LTJournal-V24N1-2014-04.pdf

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April 2014

Volume 24 Number 1

I N

T H I S

I S S U E

robust RS485/RS422

transceivers

high voltage surge stoppers

ease MIL-STD-1275D

compliance

Active Clamp Synchronous

Controllers for Forward Converters

with 6.5V to 100V+ Inputs

Wei Gu, Randyco Prasetyo and Fei Guo

boost-then-buck LED driver

for high PWM dimming

ratios

cost-effective high voltage

isoSPI™ coupling

ideal diode combines 200V

busses

The LT3752, LT3752‑1 and LT3753 are highly integrated, high

performance active clamp forward controllers that minimize external

component count, solution size and cost. Two of these controllers,

the LT3752 and LT3753, are designed for inputs up to 100V, while

the LT3752‑1 is designed for applications with input voltages greater

than 100V—suitable for HV car battery and offline isolated power

supplies, industrial, automotive and military systems. All produce

compact, versatile and efficient solutions for single‑IC output power

levels up to 400W. Higher power levels are

supported by stacking converter outputs

in series. See Table 1 (on page 4) for a

feature comparison of these devices.

NO-OPTO MODE OPERATION REGULATES WITH

ACCURATE PROGRAMMABLE VOLT-SECOND CLAMP

Figure

shows a complete

forward converter that

requires no opto-couplers thanks to the LT

3752’s accu-

rate

programmable volt-second clamp. For a forward

converter operating in continuous conduction mode

the

output voltage is V

OUT

= V

• N • D

where V

is the input

voltage

N is the secondary to primary turns ratio and D

is the duty cycle. The duty cycle clamp on the OUT pin of

the LT3752

LT3752-

and LT3753 inversely tracks V

maintain constant V

OUT

over the input voltage range.

(continued on page 4)

The LT8614 Silent Switcher™ selected for

EDN

EE Times

ACE Award (page 3)

Caption

w w w. li n e ar.co m

In this issue...

COVER STORY

Active Clamp Synchronous Controllers for Forward

Converters with 6.5V to 100V+ Inputs

Wei Gu, Randyco Prasetyo and Fei Guo

Linear in the News

NEW LINEAR VIDEO PRESENTATIONS

DESIGN FEATURES

RS485/RS422 Transceivers Operate from

3V to 5.5V Supplies and Withstand ±60V Faults

Ciaran Brennan

Two new videos at

www.linear.com

from Linear Chief Technical Officer

Bob Dobkin cover the new family of LDO+

™

linear regulators

which

add monitoring and control functions to the usual regulation features.

Here are summaries of those and two other recently released videos:

2.1A LDO+ Regulator Features Cable Drop Compensation and Monitors Current & Temperature

Bob Dobkin, Vice President Engineering & CTO—

In addition to the usual LDO regulatory

High Voltage Surge Stoppers Ease MIL-STD-1275D

Compliance by Replacing Bulky Passive Components

Dan Eddleman

functions

the LT3086 LDO+ can monitor and externally limit both temperature

and output current

has an accurate power good output and can compensate for

wire drops between the regulator and the load.

www.linear.com

solutions/4522.

1.5A LDO+ Regulator Monitors Current & Temperature

Bob Dobkin, Vice President Engineering & CTO—

The LT308

LDO+ features a wide

DESIGN IDEAS

What’s New with LTspice IV?

Gabino Alonso

Boost-then-Buck LED Drivers Enable Wide PWM

Dimming Range with Wide-Ranging Input Voltages

Keith Szolusha and Taffy Wong

safe area

allowing operation with high currents at high input-output dif-

ferentials. The LT308

’s output voltage is adjustable from 0

to 37

and

it withstands reverse voltage. It includes a current monitor and tempera-

ture monitor outputs. The LT308

is stable with no output or input capaci-

tor

a feature unique to this device.

www.linear.com

solutions/452

Single-Ended to Differential Conversion Using Differential Op Amps

Kris Lokere, Applications Manager, Signal Chain Products—

Differential op amps are

Low Cost isoSPI Coupling Circuitry for

High Voltage High Capacity Battery Systems

Jon Munson

Ideal Diode Combines 200V Busses

Mitchell Lee

new product briefs

back page circuits

important building blocks in modern analog and mixed signal circuits. For

instance

many modern ADC

require differential signals at the inputs

and dif-

ferential analog signals are used to drive signals over a cable. This video shows

how to connect differential op amps to convert a single-ended input signal to

a differential output

how common mode level shifting works and how to use

differential op amps to build active filters.

