6406730.pdf

design ideas

EDITED By BrAD ThoMPSoN

AND FrAN GrANVIllE

readerS SOLVe deSIGN PrOBLeMS

Microcontroller drives logarithmic/

linear dot/bar 20-LED display

D Is Inside

84 Optical feedback extends

white LEDs’ operating life

88 Sequencer controls power

supplies’ turn-on and turn-off

order

92 Use dual op amp

in an instrumentation amp

E What are your design problems

and solutions? Publish them here

and receive $150! Send your

Design Ideas to edndesignideas@

reedbusiness.com.

Dhananjay V Gadre and Anurag Chugh,

Netaji Subhas Institute of Technology, New Delhi, India

Available for more than 20

years, National Semiconduc-

tor’s (www.national.com) venerable

LM3914 dot/bar-display driver still en-

joys wide popularity among designers.

The LM3914 can sense an analog volt-

age level and display it on 10 LEDs by

illuminating one of 10 in dot mode or

by progressively illuminating LEDs in

bar-graph mode. Recently, an appli-

cation needed an analog-input-volt-

age display capable of displaying more

than 10 levels in linear- and logarith-

mic-scale formats. According to the

LM3914’s data sheet, you can cascade

multiple 3914s to display more than 10

levels ( Reference 1 ), but, even so, the

LM3914 offers only linear displays of

its input voltage. ( Editor’s note: Na-

tional Semiconductor also offers the

LM3915, a logarithmic, 3-dB-per-step

version, and the LM3916, which dis-

plays its input in volume units, for

audio applications.)

This application required more flex-

ibility than the LM3914 offers, and it

uses a circuit based on an Atmel (www.

atmel.com) AVR-family ATTiny13 mi-

crocontroller, which features 1 kbyte of

program memory; a four-channel, 10-

bit ADC; and six general-purpose I/O

pins. Altering the circuit’s firmware al-

lows linear or logarithmic scaling of

the 0 to 5V input-voltage range.

The circuit in Figure 1 continuous-

ly displays the input voltage in 20 lev-

els. When closed, switch S 1 freezes the

displayed reading at its then-current

level. Five of the microcontroller’s six

I/O pins control all 20 LEDs and the

switch. Configured as an ADC-input

channel, the remaining I/O pin re-

ceives the analog-input voltage. The

microcontroller uses Charlieplexing, a

method of using I/O lines to drive as

many as N3(N21) LEDs, to drive 20

LEDs with only five I/O pins ( refer-

ences 2 through 4 ).

The firmware is written in C and

compiled using AVR-GCC, a freeware

C compiler and assembler available in

Windows and Linux versions at www.

avrfreaks.net. It uses the Tiny13’s in-

ternal 10-bit ADC operating in free-

J 1

V CC

J 2

GND

INPUT 0 TO 5V

C 1

0.1 � F

(RESET)PB5

(XTAL2)PB4

(XTAL1)PB3

(SCK)PB2

(MISO)PB1

(MOSI)PB0

C 2

10 � F

IC 1

ATTINY13

VCC

GND

R 8

3.9k

D2 10 D2 9 D2 8 D2 7

D2 2 D1 1 D1 10 D1 9

D1 8 D1 7 D1 6 D1 5

D1 4 D1 3 D1 2 D1 1

D2 6 D2 5 D2 4 D2 3

R 1

100

R 2

100

R 3

100

R 4

100

R 5

100

NOTES:

ALL LEDs ARE SUPERBRIGHT, 3 MM DIAMETER.

S 1 IS NORMALLY OPEN IN RUN MODE, CLOSED TO HOLD READING.

S 1

Figure 1 Based on a low-cost microcontroller, this dot/bar LED driver operates in linear or logarithmic modes.

january 18, 2007 | EDN 83



design ideas

running, interrupt-driven mode to

convert the analog-input voltage into

a digital number. Upon completion of

each conversion, the ADC generates

an interrupt that a subroutine reads;

the interrupt stores the ADC’s con-

verted output in a shared variable.

