October 1999 - Linear Technology

Linear Technology Chronicle

• October 1999 1

Product of the Month

, LTC and LT are registered trademarks of Linear Technology

Corporation. No Latency ∆Σ, Burst Mode and Over-The-Top are

trademarks of Linear Technology Corporation. MICROWIRE is a

trademark of National Semiconductor Corporation.

A Showcase of Linear Technology’s Focus Products

Linear Technology Chronicle

October 1999 Vol. 8 No. 10

Inside This Issue:

Highly Efficient and Tiny Low Input Voltage Boost Converters—

LT1949, LT1308A/LT1308B and LT1615 ............................................................................... 2

Pin-Compatible, Micropower 12-Bit, 14-Bit and 16-Bit DACs

Provide Flexible Board Design—LTC1658/LTC1659 and LTC1448....................................... 3

A 1.1MHz Over-The-TopTM Rail-to-Rail Op Amp—LT1637 ..................................................... 3

No Latency

∆Σ

Multiplexed 24-Bit ADC—LTC2408

Figure 1. LTC2408 Block Diagram

ANALOG

INPUTS

–0.12V

REF

1.12V

REF

2408 TA01

CH0

CH1

CH2

CH3

CH4

CH5

CH6

CH7

–

8-CHANNEL

MUX

6 COM GND

1, 5, 16, 18, 22, 27, 28

MPU

SERIAL DATA LINK

MICROWIRE

AND

SPI COMPATABLE

19, 25

CSADC

CSMUX

CLK

SDO

24-BIT

∆∑ ADC

LTC2408

0.1V TO V

ADCINMUXOUT

7 4 3 2, 8 1µF

2.7V TO 5.5V

REF

= INTERNAL OSC/50Hz REJECTION

= EXTERNAL CLOCK SOURCE

= INTERNAL OSC/60Hz REJECTION

The LTC

2408 is an 8-channel,

micropower, 24-bit analog-to-digital

converter, combining the high performance

LTC2400 with an 8-channel analog input

multiplexer. The unique features of the

LTC2408, including exceptional noise per-

formance, accurate full-scale and single

cycle settling time, allow measurement of

signals ranging from microvolts on one

channel to volts on the next with no over-

head associated with the input multiplexer.

The exceptional noise performance of

the LTC2408 (1.5µVRMS) corresponds to a

resolution of 21.6 bits for a 5V input range.

Low level input signals within a 100mV

range achieve better than 16-bit resolution

without the use of a PGA. Competitive delta-

sigma ADCs are significantly noisier than

the LTC2408, requiring a PGA in order to

improve noise performance for low level

input voltages. The LTC2408 offers several

significant advantages over converters that

require a PGA. One is the ability to measure

small signals (microvolts) superimposed

upon large DC voltages (volts). For example,

a 100mV signal sitting on 2V (2V to 2.1V)

can be measured with the same accuracy and

noise performance as a 100mV signal sitting

on ground (0V to 0.1V). Conversely, a con-

verter operating with a PGA gain of 50 is

limited to an input range of 0V to 0.1V with

a 5V reference. It cannot digitize any signal

larger than 100mV.

A second advantage the LTC2408

offers is exceptional accuracy. The LTC2408

provides offset error of less than 1ppm, off-

set drift less than 0.01ppm/°C, full-scale

error less than 4ppm, full-scale drift less than

0.02ppm and integral nonlinearity of less

than 4ppm. Since the total unadjusted error

is less than 10ppm, the absolute accuracy of

any input voltage within the 0V to 5V range

is within 10ppm. Alternatively, systems us-

ing PGAs exhibit full-scale errors limited by

the matching of internal components. The

user is burdened with the removal of these

errors, usually by placing the device into a

system calibration mode. For multichannel

systems, this means full-scale and offset

calibrations for each channel. The LTC2408

does not require any system calibration. The

offset and full-scale are transparently cali-

brated during each conversion cycle.

