November 1995 - Linear Technology

Linear Technology Chronicle •

November 1995

Product of the Month

A Showcase of Linear Technology's Focus Products

Linear Technology Chronicle

November 1995 Vol. 4 No. 11

High Current, High Efficiency Step-Down Switching

Regulator Controller Has 1% Accurate Output

LOAD CURRENT (A)

100

EFFICIENCY (%)

LTC1430 • TA02

0.1 1

= 25°C

= 5V

OUT

= 3.3V

Figure 2. LTC1430 Efficiency Remains

Over 90% from 1A to 10A

Continued on page 2

, LTC and LT are registered trademarks of Linear Technology Corporation.

The LTC1430 includes a fixed fre-

quency PWM oscillator for a predictable

output noise spectrum under virtually all op-

erating conditions. The 200kHz free-running

clock frequency can be externally adjusted

from 50kHz to above 400kHz. The

LTC1430 features a low 350µA quiescent

current, allowing greater than 90% efficient

converter designs. The output drivers of the

LTC1430 can drive 10,000pF gates at

greater than 300kHz. Figure 2 shows the ef-

ficiency of the LTC1430 when used as a 5V

to 3.3V converter with 10A output current.

Shutdown mode drops the LTC1430 supply

current to 1µA.

The LTC

1430 is a high power, high

efficiency switching regulator controller op-

timized for 5V to 3.3V applications. It in-

cludes a precision internal reference and an

internal feedback system that provides 1%

output voltage accuracy over line, load and

temperature variations. The LTC1430 uses a

synchronous switching architecture with two

N-channel output devices, eliminating the

need for a high power, high cost P-channel

device. Additionally, it senses output current

across the drain source resistance of the up-

per N-channel FET, providing an adjustable

current limit without an external low value

sense resistor. Figure 1 shows a typical

LTC1430 application.



220µF

×4

OUT



330µF

×6

2.7µH/15A

CC1

FREQSET

SHDN

COMP

CC2

PGND

GND

MAX

G2NC

LTC1430 • TA01

SHUTDOWN

FB NC

M1A, M1B, M2: MOTOROLA MTD20N03HL

: AVX-TPSE227M010R0100

OUT

: AVX-TPSE337M006R0100

SENSE

LTC1430

SENSE

–

16k

100Ω



7.5k



4700pF

C1

220pF

1N4148

0.1µF

1µF

0.01µF

0.1µF

4.7µF0.1µF

3.3V

10A

M1A, M1B

2 IN PARALLEL

Figure 1. All N-Channel 5V to 3.3V, 10A Power Supply for

Desktop Computer Systems

The LTC1430 is an ideal regulator for

powering Pentium

and Pentium

Pro micro-

processors. The device is also ideal as a high

power, high efficiency 5V to 3.xV regulator

for logic supplies and battery voltage

sources. The device is available in 16-lead

plastic dual-in-line and surface mount pack-

ages. A low cost version is also available in

an 8-lead plastic dual-in-line and surface

mount package. The 8-lead version elimi-

nates soft start, current limiting and the ex-

ternally programmable clock. Devices are

specified from 0°C to 70°C. Contact your lo-

cal Linear Technology sales office for a data

sheet and free evaluation samples of the

LTC1430.

Pentium is a registered trademark of Intel Corporation.

Low Power, ±10kV

Protected RS485

Transceiver Operates

on 3.3V Supply

The LTC1480 is an ultralow power

differential line transceiver which provides

full RS485 compliance while operating

from a single 3.3V supply. This new device

also meets the requirements of RS422 and

features high speed operation to 2.5Mbps.

