November 1999 - Linear Technology

Linear Technology Chronicle

• November 1999 1

Product of the Month

, LTC and LT are registered trademarks of Linear Technology

Corporation. Hot Swap, Over-The-Top, PolyPhase and

OPTI-LOOP are trademarks of Linear Technology Corporation.

C is a trademark of Philips Electronics N.V.

A Showcase of Linear Technology’s Focus Products

Linear Technology Chronicle

November 1999 Vol. 8 No. 11

Inside This Issue:

Micropower Current Sense Amp Operates from 2.5V to 60V

with 40µV Offset Error—LT1787 ............................................................................................. 2

High Efficiency Linear and Switching Solutions for Splitting

a Digital Supply—LT1118 and LTC1504 ................................................................................ 3

High Efficiency PolyPhaseTM DC/DC Current Mode Controllers Provide 20A to 200A

Using Surface Mount Components—LTC1629/LTC1929 ....................................................... 3

A 10-Bit Voltage Output Micropower DAC in a Tiny SOT-23 Package—LTC1663 ................ 4

Low Noise, Micropower, Low Dropout Regulators

Withstand Reverse Voltage—LT1762 and LT1763

The LT

1762 and LT1763 are very low

noise, low dropout linear regulators. The

LT1762 is rated for 150mA of output cur-

rent, whereas the LT1763 is rated for

500mA. For supply flexibility and good effi-

ciency, the input voltage of these regulators

can be as low as 350mV (maximum dropout

voltage) above the output at maximum rated

current. The regulators are designed for use

in battery-powered systems with 25µA oper-

ating current for the LT1762 and 30µA for

the LT1763. Both regulators feature a 0.1µA

shutdown state. Quiescent current is well

controlled for these devices; it does not rise

in dropout—when the input to output voltage

differential is below the dropout rating—as

is the case with many other regulators.

The LT1762 and LT1763 regulators

feature low noise operation. With the addi-

tion of an external 0.01µF bypass capacitor,

output voltage noise over the 10Hz to

100kHz bandwidth is reduced to <30µVRMS

(Figures 1 and 2). Both regulators can oper-

ate with output capacitors as low in value as

2.2µF for the LT1762 and 4.7µF for the

LT1763. Small ceramic capacitors can be

used with either device without the need for

additional series resistance as is common

with other regulators. Internal protection

circuitry on both regulators includes reverse-

battery protection, current limiting, thermal

limiting and reverse-current protection. No

protection diodes are required.

Both regulators are available in fixed

output voltages of 2.5V, 3V, 3.3V and 5V or

as an adjustable device with an output range

of 1.22V to 20V. The LT1762 is packaged in

an 8-lead MSOP and the LT1763 is avail-

able in an 8-lead SO package. These regula-

tors can be paralleled for more output

current if required.

SHDN

0.01µF

10µF

1762 TA01

OUT

SENSE

3.7V TO

20V

BYP

GND

LT1762-3.3

3.3V AT 150mA

20µV

RMS

NOISE

1µF

CBYP (pF)

OUTPUT NOISE (µVRMS)

160

140

120

100

0100 1000 10000

1762 G42

COUT = 10µF

IL = 150mA

f = 10Hz TO 100kHz

LT1762-5

LT1762-3.3

LT1762-3

LT1762

LT1762-2.5

Figure 1. 3.3V Low Noise Regulator

Figure 2. LT1762 RMS Output Noise

vs Bypass Capacitor

Positive High Voltage

Hot Swap Controller—

LT1641

When a circuit board is inserted into a

live backplane, the supply bypass capacitors

on the board draw high peak currents from

the backplane power bus as they charge. The

transient currents can permanently damage

the connector pins and cause glitches on the

system supply, causing other boards in the

system to reset.

The LT1641 is designed to turn on a

board’s supply voltage in a controlled man-

ner (Figure 1), allowing the board to be

safely inserted into or removed from a live

backplane. A high side switch driver con-

trols an N-channel pass transistor in the

power path (Q1 in Figure 2) for supply volt-

ages ranging from 9V to 80V. The LT1641

is tested and guaranteed to be safe from

damage with supply voltages up to 100V and

may be connected directly to 24V and 48V

supplies without floating grounds or level

shifting of logic signals. The device provides

undervoltage and overcurrent protection and

a power good signal that indicates when the

output supply voltage is ready. It comes in

an 8-lead SO package.