To provide a flicker-free display, an

internal timer generates a 1875-Hz in-

terrupt derived from the 9.6-MHz sys-

tem clock to drive the multiplexed

LEDs at a rate exceeding 90 Hz. Di-

viding the ADC count by a constant

yields a linear display of the input

voltage. A look-up table scales the

ADC count to produce a logarithmic

display. Figure 2 shows the logarith-

mic-conversion curve that defines the

look-up table’s values. Versions of the

ATTiny13’s control programs for lin-

ear and logarithmic scales are avail-

able for downloading from the online

version of this Design Idea at www.

edn.com/070118di1. You can modify

the source code to display only a par-

ticular subrange of the input voltage

of 0 to 5V. For example, you can spec-

ify a linear-display range spanning 1

to 3V or a logarithmic scale for input

voltages of 2 to 3V. EDN

1200

1000

800

SERIES1

ADC COUNT

(0 TO 1023)

600

400

200

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

LED NUMBER

Figure 2 A linear-to-logarithmic-conversion curve defines the input voltage

required to illuminate a particular LED.

R e f e R e n c e s

Application Note 1880, Feb 10,

2003, http://pdfserv.maxim-ic.com/

en/an/AN1880.pdf.

Benabadji, Noureddine, “PIC

microprocessor drives 20-LED dot-

or bar-graph display,” EDN , Sept 1,

2006, pg 71, www.edn.com/article/

CA6363904.

LM3914 data sheet, www.national.

com/pf/LM/LM3914.html.

Lancaster, Don, “Tech Musings,”

August 2001, www.tinaja.com/glib/

muse152.pdf.

“Charlieplexing: Reduced Pin-

Count LED Multiplexing,” Maxim

Optical feedback extends white LEDs’ operating life

Bjoy Santos, Intersil Corp, Milpitas, CA

Regardless of its color, an LED’s

light output varies as a function

of forward current and ambient tem-

perature. As Figure 1 shows, an LED’s

light output can vary by as much as

150% over its operating-current

range. In response, a designer’s first at-

tempt to solve the problem focuses on

driving the LEDs with a constant cur-

rent. The most common white-LED-

driver circuits use an inductor-based

dc/dc boost-converter topology simi-

lar to the circuit in Figure 2 . A cur-

rent-feedback controller ensures that

the voltage across current-sensing re-

sistor R 1 remains constant. As a result,

the controller varies the voltage across

the entire string to maintain the LEDs’

current constant without regard to

150

100

LED-OUTPUT

CHANGE (%)

�50

�100

LED FORWARD CURRENT (mA)

Figure 1 An LED’s light output changes considerably as a function of its for-

ward current, even within the sweet spot (oval area) of its nominal operating

current.

84 EDN | january 18, 2007



design ideas

V IN

GND

L 1

22 � H

V OUT TO

ADDITIONAL

LED STRIP

D 1

BAT54C

C 3

10 � F

C 4

0.1 � F

D 2

D 3

C 1

0.22 � F

C 2

0.22 � F

GND

D 4

D 5

D 6

D 7

VIN

DIM/BRIGHT

CONTROL

INPUT

IC 1

EL7630

(SC 70 PACKAGE)

D 8

D 9

ENAB

D 10

D 11

GND

PGND

D 12

D 13

NOTE:

D 2 TO D 13 : WHITE LEDs.

R 1

R 2

Figure 2 One method of driving an LED illuminator samples current through a string and adjusts the voltage across the

entire string to maintain a constant current.

the LEDs’ actual light output.

Driving series-connected white LEDs

with a current source relies on the as-

sumption that, at constant current, an

LED’s light output remains constant.

Unfortunately, all LEDs exhibit a non-

linear decrease in brightness as a func-

tion of operating time. Although less

obvious in colored LEDs that find use

as indicators, the decrease in bright-

ness of a white-LED-illuminator-array

source becomes noticeable over an ex-

tended period. Brightness also varies as

Figure 3 Even at a

constant forward cur-

rent, an LED’s light

output correlates

strongly with tem-

perature and can vary

by as much as 100%

over the entire oper-

ating-temperature

range (upper curve).

2.5

CURRENT FEEDBACK

RELATIVE

LUMINOUS

INTENSITY

1.5

0.5

OPTICAL FEEDBACK

�40 �20

AMBIENT TEMPERATURE (�C)

V CC

L 1

22 � H

V IN

GND

V OUT TO

ADDITIONAL

LED STRIP

D 1

BAT54C

C 3

10 � F

C 4

0.1 � F

D 2

D 3

C 1

0.22 � F

C 2

0.22 � F

GND

D 4

D 5

VIN

D 6

D 7

V CC

IC 1

EL7630

(SC 70 PACKAGE)

C 5

0.1 � F

DIM/BRIGHT

CONTROL

INPUT

ENAB

D 8

D 9

GND

PGND

D 10

D 11

VCC

IC 2

ISL29000

D 12

D 13

OUT

R 1

100

GND

NOTE:

D 2 TO D 13 : WHITE LEDs.