The LTC2408 accepts any external ref-

erence voltage from 0.1V to VCC. With its

extended input conversion range of –12.5%

to 112.5% of VREF, the LTC2408 can con-

sistently resolve low level signals in the

microvolt range, regardless of the fixed DC

level (0V to VREF).

One advantage delta-sigma converter

architectures offer over conventional ADCs

is on-chip digital filtering. For low fre-

quency applications, this filtering is typically

designed to provide rejection of line frequen-

cies at 50Hz or 60Hz and their harmonics.

The disadvantage of conventional digital

filters, prior to the release of the LTC2408,

was the associated filter settling times. If the

input signal changed abruptly, the conver-

sion result was invalid for the following 3 or

4 conversion cycles, making multiplexing

extremely difficult. The LTC2408’s result is

valid for every conversion, making multi-

plexing easy. With no filter settling time and

no latency, there is a one-to-one correspon-

dence between the conversion result and the

applied input signal.

The LTC2408 is the simplest and

smallest overall solution for a 24-bit A/D

converter, making it ideal for high resolu-

tion applications such as DVMs, weight

scales, industrial process control, tempera-

ture measurement and complete system

monitoring.

Linear Technology Chronicle

• October 1999

Contact your local Linear Technology sales office for a data sheet and evaluation samples. For more information, visit our web site at www.linear-tech.com.

Four wide input voltage range, high

output voltage, step-up switching regulators

are new from Linear Technology (Figure 1).

The LT

1949 and LT1308B operate at a

fixed 600kHz for very low ripple. The

LT1615 operates with a 400ns fixed off-time

and the LT1308A has built-in Burst Mode

operation for high efficiency even at low

load currents.

shutdown and 4.5mA while operating. Its

internal NPN power switch handles an

800mA load with a voltage drop of just

410mV. An external compensation pin gives

the user the ability to optimize feedback loop

performance, permitting the use of small,

low ESR ceramic capacitors.

The LT1949 contains an internal low-

battery detector with a 200mV reference that

stays alive when the device goes into

shutdown. It is a pin-compatible higher

current upgrade to the LT1317B boost

converter (700mA internal switch).

These devices are improved versions of the

LT1308. Both versions operate with an input

voltage from 1V to 10V and deliver 5V at a

load current of up to 1A (Figure 4) from a

single Li-Ion cell or 12V at 300mA from a

3.3V supply. High efficiency is maintained

over a load range of 1mA to 1A (Figure 5)

by Burst Mode operation in the LT1308A.

The LT1308A’s no-load quiescent current is

140µA and both versions shut down to less

than 1µA.

The LT1308B’s constant frequency

600kHz operation keeps switching noise low

and away from sensitive 455kHz IF frequen-

cies, important for wireless applications.

Both versions are well-suited for applica-

tions of a low duty cycle pulsed nature, such

as two-way pagers, cell phones, camera flash

units, latching relay energizers and handheld

printers.

Fixed 600kHz current mode operation

allows the use of very small components. A

10µF ceramic bypass capacitor is the only

3.3V

LT1949

10µHD1

140k

68k

330pF

OUT

10V

175mA

1949 F01

10µF

CERAMIC

22µF

GND

SHDNSHUTDOWN

C1: AVX TAJA226M006R

C2: TAIYO YUDEN LMK325BJ106MN

D1: MBRM120LT3

L1: SUMIDA CDRH62B-100

LOAD CURRENT (mA)

20 5 10050 300

1949 F02

EFFICIENCY (%)

OUT

= 10V

3.6V

4.2V

Figure 2. 3.3V to 10V/175mA

DC/DC Converter

Figure 3. Efficiency of Circuit in Figure 2

Continued on page 4

LT1308A

LT1308B

4.7µHD1

LBO

LBI

47k R2

100k

R1*

309k

100pF

1308A/B F01

47µF

220µF

Li-Ion

CELL V

GND

SHDNSHUTDOWN

C1: AVX TAJC476M010

C2: AVX TPSD227M006

D1: IR 10BQ015

L1: MURATA LQH6N4R7

*R1: 169k FOR V

OUT

= 3.3V

887k FOR V

OUT

= 12V

Figure 4. Single Li-Ion Cell to 5V/1A

Converter with the LT1308A or LT1308B

Highly Efficient and Tiny Low Input Voltage Boost Converters—

LT1949, LT1308A/LT1308B and LT1615

Figure 1. Low Dropout Voltage, High

Frequency Boost Converters

INPUT VOLTAGE RANGE (V)