Continued on page 2

Figure 1. LT1641 Power-Up Waveforms

Linear Technology Chronicle

• November 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.

Micropower Current Sense Amp Operates from 2.5V

to 60V with 40

V Offset Error—LT1787

TIMER

SENSE GATE

876

CMPZ

5248B

GND

PWRGD PWRGD

10Ω

49.9k

24V

GND

1641 TA01

3.4k

0.68µF

R6,

1k, 5%

0.01Ω

10nF

59k

3.57k

24k

OUT

LT1641

IRF530

Short-Circuit Protection

To prevent excessive power dissipation

in the pass transistor and voltage spikes on

the input supply during short-circuit and

overcurrent conditions at the output, the

LT1641 features a programmable foldback

current limit with an electronic circuit

breaker. The current limit is set by placing a

Figure 2. 24V Input Voltage Application

sense resistor between the V

and SENSE

pins. The current folds back as a function of

the output voltage which is sensed at the FB

(feedback) pin. If the chip remains in cur-

rent limit beyond a programmable time

limit, the pass transistor latches off. The

device may be set to automatically restart

after a time-out delay.

A variable overcurrent response time

provides additional protection under severe

fault conditions. The larger the

fault voltage across the sense

resistor, the faster the gate will be

pulled down, protecting the load.

Power Good Detection

The LT1641 includes a comparator for

monitoring the output voltage. The nonin-

verting input (the FB pin) which includes

80mV of hysteresis is compared against an

internal 1.233V precision reference. The

comparator’s output (the PWRGD pin) is an

open collector capable of operating from a

pull-up as high as 100V. PWRGD can be

used to directly enable and disable a power

converter module.

Table 1 shows the entire line of

Hot Swap

controllers from Linear

Technology.

LT1641 from page 1

Table 1. Hot Swap Controllers

Device #Supplies Supplies Features

LTC®1421 2 3V to 12V Controls 3rd Rail to –12V

LTC1422 1 3V to 12V SO-8

LT1640 1 –10V to –80V –48V in SO-8

LT1641 1 9V to 80V SO-8, Foldback I

LIMIT

LTC1642 1 3V to 16.5V Fault Protected to 33V

LTC1643 4 3.3V, 5V, ±12V Four Supplies, Tiny Package

The LT1787 is a high resolution current

sense amplifier designed for applications

such as precise monitoring of the current

into or out of a battery (gas gauging) in cel-

lular telephones, portable instruments, com-

puters and wireless telecom devices. The

LT1787 has an operating voltage range of

2.5V to 36V so that the device can operate

from a single Li-Ion cell and from most

other sources. The LT1787HV version can

operate on supplies up to 60V, making it

useful for high voltage telecommunication

and industrial systems.

The LT1787 monitors bidirectional cur-

rents via the voltage across an external sense

resistor (Figure 1). A current or voltage out-

put from the device depicts the direction and

magnitude of the sensed current. The

LT1787 allows the use of a selectable sense

resistor, the value of which depends on the

current to be monitored. This allows the

input voltage to be adjusted to a 128mV full-

scale range, allowing a maximum dynamic

range. The part is generally used at ±128mV

SENSE

1787 F 01

G2A

1.25k

G2B

1.25k

G1A

1.25k

G1B

1.25k

OUT

BIAS

OUT

20k

S–

–+

Q1 Q2

1:1 MIRROR

FIL

–

S+

FIL

SENSE

LT1787HV

RSENSE

0.0016Ω

1787 TA01

1µF5V

FIL+

VS+

FIL–

VS–

DNC

VEE

15k

0.1µF

VOUT = (8 • VSENSE) + VREF

OUTPUT STEP = 13mV/Amp

I = 100A

60V

LOAD

LT1634-1.25

TO µP

VREF VCC

GND

LTC1286

CLK

DOUT

+IN

–IN

VBIAS

VOUT

ROUT

20k

full scale, although it is specified for a

±500mV minimum full scale. The 40µV

input offset voltage of the LT1787 translates

into a 12-bit minimum dynamic range in

resolving currents.