www.linear.com

solutions/4524

Wireless Power Receiver Enables Compact and Efficient Contactless Battery Charging

Trevor Barcelo, Product Line Manager, Battery Charger Products—

Wireless battery charg-

ing enables applications where it is difficult or impossible to use a wired con-

nector. Examples include products that need to operate in harsh environments

or need to be cleaned or sterilized

as well as products that are simply too

small for a connector. This video shows the LTC

20 400

wireless power

receiver buck battery charger with a wireless power transmitter to charge a

3.5

battery with a constant-current

constant-voltage charge algorithm.

It enables high efficiency charging without any of the thermal or overvoltage

problems typical of wireless power systems.

www.linear.com

solutions/4469

2 | April 2014 :

LT Journal of Analog Innovation

Linear in the news

STANFORD SOLAR CAR RACES,

MANAGEMENT SYSTEM

The Stanford solar

car uses numerous

Linear Technology

products. including

the LTC6803 and

LTC6804 multicell

battery stack

monitors.

The Stanford Solar Car Project is an

entirely student-run

nonprofit orga-

nization at Stanford University that

builds solar powered cars to race in the

2000-mile World Solar Challenge in

the Australian Outback. It provides an

opportunity for students to gain valu-

able hands-on engineering experience

while raising awareness of clean energy

vehicles. The team finished 4th overall

in the 20

3 Bridgestone World Solar

Challenge

and was the fastest under-

graduate solar car in the grueling five-

day race in Australia last October. They

experienced no mechanical failures and

did not burn a single drop of gasoline.

The Stanford solar car uses numerous

Linear Technology products

including

the LTC6803 and LTC6804 multicell battery

stack monitors. Other Linear products

in the Luminos solar car include Hot

Swap

™

controllers

precision references

and amplifiers

LED drivers

micropower

step-down regulators

micropower low

dropout regulators and synchronous

step-down switching regulators.

LINEAR PRODUCT AWARDS

EDN

EE Times

ACE Awards

Winner, Ultimate Products: Power—

error (INL)

making it a true 20-bit ADC

able to resolve down to 5

of resolu-

tion on a 5

differential input span. Its

B SNR is the industry’s highest for

any

sps

no latency ADC

providing

higher dynamic range compared to fast

delta-sigma ADC

that also have latency.

Applications include seismic monitor-

ing

energy exploration

airflow sensing

silicon wafer fabrication

medical devices

data acquisition systems

automatic test

equipment

compact instrumentation

and industrial process control systems.

Finalist, Ultimate Products: Wireless/RF—

filtering and RF shielding requirements.

The superior performance of this mixer is

ideal for applications in multicarrier GSM

4G LTE and LTE-Advanced multimode base

stations

point-to-point backhauls

mili-

tary communications

wireless repeaters

public safety radios

VHF

UHF

white-space

broadcast receivers

radar and avionics.

CONFERENCES & EVENTS

Advanced Automotive Battery Conference,

International Conference Center, Kyoto, Japan, May

19-23, Booth 40—

Linear will exhibit power

The LT86

4 Silent Switcher

™

regulator

is a 4

input capable synchronous

step-down switching regulator. It reduces

EMI

EMC emissions by more than 20

Even with switching frequencies in excess

of 2MH

synchronous rectification deliv-

ers efficiency as high as 96% while Burst

Mode

operation keeps quiescent current

under 2.5

in no-load standby conditions.

Its 3.4

to 42

input voltage range is ideal

for automotive and industrial applications.