The CMOS design offers significant power

savings without sacrificing ruggedness

against overload or ESD damage. Typical

quiescent current is only 300µA and shut-

500kHz Switching Regulator Has On-Chip 3A Power Switch,

Up to 90% Efficiency in Boost Applications ............................................................................ 2

A Circuit That Smoothly Switches Between 3.3V and 5V ....................................................... 3

5A Low Dropout Linear Regulator Has Fast Transient Response for

Microprocessor Applications.....................................................................................................4

Regulated Positive-to-Negative Voltage Converter Synchronized to System Clock ............... 4

Inside This Issue:

Linear Technology Chronicle •

November 1995

down current is only 1µA. Figure 1 shows a

3.3V powered RS485 network using the

LTC1480.

The driver and receiver of the

LTC1480 feature three-state outputs, with

the driver outputs maintaining high imped-

ance over the entire common mode range.

Excessive power dissipation caused by bus

contention or faults is prevented by a ther-

mal shutdown circuit which forces the

driver outputs into a high impedance state.

The receiver of the LTC1480 has a fail-safe

feature which guarantees a high output state

when the inputs are left open. Also, the I/O

pins are protected against multiple ESD

strikes of up to ±10kV.

The LTC1480 is an ideal RS485 inter-

face device for use in battery-powered

LTC1480 from page 1

RS485 networks or as a low power level

translator. The LTC1480 is fully specified

over the commercial and extended indus-

trial temperature range. The LTC1480 is

available in 8-pin plastic surface mount and

8-pin dual-in-line packages. Contact your

local Linear Technology sales office for a

data sheet and free evaluation samples of

the LTC1480.

RO

RE

DE

1

2

3

8

7

6

LTC1480 • TA01

LTC1480

120Ω120Ω

SHIELD SHIELD R

3.3V

RO

RE

DE

1

2

3

8

7

6

LTC1480

3.3V

1234

8765

RO RE DE D1

Figure 1. RS485 Network Runs on 3.3V Supply

500kHz Switching

Regulator Has On-Chip

3A Power Switch, Up to

90% Efficiency In Boost

Applications

The LT

1371 is a 500kHz fixed fre-

quency current mode switching regulator IC

which contains a fully protected 3A switch.

This new device provides surface mount

packaging, logic compatible shutdown, easy

synchronization capability up to 800kHz, op-

eration from 2.7V to 30V, negative or posi-

tive voltage sensing and low supply current.

The high frequency operation of the LT1371

allows small inductors and capacitors to be

used and keeps output frequency compo-

nents away from critical IF frequencies.

Similar in architecture to the 1.5A

LT1372 , the 3A LT1371 extends the avail-

able output power such that a 5V to 12V

converter can deliver up to 800mA with a

LT1371

GND

LT1371 • TA01

S/S

L1*

4.7µH

C1**

22µF

25V

C4**

22µF

25V

× 2

C2

0.047µFC3

0.0047µF



R3

R2

6.19k

R1

53.6k

OUT†



12V

D1

MBRS330T3

OFF

*



COILCRAFT DO3316P-472 (4.7µH),

DO3316P-103 (10µH) OR 

SUMIDA CD104-100MC (10µH)

AVX TPSD226M025R0200



+ +

L1

4.7µH

10µH

OUT



0.7A

0.8A

†

MAX I

OUT

OUTPUT CURRENT (A)

0.01

EFFICIENCY (%)

0.1 1

LT1371 • TA02

100 V

= 5V

Figure 1. LT1371 5V to 12V Converter

Provides 800mA Output Current

Figure 2. LT1371 Efficiency

minimum number of external components,

as shown in Figure 1. The low loss 3A

switch has a 0.25Ω typical on resistance

which allows for up to 90% efficient boost

converter designs (Figure 2). Operating qui-

escent current of the LT1371 is only 4mA

and a logic compatible Shutdown pin re-

duces supply current to 12µA.

The LT1371 is available in surface

mount 7-lead DD, 7-lead TO-220 and

20-lead SW packages enabling an all sur-

face mount design at higher power levels

than previously available. Devices are

specified for operation from 0°C to 125°C.

Contact your local Linear Technology sales

office for a data sheet and free evaluation

samples of the LT1371 high frequency

boost regulator.