R 2

Figure 4 Photosensor IC 2 , an Intersil ISL29000, resides near an LED to detect brightness fluctuations and provides com-

pensating feedback to IC 1 , the current controller, which is an Intersil EL7630 pulse-width regulator.

86 EDN | january 18, 2007

design ideas

a function of temperature, which can

affect an illuminator’s performance

over an extended-temperature range

(upper curve, Figure 3 ).

To compensate for LED-output vari-

ations due to aging and temperature

fluctuations, the control loop needs

more information in addition to volt-

age or current data. Adding an ambi-

ent-light sensor and optical feedback

to the control loop can ensure that a

white LED’s light output remains uni-

form and consistent over time and tem-

perature variations. An optical sensor

can measure the LED’s light-output in-

tensity and provide a feedback signal

for the control loop, which can adjust

the current to produce a relatively con-

stant light output. As the LEDs’ light

outputs decrease, increased current

compensates for aging and tempera-

ture-induced variations (lower curve,

Figure 3 ).

The circuit in Figure 4 includes an

optical-feedback loop based on Intersil’s

(www.intersil.com) ISL29000 light-to-

current optical sensor, IC 2 , which senses

changes in the LEDs’ light output and

decreases the feedback voltage applied

to IC 1 , the current controller, an Inter-

sil EL7630. The pulse-width-modulat-

ed controller then increases the LED-

drive current’s duty cycle, boosting the

LED current until the feedback voltage

reaches its nominal value. As ambient

temperature decreases, the LEDs’ light

output tends to increase, and IC 2 de-

livers a higher feedback voltage to the

controller, which responds by lowering

the duty cycle to decrease the LEDs’

current and thereby compensates for

the decrease in temperature. EDN

Sequencer controls power supplies’

turn-on and turn-off order

Eric Schlaepfer, Maxim Integrated Products Inc, Sunnyvale, CA

that, during shutdown, the power-sup-

ply rails switch off in reverse order. Al-

though various vendors provide pro-

grammable-sequencing ICs, these

components are usually too expensive

for cost-sensitive applications.

Offering an alternative to program-

mable-sequencing ICs, the circuit of

Figure 1 can sequence and cheaply

When a design based on mul-

tiple point-of-load dc/dc con-

verters requires a specific power-sup-

ply-start-up sequence, wiring each con-

verter’s power-good output to the next

converter’s enable input produces the

desired voltage cascade. Although this

approach works well for simple designs,

it fails to satisfy a requirement of many

modern microprocessors and DSPs:

(continued on pg 92)

C 3

0.1 �F

C 4

0.1 �F

R 7

10k

R 6

10k

R 5

10k

R 4

10k

R 3

33k

EN1

EN4

EN3

EN2

VCC

IN2

�

OUT2

�

1.2V REERENCE

3.3V

VCC

EN2 EN3 EN4 EN1

�

OUT

IN1

�

IN3

DC/DC

CONVERTER 1

OUT1

OUT3

3.3V REFERENCE

1.8V REFERENCE

�

2.5V

�

IN4

INPUT

POWER

IN4

OUT

�

DC/DC

CONVERTER 2

OUT4

2.5V REFERENCE

�

1.8V

�

IN3

OUT

�

IN1

DC/DC

CONVERTER 3

OUT3

OUT1

1.8V REFERENCE

3.3V REFERENCE

�

1.2V

�

OUT

IN2

COLY1

COLY2

COLY3

COLY4

�

DC/DC

CONVERTER 4

OUT2

IC 2

MAX16029

1.2V REFERENCE

�

COLY1

COLY2

COLY3

COLY4

IC 1

MAX16029

NR RESET

CRESET

TH1

TH0

GND

TOL

CRESET

TOL GND TH0 TH1

RESET

NC NC

C 2

R 1

100k

R 2

33k

POWER OK

C 1

POWER ON/OFF

CONTROL INPUT

(ON=HIGH)

(OFF=LOW

Figure 1 Comprising a pair of inexpensive ICs, this circuit applies four supply voltages in a specified order at power-up

and then removes them in reverse order at power-down.