Finalist, Ultimate Products: Analog ICs—

The LTC2378-20 20-bit

sps

low power

no latency SAR ADC leads the indus-

try with 0.5ppm integral nonlinearity

The LTC555

downconverting mixer’s

+36

IIP3 (input third-order intercept)

and 2.4

B conversion gain are unpar-

alleled. Other passive mixers claim-

ing similar IIP3 performance typically

have 7

B to 9

B of conversion loss. The

LTC555

’s 2.4

B of conversion gain sub-

stantially improves receiver dynamic

range. The mixer can be used over a broad

RF frequency range

from 300MH

3.5GH

. The LTC555

features an integrated

LO buffer on chip

requiring only 0

drive level

eliminating the need for a high

power buffer amplifier stage

often requir-

ing power levels of +

or higher. By

eliminating such a high power LO signal in

the user’s receiver

overall cost is lowered

and a potential source of undesirable

radiation is removed

thus simplifying

management and battery monitoring

systems. More info at

www.advanced-

autobat.com

conferences

automotive-

battery-conference-Asia-20

4/index.html

Wireless Japan 2014, Tokyo Bigsight, Tokyo,

Japan, May 28-30, West Hall 3, Booth W118—

Presenting Linear’s Dust Networks

wireless sensor network solutions. More

info at

www8.ric.co.jp

expo

wj/

Sensors Expo/Energy Harvesting Pavilion, Donald

E. Stephens Convention Center, Rosemont, Illinois,

June 24-26, Booth 920—

Linear will showcase

energy harvesting and wireless sen-

sor network solutions. Presentations:

“Energy Harvesting for Battery-

Operated Applications” by Sam Nork;

“Reliable

Low-Power Wireless Sensor

Networks for the Industrial Internet

of Things” by Joy Weiss. More info at

www.sensorsmag.com

sensors-expo

April 2014 :

LT Journal of Analog Innovation

| 3

Table 1. Feature comparison of LT3752, LT3752-1 and LT3753

PART

INPUT RANGE

ACTIVE CLAMP DRIVER

HOUSEKEEPING FLYBACK CONTROLLER

LT3753

LT3752

LT3752-1

8.5V–100V

6.5V–100V

100V–400V+

Lo-Side

Hi-Side

Yes

(

375x) continued from page 1)

In an active volt-second clamp scheme

the accuracy of V

OUT

depends heav-

ily on the accuracy of the volt-second

clamp. Competing volt-clamp solutions

use an external RC network connected

from the system input to trip an internal

comparator threshold. Accuracy of the

RC method suffers from external capaci-

tor error

part-to-part mismatch between

the RC time constant and the IC’s switch-

ing period

the error of the internal

comparator threshold and the nonlinear-

ity of charging at low input voltages.

To ensure accurate regulation part to part

the LT3752

LT3752-

and LT3753 feature

trimmed timing capacitor and compara-

tor thresholds. Figure 2 shows V

OUT

versus

load current for various input voltages.

If the resistor that programs the duty

cycle clamp goes open circuit

the part

immediately stops switching

prevent-

ing the device from running without

the volt-second clamp in place.

INTEGRATED HOUSEKEEPING

FLYBACK CONTROLLER

The LT3752/LT3752-

includes an inter-

nal constant frequency flyback con-

troller for generating a housekeeping

supply. The housekeeping supply can

efficiently provide bias for both pri-

mary and secondary IC

eliminating

the need to generate bias supplies from

auxiliary windings in the main forward

transformer

significantly reducing

transformer complexity

size and cost.

The housekeeping supply can be used to

overdrive the INTV

pin to take power

outside of the part

improve efficiency

provide additional drive current and

optimize the INTV

level. The house-

keeping supply also allows bias to any

secondary side IC before the main for-

ward converter starts switching. This

removes the need for external start-

up circuitry on the secondary side.

PRECISION UNDERVOLTAGE

LOCKOUT AND SOFT-START

The precision LT3752/LT3752-

undervolt-

age lockout (UVLO) feature can be used for

supply sequencing or start-up overcurrent

protection—simply apply a resistor divider

to the UVLO pin from the V

supply.