Linear Technology Chronicle •

November 1995

Application of the Month

A Circuit That Smoothly Switches Between 3.3V and 5V

Many subsystems require supply

switching between 3.3V and 5V to sup-

port both low power and high speed

modes. This back-and-forth voltage

switching can cause havoc to the main

3.3V and 5V supply buses. If done im-

properly, switching the subsystem from

5V to 3.3V can cause a momentary jump

on the 3.3V bus, damaging other 3.3V

devices. When switching the subsystem

from 3.3V to 5V, the 5V supply bus can

be pulled down while charging the

subsystem’s capacitors and may inadvert-

ently cause a reset.

The circuit shown in Figure 1 allows

smooth voltage switching between 3.3V

and 5V, with added protection features to

ensure safe operation. IC1 is an LTC1470

power switch matrix. This part has

on-chip charge pumps running from the

5V supply to fully enhance the internal

N-channel MOSFETs. The LTC1472 also

has guaranteed break-before-make switch-

ing to prevent cross-conduction between

buses. It also features current limiting and

thermal shutdown.

capacitor. The example shown in Figure 1

is for a 250mA subsystem. The discharge

time constant should be about 4ms. Once

the subsystem supply has dropped below

the 3.3V supply, the comparator will trip,

turning on the 3.3V switch. The compara-

tor has some hysteresis to avoid instabili-

ties. The subsystem supply will reach a

low point of about 3V before the 3.3V

switch is fully enhanced.

When switching from 3.3V to 5V,

IC1’s current limiting prevents the main

5V bus from being dragged down while

charging the holding capacitor and the

subsystem’s capacitance. Without current

limiting, the inrush current to charge these

capacitors could cause a droop in the main

5V supply.

If done improperly, supply voltage

switching leads to disastrous system con-

sequences. The voltage switch should

monitor the output voltage and have cur-

rent limiting to prevent main supply tran-

sient problems. A correctly designed

supply voltage switch avoids these pitfalls

and results in a safe, reliable system.

When switching the subsystem from

5V to 3.3V, the holding capacitor and the

load capacitance are initially charged up to

5V. Connecting these capacitors directly to

the main 3.3V bus causes a momentary step

to 5V. This transient is so fast that the

power supply cannot react in time. Switch-

ing power supplies have a particularly diffi-

cult time coping with this jump. Switching

supplies source current to raise the supply

voltage and require the load to sink current

to lower the voltage. A switching supply

will be unable to react to counter the large

positive voltage step. This jump will cause

damage to many low voltage devices.

The circuit in Figure 1 employs a com-

parator (IC2) and utilizes the high imped-

ance state of the LTC1470 to allow

switching with minimal effect on the sup-

ply. When the 3.3V output is selected,

IC1’s output will go into a high impedance

state until its output falls below the 3.3V

bus. The output capacitors will slowly dis-

charge to 3.3V, with the rate of discharge

depending on the current being pulled by

the subsystem and the size of the holding

dI1470_1.eps

IC1

LTC1470

5V 0.1µF

220µF

TANTALUM

HOLDING

CAPACITOR

TO

SUBSCRIBER

EN0

EN1

OUT

3.3V

0 = 5V

1 = 3.3V 51k

–

3.3V 1µF

1µF

IC2

LT1011

3.3V

500mV/DIV

5V/DIV

2ms/DIV dl1470_2eps

Figure 2. Oscillograph of the

Switchover Waveform Showing

Smooth Transitions

Figure 1. Schematic Diagram of 3.3V and 5V Switchover Circuit

Linear Technology Chronicle •

November 1995

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

Linear Technology

Products Are

Distributed By:

Almac/Arrow

Arrow/Schweber

Arrow/Zeus

Digi-Key

Electrosonic

Gerber Electronics

Farnell Electronics

Marshall Industries

Phase 1

5A Low Dropout Linear

Regulator Has Fast

Transient Response for

Microprocessor

Applications

The LT1585A is a new low dropout lin-

ear regulator optimized to handle fast tran-

sients commonly found in high speed

microprocessor power supply applications.