MAXIMUM OUTPUT CURRENT (A)

2468

LTC F01

10 12 14 16

LT1308A/LT1308B*

LT1949*

LT1615**

**FIXED FREQUENCY DEVICES, 600kHz

FIXED OFF-TIME OF 400ns

600kHz Fixed Frequency 1A Boost

Converter, 1.5V to 10V Input

The LT1949 is a PWM DC/DC con-

verter that operates with an input voltage

range from 1.5V to 10V. It incorporates a

1A, 30V internal switch and is capable of

producing two watts of power at output volt-

ages up to 28V. To benefit noise-sensitive

systems, the LT1949 maintains constant fre-

quency PWM operation under all loads for

very low output ripple that is easy to filter.

Its 600kHz operation allows the use of small,

low profile capacitors and inductors. The

LT1949 is available in a fused lead, 8-pin

MSOP package that saves board space.

The low output ripple minimizes noise

that causes inconsistent illumination when

powering large thin film transistor (TFT)

LCD panels. The LT1949 is also ideal for

other space-confined systems, such as GPS

receivers, diagnostic medical instrumenta-

tion and other portable equipment, as well as

board level applications operating off 3.3V

or 5V power rails.

Figure 2 shows the LT1949 configured

as a 3.3V to 10V step-up converter. Effi-

ciency reaches 87% (Figure 3). The

LT1949’s quiescent current is 25µA in Figure 5. Efficiency of Figure 4

Using the LT1308A

LOAD CURRENT (mA)

EFFICIENCY (%)

50 10 100 1000

1308A/B G03

= 4.2V

= 2.5V

= 3.6V

= 1.5V

600kHz Boost Switchers in SO-8

Deliver 5W from Single Li-Ion

Cell, 1V to 10V Input

The LT1308A/LT1308B are 600kHz

fixed frequency PWM switching regulators

that incorporate a 36V power switch capable

of handling a 2A peak current and a low-

battery detector with a 200mV reference.

Linear Technology Chronicle

• October 1999 3

Contact your local Linear Technology sales office for a data sheet and evaluation samples. For more information, visit our web site at www.linear-tech.com.

Application of the Month

Pin-Compatible, Micropower 12-Bit, 14-Bit and 16-Bit DACs

Provide Flexible Board Design—LTC1658/LTC1659 and LTC1448

µP

CONTROL

VOLTAGE

(OV TO 5V OR

0V TO 3V)

REF

3V OR 5V

CLK V

OUT

LTC1659

CS/LD GND

1659_02

IN OUT

GND

LTC1258

LT1460

0.1µF

Figure 1. 12-Bit DAC with Side Output Swing is Tiny and Very Low Power,

May Be Upgraded to 14-Bit or 16-Bit Pin Compatible DAC

A family of low power 12-bit, 14-bit

and 16-bit DACs provides flexible designs

for digital control loops and other systems

requiring low power, low cost, small size

and rail-to-rail voltage outputs. The 12-bit

LTC1659 is a single DAC in 8-pin MSOP

and SO packages that draws only 250µA

from 3V or 5V supplies. The LTC1448 is a

dual 12-bit DAC in an SO-8 package that

draws 450µA, also from 3V or 5V supplies.

In a control loop application

(Figure 1), the REF pin may be connected

to a voltage less than V

or the pins can

be tied together as shown to obtain abso-

lute accuracy and a full-scale output. The

LTC1258 (6.5µA maximum quiescent cur-

rent) and LT1460 are precision references

in 8-pin MSOP and SOT-23 packages that

offer low power and small size. In this con-

figuration, the LTC1659 has a wide output

swing of 0V to 5V or 0V to 3V. The

LTC1448 can be substituted when dual

DACs are required.