Figure 1. 150W forward converter in No-Opto mode

18V TO 72V

4.7µF

100V

×3

4:4

6.8µH

15nF

499

0.15

100nF

10k

AOUT

Si2325DS

100

FSW

CSW

100k

5.9k

OVLO

1.82k

GND

HCOMP

BLNK

IVSEC

SS1

SS2

LT3752

SYNC

UVLO_V

SEC

SENSEN

SOUT

INTV

10k

COMP

HFB

1.1k

4.7µF

2.2nF

HOUT HI

SENSE

OUT

SENSEP

SENSE

0.006

AUX

100

CSN

CSP

2.2nF

250V

OUT

12V

12.5A

INTV

2.2µF

•

AUX

2.2µF

•

20k

470µF

16V

22µF

16V

×2

2.2µF

GND

PGOOD

SYNC

560

OPTO

LT8311

INTV

PMODE

TIMER

COMP

D1, D2, D3: BAS516

D4: CENTRAL SEMI CMMR1U-02

L1: CHAMPS PQI2050-6R8

M1, M4: INFINEON BSC077N12NS3

M2: VISHAY Si2325DS

M3: FAIRCHILD FDMS86101

M5: DIODES INC. ZVN4525E6

T1: CHAMPS G45R2_0404.04D

T2: BH ELECTRONICS L00-3250

T3: PULSE PE-68386NL

220pF

INTV

•

AUX

4.7µF

499k

22.6k

49.9k

34k

7.32k

31.6k

60.4k

22nF

0.33µF

2.8k

22nF

4 | April 2014 :

LT Journal of Analog Innovation

design features

In an active volt‑second clamp scheme, the accuracy of V

OUT

depends

heavily on the accuracy of the volt‑second clamp. Competing volt‑clamp

solutions use an external RC network which suffers from a number of error

sources. To ensure accurate regulation part to part, the LT3752, LT3752‑1 and

LT3753 feature trimmed timing capacitor and comparator thresholds.

The UVLO pin features adjustable input

hysteresis

allowing the IC to resist input

supply droop before engaging soft-stop.

During soft-stop the converter continues

to switch as it folds back the switch-

ing frequency

volt-second clamp and

COMP pin voltage. The LT3752

LT3752-

and LT3753 have a micropower shut-

down threshold of approximately

400

at the UVLO pin—V

quiescent

current drops to 40μ

or lower.

Adding capacitors to the soft-start pins

(SS

and SS2) implements the soft-start

feature

which reduces the peak input

current and prevents output voltage

overshoot during start-up or recovery

from a fault condition. The SS

/2 pins

reduce the inrush current by lowering the

current limit and reducing the switching

frequency

allowing the output capacitor

to gradually charge toward its final value.

SHUTDOWN WITH SOFT-STOP

In a reversal of soft-start start-up

the

LT3752/LT3752-

and LT3753 can gradu-

ally discharge the SS

pin (soft-stop)

during shutdown. Figure 3 shows

shutdown waveforms of the converter

shown in Figure 5. Without soft-stop

the self-driven synchronous rectifier

feedback transfers capacitor energy

to the primary

potentially causing

shutdown oscillation and damaging

components on the primary side.

Figure 4 shows shutdown waveforms

with soft-stop. The converter continues to

switch as it folds back switching fre-

quency

volt-second clamp and COMP pin

voltage

resulting in clean shutdown.

CURRENT MODE CONTROL

fewer compensation components than

voltage mode control architectures

mak-

ing it much easier to compensate a broad

range of operating conditions. For opera-

tion in continuous mode and above 50%

duty cycle

required slope compensation

can be programmed by a single resistor.

PROGRAMMABLE FEATURES

SIMPLIFY OPTIMIZATION

The LT3752/LT3752-

and LT3753 include

a number of programmable features that

allow the designer to optimize them for

a particular application. For instance

programmable delays between various

gate signals can be used to prevent cross-

conduction and to optimize efficiency.

Each delay can be set with a single resistor.

Programmable turn-on current spike

blanking (adaptive leading edge blank-

ing plus programmable extended blank-

ing) of the main MOSFET greatly improves

the converter’s noise immunity. During

gate rise time

and sometime thereafter

The LT3752/LT3752-

and LT3753 use a cur-

rent mode control architecture to increase

supply bandwidth and response to line

and load transients over voltage mode

controllers. Current mode control requires

Figure 3. Shutdown waveforms of circuit in Figure 5

without soft-stop show oscillations.

Figure 2. Output voltage vs load current at various

input voltages

14.0

13.5

13.0

12.5

OUT

(V)

12.0

11.5

11.0

10.5

10.0

LOAD CURRENT (A)

= 70V

= 60V

= 48V

= 36V

= 20V

Figure 4. Shutdown waveforms of circuit in Figure 5

showing soft-stop in action

PRIMARY

NFET

DRAIN

VOLTAGE

(50V/DIV)

PRIMARY

NFET

DRAIN

VOLTAGE

(50V/DIV)

OUT

2V/DIV

OUT

2V/DIV

500µs/DIV

April 2014 :

LT Journal of Analog Innovation

| 5

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