December 1999 - Linear Technology

Linear Technology Chronicle

• December 1999 1

Product of the Month

, LTC and LT are registered trademarks of Linear Technology

Corporation. Over-The-Top, C-Load, UltraFast and Burst Mode

are trademarks of Linear Technology Corporation. MICROWIRE

is a trademark of National Semiconductor Corporation. Pentium

is a registered trademark of Intel Corporation.

A Showcase of Linear Technology’s Focus Products

Linear Technology Chronicle

December 1999 Vol. 8 No. 12

Inside This Issue:

Linear Phase, DC Accurate, 10th Order Lowpass Filters. Cutoff Frequency

Set by a Single Resistor—LTC1569-6 and LTC1569-7 ........................................................... 2

High Speed, Low Power, Dual Comparator Creates 1ns Pulse Stretcher—LT1720 .............. 3

Micropower SOT-23 Charge Pump Delivers 5V at 50mA from Single Li-Ion—LTC1754-5 .... 3

Ultraprecise, Micropower, Low Dropout 2.5V Reference—LT1461-2.5 .................................. 4

Important Update: Micropower Voltage References Available in SOT-23—LT1460 .............. 4

For Mobile VID Processors: Dual Output 2-Phase DC/DC Converter

Combines Fast Transient Response and Small Solution Size—LTC1703 .............................. 4

Rugged, High Precision, Wide Operating Range—

LT1782/LT1783 SOT-23 Op Amps

Continued on page 2

The LT

1782 and LT1783 are

micropower, rail-to-rail, Over-The-Top

single op amps in SOT-23 packages. The

combination of 400µV offset voltage,

1500V/mV gain and 100dB CMRR (typical

values) places them among the highest preci-

sion micropower op amps in SOT-23. The

LT1782 and LT1783 feature a wide supply

range of 2.7V to 18V. Competing products

have operating voltages that are restricted to

6V or less, limiting their use in many appli-

cations.

The LT1782 and LT1783 are the first

SOT-23 op amps to operate with either or

both inputs above the positive rail (Over-

The-Top). This LTC innovation is important

in many high side current sensing applica-

tions (Figure 1) and in other cases where the

input is at or up to 6V above the supply

voltage.

Several features make the LT1782 and

LT1783 the toughest SOT-23 op amps in the

market. In addition to Over-The-Top opera-

tion, the inputs can be taken 9V below the

negative supply without phase reversal or

damage. This prevents false outputs from

occurring when the inputs are below the

negative rail. The LT1782 and LT1783

require no special protection when operating

on batteries, since they are internally pro-

tected for 18V of reverse supply. Competing

products require external protection circuitry

or will be damaged if the supplies are acci-

dentally reversed.

Unlike other SOT-23 op amps that

require high input overdrive to swing near

the rails, a few millivolts of overdrive is

enough to swing the outputs of the LT1782/

LT1783 to their guaranteed value (Table 1).

Both input and output operate rail-to-rail.

The LT1782 has a 200kHz bandwidth

and draws less than 55µA supply current.

The LT1783 is faster with a 1.25MHz band-

width, yet its supply current is still less than

300µA. Each device can drive loads of up to

18mA. The excellent DC characteristics

combined with the micropower rail-to-rail

operation make these op amps an excellent

choice in portable, battery-operated applica-

tions where performance is critical.

The LT1782 and LT1783 are available

in 5-lead and 6-lead SOT-23 packages. The

6-lead version includes a shutdown function

that limits the supply current to a maximum

of 5µA.

–

LT1782

LOAD

OUT

= 2Ω(I

LOAD

)

0V TO 4.3V

200Ω

1782 TA01

0V TO 18V

200Ω

0.2Ω

LOAD

2N3904

Figure 1. Positive Supply Rail

Current Sense

Table 1. LT1782/LT1783 Performance,

VS = 3V/0V or 5V/0V, TA = 25°C

Parameter* LT1782 LT1783

Supply Voltage Range 2.7V to 18V 2.7V to 18V

Supply Current 55µA 300µA

Input Offset Voltage 800µV 800µV

Input Bias Current 15nA 80nA

Input Offset Current 2nA 8nA

Open-Loop Gain, 200V/mV 200V/mV

= 10k

PSRR 90dB 90dB

CMRR 90dB 90dB

Common Mode Range 0V to 18V 0V to 18V

Output Swing, Low 8mV 8mV

Output Swing, High 90mV 90mV

Slew Rate (typ) 0.07V/µs 0.42V/µs

Gain Bandwidth 200kHz 1.25MHz

Product (typ)

C-LoadTM Stability 500pF 500pF

Input Noise 50nV/

√Hz

20nV/

√Hz

Voltage (typ)

Input Noise 0.06pA/

√Hz

0.14pA/

√Hz

Current (typ)

*Guaranteed value unless noted as typ (typical).