The 5A output current capability of the

LT1585A allows it to handle the combined

power demand of a high speed microproces-

sor and cache RAM. The dropout voltage of

the LT1585A is guaranteed to be less than

1.4V over full load and temperature, opti-

mizing this new device for 5V to 3.3V appli-

cations. Figure 1 shows the dropout voltage

vs load current characteristics of the

LT1585A.

OUTPUT CURRENT (A)

INPUT/OUTPUT DIFFERENTIAL (V)

1.5

1.4

1.3

1.2

1.1

1.0

0.9

0.8

0.7

0.6

0.5

LT1585A TA02

FULL LOAD

Figure 1. LT1585A Dropout Is Less

Than 1.4V at Full Load

Current limit is trimmed to ensure con-

trolled short-circuit current. On-chip thermal

limiting provides protection against any

combination of overload that would create

excessive junction temperatures.

The fast transient response and low

dropout voltage of the LT1585A make it an

ideal regulator for powering Pentium,

PowerPC

and other high speed micropro-

cessors. The device can also be used to pro-

vide regulated power for low voltage logic

supplies or as a post regulator for switching

power supplies.

The LT1585A is available in the TO-

220 package with a fixed 3.3V or user ad-

justable output voltage. Devices are

specified for operation from 0°C to 125°C.

Contact your local Linear Technology sales

office for a data sheet and free evaluation

samples of the LT1585A.

The LT1585A has lower dropout and

faster transient response than its LT1083/

84/85 predecessors. Output regulation is

less than ±2% over fast changing line and

load conditions. These improvements make

the LT1585A ideal for low voltage micro-

processors requiring a regulated 2.5V to

3.6V supply.

Regulated Positive-to-

Negative Voltage

Converter Synchronized

to System Clock

The LTC1429 is a switched capacitor

voltage inverter that provides a regulated

negative output voltage from a single posi-

tive supply without using inductors. This

new voltage converter IC features user con-

trolled clock synchronization for sensitive

RF applications. Supply current is only

600µA and a shutdown mode reduces cur-

rent to 0.2µA for portable applications.

The 14-lead LTC1429CS operates

from a single 3V to 8V supply and provides

an adjustable output voltage from –1.25V to

–8V. An on-chip resistor string allows the

LTC1429CS to be configured for output

voltages of –3.5V, –4V, –4.5V or –5V.

The 8-lead LTC1429CS8 is optimized for

C2

0.1µF

C1

0.1µF

C3 

0.001µF

OPTIONAL

OUT

–4V AT 10mA

POWER VALID

C4

3.3µF

GND

–

SYNC/SD

COMP

REG

OUT

LTC1429-4CS8

10k

LTC1429 • TA01

Figure 1. LTC1429 GaAs Bias with

Power Valid Output

LTC1261 contains an onboard oscillator.

All these devices are ideal for use as GaAs

transistor bias voltage generators commonly

used in portable RF applications such as RF

wireless LANs, cellular phones and CDPD

data transfer applications.

The LTC1429 is available in 8-lead

surface mount and 14-lead surface mount

packages. Devices are specified over the

0°C to 70°C range. Contact your local Lin-

ear Technology sales office for a data sheet

and free evaluation samples of the

LTC1429.

applications requiring a fixed –4V output

from a 5V supply and only requires a single

external 0.1µF flying capacitor. Output

ripple is typically as low as 5mV and an op-

tional compensation pin is provided on both

versions to reduce output ripple. Figure 1

shows the LTC1429CS8 used as a 5V to –

4V converter for GaAs power transistor

bias.

Other family members include the

LTC1550/51 and the LTC1261. The

LTC1550/51 provide a negative output

voltage with 1mV output ripple and the

PowerPC is a trademark of IBM Corporation.