The LT1787 has a fixed voltage gain of

8 from input to output, set by onboard preci-

sion resistors (Figure 2). It is self-powered

from the supply that it is monitoring and

requires only 60µA supply current. Addi-

tional features include provisions for input

noise filtering (both differential and common

mode) and a PSRR of greater than 120dB.

The device is available in both 8-lead SO

and MSOP packages.

Figure 1. 12-Bit Dynamic Resolution Bidirectional Output to an ADC Figure 2. LT1787 Functional Diagram

Linear Technology Chronicle

• November 1999 3

1629 TA01

TG1

BOOST1

SW1

BG1

PGND

SENSE1

–

TG2

BOOST2

SW2

BG2

INTV

SENSE2

–

RUN/SS

EAIN

DIFFOUT

OS–

OS+

LTC1629

SGND

0.1µF

16k

1000pF

10Ω

10k 100pF

16k

10µF ×4

35V

CERAMIC

OUT

1000µF ×2

OUT

: T510E108K004AS

L1, L2: CEPH149-IROMC

OUT

1.6V/40A

1µH0.002Ω

5V TO 28V

1µH

×2

0.47µF

10µF

0.002Ω

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 Efficiency Linear and Switching Solutions for

Splitting a Digital Supply—LT1118 and LTC1504

This note presents two methods of cre-

ating a split supply with good transient

response and minimal supply current in a

5V-only system.

The LT1118 is a linear regulator

designed to source or sink current to keep

its output in regulation. It draws 600µA of

quiescent current typically and can source

800mA or sink 400mA, enough to satisfy

most analog subsystems. The LT1118

requires only two external components

(Figure 1) and has a DC output impedance

below 0.1Ω. The LT1118 provides nearly

50% power efficiency over a wide range of

load currents (Figure 2). Load transient

response is excellent with less than 5µs

recovery time from a ±400mA load step.

At low current levels the LT1118 is the opti-

mum solution.

The LTC1504 is a synchronous switch-

ing regulator that provides as much as 90%

efficiency (Figure 2) while sourcing or sink-

ing up to 500mA. It will draw only 56mA

from the 5V supply while sourcing 100mA

from its 2.5V output. The switching architec-

ture of the LTC1504 requires more external

components than the LT1118 (Figure 3) and

generates a small amount of output noise

(ripple) at its 200kHz switching frequency.

With a 47µH inductor, switching noise is

minimal and the circuit recovers from a

±400mA output load step in 30µs. A shut-

down function drops quiescent current

below 10µA when the split supply is not

required. The LTC1504 is the best solution

where efficiency, especially at high current

levels, is the overriding concern. (See

Design Note 172)

LT1118-2.5

GND

0.1µF

0.6mA

OUT

1µF

2.5V

VIRTUAL GROUND

OUT

< 0.1Ω

DN172 F02

High Efficiency PolyPhase DC/DC Current Mode Controllers Provide 20A to

200A Using Surface Mount Components—LTC1629/LTC1929

The LTC1629 and LTC1929 are high

power true current mode DC/DC controllers.

These dramatically simplify the design of

DC/DC converters for powering today’s

high end microprocessors, large memory

arrays and other high power loads. Using a

unique PolyPhase switching technique, the

LTC1629 can be configured to supply load

currents up to 200A with inherent current

sharing using readily available surface

mount components. This PolyPhase tech-

nique allows a significant reduction of input

and output ripple currents, resulting in high

efficiency, low EMI DC/DC converter

designs that require very little PC board

space.

Unlike traditional switching regulators,

the LTC1629 and LTC1929 interleave the

clock signals of their two paralleled power

stages 180° out of phase. Ripple current

Continued on page 4

Figure 1. High Current Dual Phase Step-Down Converter

LOAD CURRENT (SINK/SOURCE)(mA)

EFFICIENCY (%)

10 100 1000

DN172 F03

100

A: LT1118 SOURCE

B: LT1118 SINK

C: LTC1504/47µH SOURCE

D: LTC1504/47µH SINK

E: LTC1504/22µH SOURCE

F: LTC1504/22µH SINK

MAX

SHDN

SS COMP

GND

4.7µF

CERAMIC

SPLIT SUPPLY

2.5V ±500mA

SHDN

7.5k

0.01µF

220pF

11.8k

12.1k

OUT

EXT

OUT

: AVX TAJC476M016R

EXT

: SUMIDA CDRH73-470 (47µH: LOW RIPPLE, HIGH EFFICIENCY)