See data sheets.

16-Bit Rail-to-Rail

Micropower DAC Runs

On Single 3V Supply—

LTC1655L

The LTC

1655L is a single, 16-bit,

rail-to-rail, micropower DAC powered by a

single 3V supply. A lower supply voltage

and lower power version of the LTC1655,

the LTC1655L typically draws only 600µA

of supply current. With ±1LSB maximum

DNL error, the LTC1655 offers 16-bit

monotonicity over its full temperature range,

critical for closed-loop feedback systems

such as industrial control. The LTC1655L’s

low power and high performance make it

Linear Technology Chronicle

• December 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.

Figure 1. LTC1655L 16-Bit Rail-to-Rail VOUT DAC

LTC1655L from page 1

ideal for applications such as automatic test

equipment, instrumentation and digital cali-

bration.

A built-in 1.25V bandgap reference sets

the output voltage range from 0V to 2.5V.

This reference pin can also be overdriven to

higher voltages to achieve larger output

swings (up to the positive supply rail). The

LTC1655L also includes an internal rail-to-

rail output buffer, eliminating the need for

any external op amps.

The LTC1655L features a 3-wire serial

interface compatible with SPI/QSPI and

MICROWIRE

protocols. The inputs are

TTL/CMOS level compatible for ease of use

and the CLK input has an internal Schmitt

trigger, providing the noise immunity neces-

sary for direct optocoupler connection to the

inputs.

–

16-BIT

DAC

2.7V TO 5.5V 1.25V

GND

POWER-ON

RESET

OTHER

DACS

16-BIT

SHIFT

REG

AND

DAC

LATCH

µP

REF

OUT

1655 TA01

SCK1

CS/LD3 7RAIL-TO-RAIL

VOLTAGE

OUTPUT

(0V TO 2.5V)

OUT

REF

With its standard pinout, the

LTC1655L provides a convenient pin

compatible upgrade path for users of LTC’s

12-bit voltage output DAC family. The

LTC1655L is available in SO-8 and N8

packages in both commercial and industrial

temperature ranges.

1µF

1/4

1/16

1/1

EXT

= 10k

100pF

1µF

3.48k

LTC1569-6

1569-6 TA01

–

GND

–

OUTV

OUT

DIV/CLK

FREQUENCY (kHz)

GAIN (dB)

–20

–40

–60

–80

–100 10 100 1000

1569-6 TA01a

Figure 1. LTC1569-6 Lowpass Filter Figure 2. LTC1569-6 Frequency Response

Linear Phase, DC Accurate, 10th Order Lowpass Filters.

Cutoff Frequency Set by a Single Resistor—LTC1569-6 and LTC1569-7

The LTC1569-6 and LTC1569-7 are

10th order lowpass filters featuring linear

phase, a root raised cosine amplitude

response and DC accuracy. This unique

family of switched capacitor filters is the

first in the industry not to require an exter-

nal clock to set the filter’s cutoff frequency.

The LTC1569-7 is designed for higher fre-

quency applications with cutoff frequencies

up to 300kHz on a single 5V supply. The

LTC1569-6 is the lower power model with

power consumption as low as 8mW on a

single 3V supply.

The LTC1569-6 should be

used in applications with cutoff frequencies

below 64kHz. Both versions can operate on

power supplies from 3V to ±5V.

Most other sampled data filters use an

external clock to set the cutoff frequency.

Clock generation can therefore become

quite difficult, requiring a crystal based

master clock to be divided down to precisely

the desired frequency. The LTC1569 family

of filters avoids these complications. The

cutoff frequency is set with a single external

resistor. The filter cutoff frequency is accu-

rate to ±3% for a given resistor value. Addi-

tionally, with a single pin the user can

divide the cutoff frequency by 1, 4 or 16.

This allows up to three cutoff frequency

options to be obtained with each resistor

value. Using various resistor values and

divider settings, the cutoff frequency can be

programmed over a range of six octaves.

Alternatively, the cutoff frequency can be

set with an external clock.

The LTC1569 family has excellent DC

performance and dynamic range. An offset

voltage of less than 5mV, DC linearity of 12

bits and common mode rejection ratio of

80dB make the LTC1569 well suited for DC

accurate systems. Both filters offer differen-

tial or single ended inputs.

The high selectivity of the LTC1569-6

combined with its linear phase in the pass

band makes it suitable for filtering both in

data communications and data acquisition

systems. Furthermore, its raised root cosine

response offers the optimum pulse shaping

for PAM data communications. The

LTC1569-6 and LTC1569-7 are available in

SO-8 packages.

Linear Technology Chronicle

• December 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

High Speed, Low Power, Dual Comparator

Creates 1ns Pulse Stretcher—LT1720

For detecting short pulses from a

single sensor, a pulse stretcher is often

required. The circuit of Figure 1 acts as a

one-shot, stretching the width of an incom-

ing pulse to a consistent 100ns. Unlike a

logic one-shot, this circuit triggers on only

100pV-s of stimulus. The UltraFast

LT1720 dual comparator draws a low 4mA

per comparator.

Comparator C1 functions as a thresh-

old detector, whereas comparator C2 is con-

figured as a one shot. The first comparator

is prebiased with a threshold of 80mV to

overcome comparator and system offsets

and to establish a low output in the absence

of an input signal. An input pulse causes the

output of C1 to go high, which, in turn,

latches C2’s output high. The output of C2

is fed back to the input of C1, causing

regeneration and latching both outputs high.

Timing capacitor C now begins charging

through R and after 100ns, C2 resets low.

The output of C1 also goes low, latching

both outputs low. A new pulse at the input

of C1 can now restart the process. Timing

capacitor C can be increased without limit

for longer output pulses.

This circuit has an ultimate sensitivity

of better than 14mV with 4ns to 10ns input

pulses. It can even detect an avalanche

–

PULSE SOURCE C1

1/2 LT1720

50Ω

51Ω

6.8k

1N5711

24Ω

15k

2k 2k

1720 F15

100pF

0.01µF

OUTPUT

100ns

generated test pulse of just 1ns duration

with sensitivity better than 100mV. (See

Linear Technology magazine, November

1998 and Application Note 47.) New single

and quad versions of the LT1720 are also

available (LT1719 and LT1721).

Figure 1. Low Power 1ns Pulse Stretcher

Micropower SOT-23 Charge Pump Delivers 5V at

50mA from Single Li-Ion—LTC1754-5

The LTC1754-5 is a micropower 5V

charge pump in a 6-lead SOT-23 package

that delivers up to 50mA of output current

from a single Li-Ion cell. Its tiny size and

extremely low operating and shutdown cur-

rents (13µA and less than 1µA typically)

LOAD CURRENT (mA)

0.001

EFFICIENCY (%)

0.01 0.1 1 10 100

1754-5 G05

100 VIN = 3V

TA = 25°C

CFLY = 1µF

LTC1754-5

ON/OFF

OUT

1754-5 TA01

2.7V TO 5.5V

OUT

= 5V ±4%

OUT

= 0mA TO 25mA, V

> 2.7V

OUT

= 0mA TO 50mA, V

> 3.0V

10µF

1µF

10µF

GND

SHDN

–

Figure 1. Efficiency vs Load Current

for the LTC1754-5 Figure 2. Regulated 5V Output from a 2.7V to 5.5V Input

extend battery operating time and make it

an excellent choice for a wide variety of

space-restricted applications, such as cellu-

lar phones, bar code and smart card readers,

handheld computers and portable instru-

ments. The LTC1754-5 is also useful for

battery back-up supplies and local 3V-to-

5V conversion.

The LTC1754-5 operates over an input

voltage range of 2.7V to 5.5V at an operat-

ing frequency of 700kHz. It produces a

regulated output voltage of 5V with a ±4%

tolerance. Efficiency is greater than 82%

from 1mA to 50mA (Figure 1). This

inductorless DC/DC converter requires only

three small capacitors to form a complete

boost converter circuit that fits into a PC

board space of just 0.05 square inches

(Figure 2). The high frequency also means

lower ripple that is easier to filter for RFI

suppression.

The device offers short-circuit and ther-

mal protection—important fault tolerant

features—and the load is disconnected from

the input in shutdown which eliminates any

battery drain. The LTC1754-5 is available

in the tiny 6-lead SOT-23 package. It’s an

excellent choice whenever an ultrasmall 5V

supply is required using a single Li-Ion

cell.

Linear Technology Chronicle

• December 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.

For Mobile VID Processors: Dual Output 2-Phase DC/DC Converter

Combines Fast Transient Response and Small Solution Size—LTC1703

The LTC1703 is a 2-phase DC/DC syn-

chronous dual controller for 5V or 3.3V sup-

plies that allows the smallest solution size.

Mobile PCs using the most recent Intel

Pentium

III processors require very high

performance DC/DC conversion coupled

with a DAC-controlled voltage to the core

supply. The LTC1703 is designed specifi-

cally for this application and includes a 5-bit

DAC controlling one of the outputs. The

LTC1703 creates two high current outputs at

voltages as low as 0.8V.

The LTC1703 enables the use of

smaller, less expensive external components

than competing solutions. Its 25MHz GBW

error amplifier provides faster transient

response than conventional solutions, thus

reducing output capacitance requirements.

The 2-phase architecture runs each output

stage 180° out-of-phase to lower the input

ripple current; this minimizes input capaci-

tance, lessens EMI radiation and simplifies

filtering. The LTC1703’s 550kHz constant

frequency operation and compact 28-lead

SSOP package also shrink solution size.

The high power, all N-channel synchro-

nous design (Figure 1) allows the LTC1703

to drive large MOSFETs with high gate

capacitance: each output channel can sup-

port up to 25A of output current with excel-

lent total output accuracy of 1%. It also

features Burst Mode

operation at light

Ultraprecise, Micropower, Low Dropout 2.5V Reference—LT1461-2.5

The LT1461-2.5 is the first low dropout

reference to combine the high precision of

power hungry references with the low power

consumption of micropower references. The

LT1461 draws only 35µA of supply current,

making it ideal for low power and portable

applications. However, its high 50mA output

drive makes it suitable for higher power

requirements such as precision regulators.

In low power applications, a dropout

voltage of less than 300mV ensures maxi-

mum battery life while maintaining full ref-

erence performance. Line regulation is

nearly immeasurable, while the precise load

and thermal regulation will not add signifi-

cantly to system error budgets. A shutdown

pin further reduces power when output is

not required.

The LT1461-2.5 is available in four

performance grades and three temperature

ranges, as shown in Table 1.

Table 1. LT1461-2.5 Performance Grades

Performance Grade Temperature Coefficient Tolerance Temperature Range

A 3ppm/°C 0.04% C (0°C to 70°C) and I (–40°C to 85°C)

B 7ppm/°C 0.06% C (0°C to 70°C) and I (–40°C to 85°C)

C 12ppm/°C 0.08% C (0°C to 70°C) and I (–40°C to 85°C)

D 20ppm/°C 0.15% H (–40°C to 125°C)

1µF

≥ 2.8V 2.5V

2µF

1461 TA01

LT1461-2.5

Figure 1. LT1461-2.5 Voltage Reference

Important Update: Micropower Voltage References

Available in SOT-23—LT1460

The LT1460 family of micropower

series voltage references is now available in

the tiny SOT-23 package, offering high ac-

curacy in a small footprint. Further reducing

board area, the LT1460 does not require an

output compensation capacitor and the out-

put voltage is stable for a wide range of out-

put capacitance values. Fixed output

voltages of 2.5V, 3V, 3.3V, 5V, and 10V are

available in the SOT-23 package.

The LT1460 is available in three perfor-

mance grades. The high performance “A”

grade parts have an initial accuracy of 0.2%

and a temperature coefficient of 20ppm/C.

All grades are micropower references with

supply currents as low as 180µA.

BOOST

PGND

OUT

1703 F02

OUT

EXT

1µF

LTC1703

Figure 1. LTC1703 Switching

Architecture

loads for better than 85% efficiency over a

wide range of output currents. Onboard

power good and overvoltage (OV) fault flags

indicate when the output is in regulation or

an overvoltage fault has occurred. Either out-

put can be shutdown without affecting the

operation of the other side. The LTC1703’s

quiescent current drops below 100µA when

both outputs are shut down.

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