In the same MSOP package and pinout

as the LTC1659, the LTC1658 provides

14-bit performance at very low power

(270µA supply current) on 3V or 5V supply

voltages. One compact board design can thus

support 12-bit and 14-bit applications.

For additional flexibility that includes a

16-bit option, use the 12-bit LTC1659 in the

SO-8 package. The 14-bit LTC1658 and the

16-bit LTC1655L (600µA), both available

in SO-8, fit the same layout and can be

used in 3V or 5V supply systems.

Each of these converters achieves

1LSB or better maximum DNL and

includes a 3-wire cascadable serial inter-

face. For more information, see the Febru-

ary, 1999, issue of Linear Technology

magazine.

A 1.1MHz Over-The-Top

Rail-to-Rail Op Amp—

LT1637

The LT1637 provides a unique and

versatile combination of micropower, preci-

sion and durability. Drawing only 250µA

supply current in normal operation, the

LT1637 is ideal for battery-operated, low

power applications and portable instru-

ments. The LT1637 offers excellent DC and

noise specifications, with a maximum offset

voltage of 350µV, maximum input bias cur-

rent of 50nA and a typical input voltage

noise of 27nV/ . In shutdown mode, the

op amp draws only 12µA of current and

places the output in high impedance mode

for easy multiplexing. At a typical gain

bandwidth of 1.1MHz and slew rate of

0.4V/µs, the LT1637 provides an excellent

speed-to-power ratio.

The true rail-to-rail input and output of

the LT1637 maximizes the dynamic range of

operation for the amplifier. This is especially

useful for low voltage, single supply applica-

tions powered by batteries, giving maximum

signal swing from limited supply voltages.

Over-The-Top operation allows for common

mode inputs of up to 44V, independent of

the positive supply. This feature is

extremely important for applications such as

high side current sensing or monitoring volt-

ages above the supply rails.

√Hz

The LT1637 provides tremendous dura-

bility, offering a wide supply range and

built-in input overvoltage protection fea-

tures. The supply voltage range extends from

2.7V to 44V, with an input stage designed to

take common mode voltages above the posi-

tive supply rail (up to 44V above the nega-

tive rail). Input voltages can also fall 22V

below the negative supply rail without harm

to the op amp. The LT1637 is protected

against reverse supply voltages up to 25V.

These protection features make the LT1637

a good choice for applications in high volt-

age industrial control systems.

The LT1637 is now available in the

8-lead small outline package and in the

8-lead micro-SO package (MSOP).

Linear Technology Chronicle

• October 1999

Linear Technology Corporation • 1630 McCarthy Blvd. • Milpitas, CA 95035-7417 • (408) 432-1900 • FAX: (408) 434-0507 • www.linear-tech.com • For Literature Only: 1-800-4-LINEAR

Contact your local Linear Technology sales office for a data sheet and evaluation samples. For more information, visit our web site at www.linear-tech.com.

Linear Technology Products

Are Distributed By:

Arrow Electronics

Arrow/Zeus Components

Digi-Key

Gerber Electronics

Marshall Industries

Wyle Electronics

DC/DC Converters from page 2

input capacitance required when operating

from a low impedance source such as a NiCd

or NiMH battery. Low-battery detector

accuracy of the LT1308A and LT1308B is

±2%. The 36V switch rating allows up to

34V outputs in boost mode and high input

and output voltages in SEPIC circuits.

A common requirement for GSM

terminals is an efficient, compact converter

that develops 5V from a single Li-Ion cell to

power the RF amplifier. The LT1308B

performs this function with few external

components, with efficiency that reaches

90%. Transient response of a 0A to 1A load

step with typical GSM profiling produces a

voltage droop of only 200mV.