88 EDN | january 18, 2007



design ideas

(continued from pg 88)

and effectively monitor four power-

supply rails. Four dc/dc power supplies

provide the application circuit with

3.3, 2.5, 1.8, and 1.2V. A quad super-

visor circuit, IC 1 , monitors each rail,

generating the master POK (power-

OK) signal and ensuring that, during

power-up, the next supply in the se-

quence does not turn on until the pre-

ceding supply voltage is valid. Using an

RC circuit comprising R 1 , R 2 , R 3 , and

C 1 , a second quad supervisor, IC 2 , cre-

ates the power-up and power-down se-

quences. Each supervisor’s internal-

ly preset voltage threshold eliminates

the need for external resistive-voltage

dividers.

Connecting the power-on/off signal

to 5V initiates a power-up sequence,

which charges C 1 through R 2 . As the

capacitor’s voltage gradually exceeds

1.2, 1.8, 2.5, and 3.3V, each of IC 2 ’s

corresponding open-drain outputs

floats, thereby allowing the power sup-

plies to turn on in the prescribed se-

quence. After a time delay, which C 2

sets, and after all four supplies turn on,

the POK signal asserts—that is, goes

high.

To monitor the supply rails, allow

Figure 2 Beginning with a dc/dc converter, the circuit in Figure 1 switches on

three additional converters in sequence and generates a POK signal. Pulling

the circuit’s on/off input low removes the POK signal and switches off all four

converters in reverse order.

the power-on/off-control input to float.

Resistors R 1 and R 3 sustain the voltage

across C 1 and maintain the POK sig-

nal high to keep the power supplies on.

When an output-voltage fault occurs,

POK rapidly deasserts, discharging C 1

through R 1 and shutting off all of the

power supplies. To remove power in an

orderly sequence, connect the power-

on/off signal to ground. Capacitor C 1

discharges through R 2 and also through

R 1 when POK deasserts, turning off

each power supply in reverse order

( Figure 2 ). EDN

Use dual op amp

in an instrumentation amp

Jerald Graeme, Burr-Brown Corp, Tucson, AZ

a voltage (V 1 1 V 2 ) across the gain-set-

ting resistor R G . Signal current in the

combined feedback path is thus propor-

tional to the differential input voltage

and inversely related to R G . The out-

put voltage, V OUT , equals G(V 1 1 V 2 )—

that is, V OUT =2(1+R/R G )(V 1 1 V 2 ).

You choose R G for the desired gain

G, which may range from a value of

2 (R G omitted) to a maximum that

is limited only by the op amps’ open-

loop gain, the allowable gain error, and

the required bandwidth. The Figure 1

circuit provides a 2-kHz bandwidth at

a gain of 2000; in general, the band-

width is about 2 MHz/G. What’s more,

the output offset equals the difference

in op-amp offsets multiplied by G.

The dc CMR (common-mode rejec-

tion) is an important spec for instru-

mentation amps; in Figure 1 , CMR de-

pends primarily on matching values for

the four resistors labeled R. DC CMRR

Editor’s note: Here’s an oldie but good-

ie. EDN editors regularly field requests for

copies of articles that predate our online

archives (www.edn.com/archives). But

this Design Idea from our Feb 20, 1986,

issue has generated many more requests

than normal. We aren’t sure how readers

know of this Design Idea, but its endur-

ing popularity has led us to publish it once

again, and now it will be available in our

online archives.

Although monolithic instru-

mentation amplifiers are more

cost-effective than their discrete and

modular predecessors, the limited va-

riety of monolithic instrumentation

amps restricts their use. You can widen

your options, however, by deriving the

differential response of an instrumenta-

tion amplifier from a dual op amp ( Fig-

ure 1 ). The circuit uses FET-input op

amps to provide lower noise and lower

input-bias currents than monolithic

instrumentation amps can offer.

In Figure 1 , feedback networks for

the two op amps are interconnected to

establish IC 1B as an inverting amplifier

in the feedback path of IC 1A . Each am-

plifier provides an external signal input

with the high impedance expected of

an instrumentation amplifier. (Input-

bias currents for this circuit are 2 pA

at 258C.)

Feedback from each amplifier forces

92 EDN | january 18, 2007



Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: