LTJournal-V24N4-2015-01.pdf

(4538 KB) Pobierz

January 2015

Volume 24 Number 4

I N

T H I S

I S S U E

Mike Engelhardt shows

how LTspice

is head of the

SPICE pack

Temperature-to-Bits: One IC for All

Sensor Types, 0.1°C Conformity

Michael Mayes

clocking solutions: 1.4GHz

low jitter PLL with clock

distribution

1.5A monolithic buck-boost

DC/DC, 2.5V–15V V

and

OUT

DC/DC for hundreds of

watts, 60V V

or V

OUT

analysis of Hot Swap™

circuits with foldback

current limit

Temperature measurement is not new. Galileo invented a

rudimentary thermometer capable of detecting temperature

changes, and two hundred years later, Seebeck discovered the

thermocouple. Given the long history of temperature measurement

and its extensive use today, one would think that accuracy

problems were all but eliminated. Not so. Even though methods

for extracting temperature from sensor elements are well known,

accurately measuring temperatures to better than 0.5°C or

0.1°C accuracy remains a challenge. The LTC 2983 enables up

to 0.1°C temperature conformity (measured accuracy against

a precision temperature calibrator), as shown in Figure 1.

Thermocouples, temperature dependent resistance elements (RTD

and thermistors) and semiconduc-

tor elements (diodes) are widely used to electrically measure temperature. Digitizing the electrical

signals of these sensor elements requires

significant expertise in a number of areas:

sensor behavior, analog circuit design, digital

circuit design and firmware development.

The LTC2983 packs this expertise into a single

IC and solves each of the unique challenges

associated with thermocouples, RTD

, therm-

istors and diodes. It combines all analog

circuitry necessary for each sensor type

with temperature measurement algorithms

and linearization data to directly measure

each sensor and output the result in °

(continued on page 4)

The LTC2983 solves the unique problems presented by all standard temperature sensors to produce

unmatched conformity and ease-of-use.

w w w. li n e ar.co m

In this issue...

COVER STORY

Temperature-to-Bits: One IC for All

Sensor Types, 0.1°C Conformity

Michael Mayes

Linear in the News

LINEAR AT ELECTRONICA

In November, Linear had a major presence at the biannual Electronica Show in

Munich. The Linear booth featured an ambitious array of 20 live, system-based

demos, showcasing the breadth of the company’s product line. These demos

included:

• Power over Ethernet port forwarding using 90

LTPoE++

™

• High performance amplifiers and converters enabling small, precise instruments

• Complete backup power—when you need it

• Maximize battery pack safety, life and capacity

• Robust IO-

ink, PHY solutions

• Highly reliable wireless mesh

network running on solar power

• Wireless power for battery

charging in challenging

environments

• Bidirectional converter allows

backup power

• Precision electrocardiogram signal

chain

• Solar power charging with True

Maximum Power Point Tracking

• Power management for video and

broadcasting

• µModule

regulator with

The Linear booth at Electronica was a hive of activity

with 20 live system-based demos.

READ/WRITE telemetry

• Universal temperature sensor IC

• E

-M

otorbike battery management

• Batteries of BMW i8 monitored by LTC6802 battery stack monitor

NEW VIDEOS

DESIGN FEATURES

SPICE Differentiation

Mike Engelhardt

1.4GHz Low Jitter PLL with Clock Distribution

Solves Difficult Clocking Problems: Multi-Clock

Synchronization and Data Converter Clocking

Chris Pearson

1.5A Monolithic Buck-Boost DC/DC Converter with

Up to 95% Efficiency Features 2.5V–15V

Input and Output Voltage Ranges

Richard Cook

Hundreds of Watts, 60V In or Out: Synchronous

4-Switch Buck-Boost Converter is Easy to

Parallel to Minimize Temperature Rise

Keith Szolusha

DESIGN IDEAS

What’s New with LTspice IV?

Gabino Alonso

Analysis of Hot Swap Circuits with

Foldback Current Limit

Vladimir Ostrerov and Josh Simonson

new product briefs

back page circuits

Linear has posted several new videos at www.linear.com, including:

Universal Temperature Measurement System—

Michael Mayes presents a series

of lab-based demonstrations that highlight the capabilities of the innova-

tive LTC2983 universal temperature sensor IC. These TechClips show how the

LTC2983 can be used to directly digitize RTD

, themocouples and thermis-

tors to measure temperature to an accuracy of 0.

and report the results

in °

or °

. View the TechClips at

www.linear.com

solutions/5500

High Power 3-Channel LED Driver—

Keith Szolusha shows how the LT

3797

powers three high power strings of LED

with flexible topolo-

gies and major fault protection such as open LED strings and shorted

LED strings. View video at

www.linear.com

solutions/5466

2 | January 2015 :

LT Journal of Analog Innovation

Linear in the news

More Power, Better Heat Dissipation, Smaller

PCB Area—

Video comparison of three

converter options: switching controllers,

monolithic regulators, and µModule regu-

lators in terms of component count, area,

design effort and thermal performance.

The LTM

4633 and LTM4634 µModule

regulators require the lowest component

count, design effort, PCB area and address

thermal concerns with an innovative

package design that includes an integrated

top side heat sink, creating a low thermal

impedance path to the ambient air. See

video at

www.linear.com

solutions/5467

A Very Low Power, High Performance

Modulator—

This video discusses the LTC5599,

many applications, the input voltage range

can vary from below the output battery

voltage to above the battery voltage,

requiring both step-down and step-up

capability. This video shows how the

LTC4020 controller meets these challenges,

how it works and its main features. See the

video at

www.linear.com

solutions/5430

AWARDS

EDN Hot 100 Products for 2014

Linear Products Receive Awards in

China

EDN

in November announced the selec-

tion of several Linear Technology products

EDN

Hot

00 Products for 20

• LTC6268 ultralow input bias current op

amp

• LT3669 IO-

ink PHY-compatible

industrial transceiver

• LTC5599 direct conversion

Q modulator

Wireless Battery Charger Selected for

Two Awards

Two Linear Technology products received

awards from major publications in China.

The LTC2338-

8-bit,

sps

SAR ADC was

selected by

EEPW

in their Editors’ Choice

Awards 20

4 for Best Mixed-Signal Chip

Award. And

Electronic Products China

chose the LTC3330 nanopower buck-boost

converter with energy harvesting battery

life extender for their Top

0 Product

Award and their Green Energy Award.

CONFERENCES & EVENTS

CAR-ELE Japan, 7th International Automotive

Electronics Technology Expo, Tokyo Big Sight, Tokyo,

Japan, January 14-16, West 1 Hall, Booth West

8-12—

Linear showcases its high perfor-

a very low power modulator with excel-

lent sideband rejection and carrier

leakage suppression, and with on-chip

capability to adjust these performance

metrics to unprecedented levels. The

device has an exceptionally low output

noise floor and high linearity, producing

superior dynamic range performance

for mission-critical transmitters. View

www.linear.com

solutions/5429

High Efficiency Buck-Boost Battery Charger—

mance analog IC solutions for automotive,

including battery management systems.

More info at

www.car-ele.jp

Home/

APEC 2015, Applied Power Electronics Conference,

Charlotte Convention Center, Charlotte, NC,

March 16-18, Booth 201—

Presenting Linear’s

Today’s battery chargers are expected to

support a variety of battery chemistries

and accept a range of input voltages. In

Linear’s LTC4

20 wireless battery charger

was selected by UK’s

Electronic Product

Design & Test

magazine for their 20

E-Legacy Awards in the Environmental

category. The LTC4

20 also brought

home the award for Italy’s

Selezione di

Elettronica’s

4 Innovation Award.

The LTC6268

ultralow input

bias current

op amp was

selected as an

EDN Hot 100

Product.

broad range of power management

solutions, including innovative switch-

ing regulators, linear regulators, digital

power system management products

and µModule regulators. Michael Jones

is presenting “PMBus Firmware: Using

Arduino for Prototyping and Designing

PMBus Solutions” at 8:30 am, March

7. More info at

www.apec-conf.org/

Electronica China, Shanghai New International

Expo Centre, Shanghai, China, March 17-19, Hall E3,

Booth 3318—

Linear will exhibit its broad

portfolio of analog and power solutions,

including Silent Switcher

regulators,

LDO+

™

linear regulators, high power

LED drivers, µModule regulators, supercap

chargers, universal digital temperature

measurement system, SAR ADC

, automo-

tive battery management systems, wireless

sensor network products and RF prod-

ucts. More info at

www.electronica-

productronica-china.com

home

January 2015 :

LT Journal of Analog Innovation

| 3

The LTC2983 has these polynomials built in for all eight standard

thermocouples (J, K, N, E, R, S, T and B) as well as user programmed table

data for custom thermocouples. The LTC2983 simultaneously measures the

thermocouple output and the cold junction temperature, and performs all

required calculations to report the thermocouple temperature in °C.

(

LTC

2983, continued from page

)

THERMOCOUPLES: OVERVIEW

LONG LEADS SUSCEPTIBLE TO

50kHz NOISE AND ESD EVENTS

OPEN CIRCUIT FAULT

DETECTION REQUIRED

INPUT CAN GO

BELOW GND

Thermocouples generate voltage as a func-

tion of the temperature difference between

the tip (thermocouple temperature) and

the electrical connection on the circuit

board (cold junction temperature). In

order to determine the thermocouple tem-

perature, an accurate measurement of the

cold junction temperature is required; this

is known as cold junction compensation.

The cold junction temperature is usually

determined by placing a separate (non-

thermocouple) temperature sensor at the

cold junction. The LTC2983 allows diodes,

RTD

, and thermistors to be used as cold

junction sensors. In order to convert the

voltage output from the thermocouple into

a temperature result, a high order poly-

nomial equation (up to

4th order) must

be solved (using tables or mathematical

functions) for both the measured voltage

and the cold junction temperature. The

LTC2983 has these polynomials built in for

Figure 1. Typical temperature error conformity of the

LTC2983 with various sensors

0.5

0.4

0.3

0.2

ERROR (°C)

0.1

–0.1

–0.2

–0.3

–0.4

–0.5

–200 0

200 400 600 800 1000 1200 1400

TEMPERATURE (°C)

3904 DIODE

RTD

THERMISTOR

VOLTAGE VS TEMPERATURE

IS HIGHLY NON-LINEAR

AND LOW LEVEL

COLD JUNCTION

TEMPERATURE REQUIRED

INPUT PROTECTION AND

ANTI-ALIASING REQUIRED

Figure 2. Thermocouple design

challenges

all eight standard thermocouples (J, K, N, E,

R, S, T and B) as well as user-programmed

table data for custom thermocouples.

The LTC2983 simultaneously measures

the thermocouple output and the cold

junction temperature, and performs

all required calculations to report the

thermocouple temperature in °

THERMOCOUPLES:

WHAT’S IMPORTANT

of a second negative supply or an input

level shifting circuit. The LTC2983 incor-

porates a proprietary front end capable

of digitizing signals below ground on

a single ground-referenced supply.

In addition to high accuracy measure-

ments, thermocouple circuits need to

incorporate noise rejection, input protec-

tion, and anti-alias filtering. The LTC2983

input impedance is high, with a maximum

input current of less than

. It can

accommodate external protection resistors

and filtering capacitors without introduc-

ing extra errors. It includes an on chip

digital filter with 75

rejection of both

and 60

of 50

or 60

Fault detection is an important feature of

many thermocouple measurement sys-

tems. The most common fault reported

is an open circuit (broken or unplugged

thermocouple). Historically, current

sources or pull-up resistors were applied

to the thermocouple input in order to

detect this type of fault. The problem

with this approach is that these induced

THERMOCOUPLE

A thermocouple’s generated output

voltage is small (<

full-scale) (see

Figure 2). A

a result, the offset and noise

of the ADC making the voltage measure-

ment must be low. Furthermore, it is

an absolute voltage reading requiring

an accurate

low drift reference voltage.

