
In fact, you’ll find that the LTC6268-
1
0 is
able to find and use tiny parasitic capaci-
tances that other op amps miss. Normally,
high value resistors begin to reduce their
net impedance at high frequency due to
their end-to-end capacitance. The key
to exploiting the 4
GH
z
gain bandwidth
of the LTC6268-
1
0 with higher gain
TIA
s
PD: OSI FCI-125G-006
200ns/DIV
Figure 4. Time domain response of 402kΩ TIA
without extra effort to reduce feedback capacitance.
Rise time Is 88ns and BW is 4MHz.
Figure 3. LTC6268-10 and low capacitance
photodiode in a 402kΩ TIA
is to minimize the feedback capacitance
around the main feedback resistor. Though
minimized, the LTC6268-
1
0 can use the
tiny residue feedback capacitance to
compensate the feedback loop, extend-
ing resistor bandwidth to several
MH
z
.
Following is a design example at 402
k
.
BOTTOM
CERAMIC R SUBSTRATE E
ENDCAP
I
PD
Figure 5. A normal layout (a) and a field-
shunting layout (b). Circuit board in (c)
shows actual layout with extra shunting
at R9, less at R12. Simply adding a
ground trace under the feedback
resistor does much to shunt field away
from the feedback side, dumping it
to ground. Note that the dielectric
constant of FR4 and ceramic is
typically 5, so most of the capacitance
is in the solids and not through the
air. Such field shunting techniques
reduced feedback capacitance from
approximately 100fF in Figure 4 to
11.6fF in Figure 6. Note also that the
feedback trace is exposed in upper (c)
but entirely shielded in lower (c).
A
G
+2.5
K
(c)
RESISTIVE
ELEMENT
TOP
–
LTC6268-10
FR4
V
OUT
MUCH
LIGHTER
DOSE
+
(a)
E FIELD
⇒
C
CERAMIC R SUBSTRATE E
ENDCAP
I
PD
A
G
+2.5
K
RESISTIVE
ELEMENT
EXTRA GND
TRACE UNDER
RESISTOR
V
OUT
HEAVY DOSE
OF SHUNTING
C
F
≈ 11.6fF
–
+
(b)
FR4
LTC6268-10
TAKE E FIELD TO GND,
MUCH LOWER C
July 2015 :
LT Journal of Analog Innovation
| 5

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika:

Zgłoś jeśli naruszono regulamin