SUMIDA CDRH73-220 (22µH: FAST TRANSIENT RESPONSE)

DN172 F05

LTC1504

Figure 3. LTC1504 Supply Splitter

Figure 2. Efficiency Comparison

Figure 1. LT1118-2.5 Supply Splitter

cancellation occurs in both the input and

output circuits due to properly phased cur-

rent pulses. Their multiple phase technique

effectively multiplies the fundamental fre-

quency by the number of channels used,

Linear Technology Chronicle

• November 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

LTC1629/LTC1929 from page 3

improving transient response while operating

each channel at an optimum frequency for

efficiency. This allows a dramatic reduction

in input and output capacitance requirements

and EMI generation while improving overall

efficiency.

The LTC1929 operates in a 2-phase

configuration and can support load currents

up to 40A. The LTC1629 adds additional

internal clock management circuitry that

allows multiple devices to be connected in

parallel to support higher currents. Using

this technique, the LTC1629 can be used in

DC/DC converter circuits incorporating up

to twelve phases to provide load currents up

to 200A.

A wide 4V to 36V input range allows

the LTC1629 and LTC1929 to operate from

a variety of input sources and OPTI-LOOP

compensation allows the transient response

to be optimized for the specific output

capacitors used in each application. The

LTC1629 and LTC1929 provide output volt-

ages from 0.8V to 6V. An internal differen-

tial amplifier provides true remote sensing of

the regulated supply’s positive and negative

output terminals as required in high current

applications. Internal current foldback with a

shutdown timer and a soft-latch overvoltage

protection feature helps protect the load

against harmful voltage spikes and

overcurrent conditions.

The LTC1629 and LTC1929 are both

offered in a 28-lead SSOP package, in com-

mercial and industrial temperature grades.

Where previous high power applications

required the use of an expensive and bulky

DC/DC converter module, the LTC1629

and LTC1929 offer a smaller and more

efficient alternative that can be easily inte-

grated into even the most space-constrained

system.

A 10-Bit Voltage Output

Micropower DAC in a

Tiny SOT-23 Package—

LTC1663

The LTC1663 is a single, 10-bit, volt-

age output DAC with true buffered rail-to-

rail output voltage capability, all in a tiny

SOT-23 package. Operating from supply

voltages between 2.7V and 5.5V, the

LTC1663 consumes only 60µA of supply

current and a mere 10µA in shutdown

mode. The part also features a simple

2-wire serial interface compatible with

SMBus and I

, allowing multiple de-

vices to drive and monitor the bus without

contention. In the MS8 package, eight user-

selectable addresses allow up to eight

LTC1663s to be placed on the same bus. Its

ultralow power consumption and small size

make the LTC1663 ideal for portable, bat-

tery-powered applications where supply

current directly affects the operation time

and board space is at a premium.

The LTC1663 allows the user to select

either an internal 1.25V bandgap reference

or the positive supply as the DAC’s refer-

ence voltage. Selecting the internal reference

will set the full-scale output voltage to

0V–2.5V. Selecting VCC as the reference

10-BIT

DAC LATCH

INPUT

LATCH

2-WIRE INTERFACE

SDA

AD0

AD1

AD2

SCL

GND

VOUT 8

1663 BD

VCC

1.25V

BANDGAP

REFERENCE

SELECT

–

COMMAND

LATCH

extends the full-scale output to the positive

supply voltage, adding extra flexibility for

ratiometric applications and maximizing the

output voltage range.

The LTC1663 includes a rail-to-rail

output buffer, eliminating the need for exter-

nal op amps. A DNL of ±0.75LSB (max)

guarantees true 10-bit monotonic perfor-

mance over the full temperature range, criti-

cal in control loop applications. A power-on

reset function ensures that the DAC output is

at 0V when power is initially applied and all

internal registers are cleared.

The LTC1663 is available in MS8 and

5-lead SOT-23 packages.

Figure 1. LTC1663 Block Diagram