Power supplies for digital cameras must

be small and efficient while generating

several voltages free of low frequency noise,

so that post filtering can be done easily. A

single LT1308A, along with an inexpensive

transformer in a coupled-flyback scheme,

generates 3.3V/200mA, 5V/200mA,

18V/10mA and –10V/10mA from a pair of

AA or AAA cells.

The LT1308A and LT1308B are offered

in an SO-8 package and are pin compatible

with the LT1307A and LT1307B which have

a 600mA switching current capability. This

permits the same board design to support

two different power levels.

Versatile SOT-23 Boost Switching

Regulator for 1.2V to 15V Input

The LT1615 is a micropower step-up

DC/DC converter in a 5-lead SOT-23 pack-

age. With an input voltage range from 1.2V

to 15V, this converter is ideal for a wide

range of applications and input sources. The

LT1615 features typical quiescent current of

20µA with no load, which further reduces to

0.5µA in shutdown. The LT1615 uses a cur-

rent limited, fixed off-time control scheme

that helps achieve high efficiency over a

broad range of load currents. Its short off-

time of 400ns permits the use of tiny, low

profile inductors and capacitors to minimize

the overall system footprint and cost

(Figure 6).

The LT1615 includes an internal 36V

switch that allows the device to generate

output voltages of up to 34V without the use

of costly transformers. This makes the

LT1615 attractive for applications such as

LCD biasing which need a high output volt-

age at a relatively low current. The converter

in Figure 7 demonstrates the input and out-

put voltage range of the LT1615. A 33V out-

put is generated from a wide range input

voltage using a simple boost topology. A

small 1µF ceramic capacitor is all that is

needed at the output. The circuit can deliver

up to 1.32W (40mA at 33V) of power from

an input of 2.5V to 4.2V with an efficiency

of 85%, all from a tiny SOT-23 package.

A typical requirement is for a 15V out-

put at 15mA from a single Li-Ion cell. Using

the circuit in Figure 7 with different resistor

values, efficiency for this LT1615 converter

reaches 82% and exceeds 70% even as the

battery voltage drops to 2.5V and for load

currents under 1mA.

A dual-output converter, with 20V and

–20V outputs, for example, may also be

generated by the LT1615 with efficiency

reaching 77% from a single Li-Ion cell

LT1615

3.3V

TO 12V

10µHD1

SHDN

76.8k

2M C2

1µF

33V

8mA

1615 TA03

GND

4.7µF

C1: TAIYO YUDEN EMK316BJ475 (408) 573-4150

C2: TAIYO YUDEN GMK316BJ105 (408) 573-4150

L1: MURATA LQH3C100K24 (814) 237-1431

D1: MOTOROLA MBR0540 (800) 441-2447

Figure 7. 1-Cell Li-Ion to 33V

Converter for LCD Bias

Figure 6. High Frequency and SOT

Package Allow a Very Small Circuit

(Shown 150% of Actual Size)

LT1615

1.5V TO 5V

10µHD1

SHDN

130k

2M C2

1µF

20V

4mA

–20V

4mA

1615 TA04

GND

1µF

4.7µF

C1: TAIYO YUDEN LMK316BJ475 (408) 573-4150

C2, C3, C4: TAIYO YUDEN TMK316BJ105 (408) 573-4150

L1: MURATA LQH3C100K24 (814) 237-1431

D1, D2, D3: MOTOROLA MBR0530 (800) 441-2447

15 4.7pF

1µF

Figure 8. ±20V Dual Output Converter Provides 8mA Output From 1.5V Input

(Figure 8). The addition of a diode and ca-

pacitor to this circuit allows both outputs to

be disconnected from the input in shutdown.

(See data sheet.)

Another popular application is a single-

cell Li-Ion battery to a 3.3V output using a

SEPIC (single-ended primary inductance

converter) topology. A simple SEPIC circuit

using the LT1615 will provide 100mA with

a typical efficiency of 70% using two small

10µH inductors or a single, dual-winding

inductor.

No other IC solution offers the high

output voltage, high efficiency and very

small size of the LT1615.