The LTC2983 contains a low noise, con-

tinuously offset calibrated 24-bit delta-

sigma ADC (offset and noise <

) with a

0ppm/°

max reference (see Figure 3).

A thermocouple’s output voltage can

also go below ground when the tip is

exposed to temperatures below the cold

junction temperature. This complicates

systems by either forcing the addition

4 | January 2015 :

LT Journal of Analog Innovation

design features

If two perfectly matched excitation current sources of known ratio are applied to the

diode, a voltage of known proportionality to absolute temperature (PTAT) is output. The

LTC2983 automatically generates the ratioed currents, measures the resultant diode

voltage, calculates the temperature using the programmed non-ideality and outputs

the results in °C. It can also be used as the cold junction sensor for thermocouples.

2.85V TO 5.25V

LTC2983

“PULSED”

BURNOUT

1nA MAX

J, K, N, R, S,T , E, B

CUSTOM

THERMOCOUPLE

DIODE

CURRENT

GENERATOR

REF

(10ppm/°C MAX)

sensor connections are used in industrial

environments. The LTC2983 indicates

via fault reporting if the measured tem-

perature is above

below the expected

range for the specific thermocouple.

DIODES: OVERVIEW

24-BIT

∆∑ ADC

COLD JUNCTION

CAN BE DIODE OR

RTD OR THERMISTOR

SPI

INTERFACE

24-BIT

∆∑ ADC

°C/°F

AND

FAULTS

24-BIT

∆∑ ADC

Diodes are inexpensive semiconductor-

based devices that can be used as tempera-

ture sensors. These devices are typically

used as the cold junction sensor for a

thermocouple. When an excitation cur-

rent is applied to a diode, they generate a

voltage as a function of temperature and

the current that is applied. If two per-

fectly matched excitation current sources

of known ratio are applied to the diode,

a voltage of known proportionality to

absolute temperature (PTAT) is output.

DIODES: WHAT’S IMPORTANT

Figure 3. Thermocouple

measurement using diode

cold junction compensation

PROPRIETARY

NEGATIVE VOLTAGE

GENERATOR

THERMOCOUPLE POLYNOMIALS,

COLD JUNCTION CALCULATION,

FAULT DETECTION

signals lead to errors and noise, and

interact with input protection circuitry.

The LTC2983 includes a unique open

circuit detection circuit that checks for

a broken thermocouple just prior to the

measurement cycle. In this case, the open

circuit excitation current does not inter-

fere with measurement accuracy. The

LTC2983 also reports faults related to the

cold junction sensor. It can detect, report

and recover from electrostatic discharge

(ESD) events that may occur when long

PARASITIC LEAD

RESISTANCE

REQUIRES PRECISELY MATCHED

CURRENT SOURCES

Figure 4. Diode design

challenges

NEED PROGRAMMABLE

NON-IDEALITY FACTOR

OPEN/SHORT/DIRECTION

FAULT DETECTION

AUTOMATICALLY USE AS

COLD JUNCTION COMPENSATION

In order to generate a PTAT voltage with

known proportionality, two highly

matched, ratioed current sources are

required (see Figure 4). The LTC2983

accurately generates this ratio by relying

on delta-sigma oversampling architecture.

Diodes and the leads connecting to the

ADC contain unknown parasitic diode

effects. The LTC2983 contains a 3-current

measurement mode that removes parasitic

lead resistances. Various diode manufac-

turers specify different diode non-ideality

factors. The LTC2983 allows individual pro-

gramming of each diode’s non-ideality fac-

tor. Since absolute voltages are measured,

the value and drift of the ADC reference

voltage are critical. The LTC2983 includes a

factory trimmed

0ppm/°

max reference.

January 2015 :

LT Journal of Analog Innovation

| 5

Plik z chomika:

TirNaNog

LTJournal-V24N4-2015-01.pdf

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: