101603di.pdf

design

ideas

Edited by Bill Travis

High-fidelity triangle-wave generator

consumes only 6 A

Glen Brisebois, Linear Technology Corp, Milpitas, CA

I DEAL TRIANGLE WAVES involve infinite

C 1

d 2 V/dt 2 , so high-fidelity triangle waves

entail very high bandwidths. Micro-

power circuits have fairly low bandwidth,

so generating good triangle waves with

such circuitry becomes problematic. The

circuits of Figure 1 show two methods of

generating triangle waves. The solitary-

comparator circuit uses a relaxation-os-

cillator approach with the triangle ap-

proximation assuming an RC (expo-

nential) nature ( Figure 1a ). When you

need better linearity, adding an integra-

tor improves the triangle approxi-

mation ( Figure 1b ). Both circuits

R 1

10 nF

R 2

R 3

COMPARATOR

C 1

R 1

OP AMP

TRIANGLE

R 1

R 2

100k

R 3

1. 5 V

(a)

TRIANGLE

SQUARE

COMPARATOR

Figure 1

(b)

SQUARE

–1.5V

1.5V

These popular methods of generating triangle waves have drawbacks, especially when your design

requires low-power operation.

TC7S14F

include a hysteretic-feedback path, as well

as an RC or integrator feedback path

comprising R 3 and C 1 . The hysteretic-

feedback path keeps changing the direc-

tion of the RC integrator and setting the

new target voltage, and the RC integrator

sets the rate of change toward the new

target. These circuits are robust and find

wide usage.

The problem arises when you simul-

taneously require ultralow power con-

sumption and relatively high-frequency

operation. This scenario makes phenom-

enal demands on the micropower op

amp. Consider that, every time the com-

parator reverses direction, it slams an in-

stantaneous current into or out of the op

amp’s output through the two feedback

paths. This situation would be acceptable,

except that the amount of current it

slams is greater than the op amp’s total

supply current. The result is a disastrous-

looking waveform with enormous glitch-

es stemming from the fact that the op

amp cannot provide the instantaneous

output-switch current demands. You can

gain some improvement by increasing

the resistor values and reducing the ca-

pacitor values. However, the improve-

ments are only incremental, and the cir-

cuit becomes noisier and more

susceptible to interference.

Figure 2

1.5V

R 4

536k

10 nF

R 3

1.5V

High-fidelity triangle-wave

generator consumes only 6

A.................. 81

OP AMP

R 2

High-current driver

serves home-power-line modems ............ 82

Circuit forms fast, portable

light pulser ...................................................... 84

3V supply delivers

12V p-p to piezo speaker.............................. 86

Code provides adjustable

differential drive for robots .......................... 88

Microcontroller produces

analog output ................................................ 88

Single transistor provides

short-circuit protection .................................. 90

Publish your Design Idea in EDN . See the

What’s Up section at www.edn.com.

R 1

100k

COMPARATOR

1.5V

LTC1542

A simple CMOS inverter provides high-band-

width current assist, improving the waveform

drastically with minimal impact on supply cur-

rent.

www.edn.com

OCTOBER 16, 2003 | EDN 81

design

ideas

But take heart; a simple and inexpen-

sive solution is at hand. Why not let a

CMOS inverter provide the instanta-

neous current and let the op amp simply

provide the precision linearizing current?

Figure 2 shows the method. The circuit

is identical to that of Figure 1b ,except

that the op amp need not provide the in-

stantaneous switch current. Instead of

the drastic change in output-current po-

larity at the triangle peaks, the op amp’s

output current slowly crosses zero at

midsupply. In the improved waveform,

the total supply current at 280-Hz oper-

ating frequency is 6.2

High-current driver

serves home-power-line modems

Ryan Metivier, Analog Devices, Wilmington, MA

networking signals are

similar to xDSL (digital-

subscriber-line) signals in that

they both typically employ a

form of OFDM (orthogonal

frequency-division multiplex-

ing). Both applications re-

quire high output current,

wide bandwidth, and good

linearity. This Design Idea de-

scribes a simple line-driver

circuit, designed with an

xDSL driver, to drive high-

speed data over a home pow-

er line. Figure 1 shows the

AD8391 current-feedback

amplifier connected in a neg-

ative-feedback circuit to drive

wideband, discrete multitone-

based signals through

home power lines.

The advantage of current

feedback is that it allows you

the flexibility of increasing the

gain beyond unity without be-

ing limited by the gain-band-

width product. The AD8391

has 60-MHz bandwidth,

600V/

0.1 F

R G

205

R F

412

0.1 F

49.9

IN2

V MID

V S

OUT2

POWER

LINE

FROM DAC/MODEM

AD8391

Figure 1

INI

PWDN

V S

OUT1

0.1 F

R G

205

R F

412

49.9

0.1 F

sec slew rate and 250-

mA output-drive-current ca-

pability, making it ideal for

driving home power lines.

The circuit in Figure 1 op-

erates with a 5V supply, has a voltage gain

An xDSL driver uses current-feedback technology to make an efficient home-power-line driver.

with a peak-to-average ratio of 4V/V. The

feedback resistor, R F , and the gain resis-

tor, R G , maximize circuit bandwidth and

stability. For this circuit, an acceptable

bandwidth is approximately 30 MHz.

The following equation shows the rela-

tionship between closed-loop bandwidth

(f CL ), R G , and R F for current-feedback am-

plifiers.

R F /R G ), and drives a 33

load.

−

load emulates the worst-case

impedance of a home power-line net-

work, which can vary greatly from home

to home. The driver is transformer-cou-

pled to the power line. The amplitude of

the output signal is 2.8V p-p into the dif-

ferential power line (hot and neutral)

C P , the internal capacitance, sets the

corner frequency of the open-loop tran-

simpedance function, and R IN is the in-

put impedance of the inverting terminal

of each amplifier. ( Figure 1 does not

82 EDN | OCTOBER 16, 2003

www.edn.com

H OME-BASED power-line

The 33

design

ideas

AMPLITUDE

(10 dB/DIV)

START 3 MHZ

1.9 MHZ

FREQUENCY

STOP 22 MHZ

Figure 3

100 nSEC

Figure 2

The output spectrum of the power-line driver in Figure 1 shows that the

worst-case empty-tone distortion is 35 dBc.

This plot represents the time-domain characteristic of the

power-line driver.

show C P and R IN .) It is important to note

that R F dominates the expression, thus

controlling the closed-loop bandwidth.

The 49.9

provide ac coupling on the input and

output lines. The test signal is a compos-

ite waveform constructed from the sum

of 75 sinusoids of pseudorandom phase.

Each tone in the test waveform may have

one of four phases to emulate QPSK

(quadrature phase-shift keying). The si-

nusoids are orthogonally spaced from 4

to 21 MHz, leaving the amateur-radio

bands empty. Figure 2 shows that the

worst-case empty-tone distortion is

F capacitors

Circuit forms fast, portable light pulser

SK Kaul and IK Kaul, Bhabha Atomic Research Centre, Trombay, Mumbai, India

T HE ABSENCE of a

fast one-shot mul-

tivibrator in the

entire TTL family, as

well as the

low-voltage

swing and unwieldy

supply requirements

of ECL, drove us to ex-

ploit the fast transition

times and low propa-

gation delays of F-se-

ries gates. The applica-

tion called for the

implementation of a

compact, portable, fast

light pulser for field

testing fast photomultipliers in gamma-

ray astronomy work. The use of only two

ICs helped to minimize the size and

power consumption ( Figure 1 ). The

normally high pulses at the output gate,

G4, in IC 2 have rise and fall times of ap-

proximately 2.5 nsec and a duration of

56k 22k

0.1 F

Figure 1

IC 2

HLMP-CB-15

IC 1

7555

0.1

74F00

This circuit provides fast light pulses in a blue LED.

less than 10 nsec, corresponding to three

gate delays. These pulses are ideally suit-

ed to pull low the cathode of a fast, blue

HLMP-CB-15-type LED with the anode

clamped at 5V. The gate forces almost

the entire 5V supply voltage across the

LED. This high swing ensures optimum

brightness of the LED, which is soldered

to the edge of a small pc-board strip.

Rechargeable batteries are clamped onto

the other side of the pc board. Using a

CMOS version of the timer, IC 1 , the cir-

cuit has a current drain of less than 4

mA.

84 EDN | OCTOBER 16, 2003

www.edn.com

resistors on the inputs of the

circuit terminate the signal source. You

should adjust these values based on each

application. The four 0.1-

dBc. This figure is adequate for most

practical power-line applications. Figure

3 shows the output in the time domain.

design

ideas

3V supply delivers 12V p-p to piezo speaker

Royce Higashi and Tony Doy, Maxim Integrated Products, Sunnyvale, CA

ers can provide quality sound for

portable electronic devices, but they

require voltage swings greater than 8V p-

p across the speaker element. Yet, most

portable devices include only a low-volt-

age power source, and conventional am-

plifiers operating from batteries cannot

provide enough voltage swing to drive a

piezoelectric speaker. One approach to

this problem is to use IC 1 in Figure 1 ,

which you can configure to drive a piezo-

electric speaker with as much as

12V p-p and operate from a single

3V supply. IC 1 , a MAX4410, combines a

stereo-headphone driver with an invert-

ing charge pump that derives a negative

10k

OPTIONAL

RL NETWORK

AUDIO

INPUT

1 F

10k

INR

OUTR

100 H

10k

INL

10k

OUTL

Figure 1

This bridge-tied-load

configuration multiplies

the amplifiers’ voltage-

swing capability.

IC 1

MAX4410

100

3V supply from the positive 3V supply.

Thus, providing the drive amplifiers with

an internal

speaker appears to the amplifier as a ca-

pacitor, the speaker’s impedance decreas-

es as frequency increases, resulting in a

larger current draw from the amplifier.

IC 1 remains stable with the speaker, but a

speaker with different characteristics

might cause instability ( Figure 4 ),. In that

case, you can isolate the speaker’s capaci-

tance from the amplifier by adding a sim-

ple inductor/resistor network in series

with the speaker (within the dotted lines

on Figure 1 ). The network maintains sta-

bility by maintaining a minimum high-

frequency load of approximately 10

3V supply allows each out-

put of IC 1 to swing 6V p-p. Configuring

IC 1 as a BTL (bridge-tied load) driver

again doubles the maximum swing at the

load to 12V p-p. In the BTL configura-

tion, IC 1 ’s right channel serves as the

master amplifier. It sets the gain of the

device, drives one side of the speaker, and

provides a signal to the left channel. If

you configure IC 1 as a unity-gain follow-

er, the left channel inverts the output of

the right channel and drives the other leg

of the speaker. To ensure low distortion

and good matching,

you should set the

left-channel gain us-

ing precision resis-

tors.

We tested the cir-

cuit with a Panasonic

(www.panasonic.

com) WM-R57A pi-

ezoelectric speaker,

yielding the THD

THD+N (%)

0.1

f=10 kHz

0.01

f=1 kHz

0.001

f=100 kHz

0.0001

0 2 4 6 8 10 12 14

OUTPUT VOLTAGE

(V PP )

the IC’s output.

Figure 2

Testing the circuit yields this

THD

N versus output voltage

for the Figure 1 circuit.

V OUT =2V CC

0.1

OUTR

2V/DIV

THD+N (%)

0.01

N (total-harmonic-

distortion-plus noise)

curves ( figures 2 and

3 ). Note that

THD

0.001

10 100 1000 10000 100000

N in-

creases as frequency

increases in both

graphs. Because the

Figure 4

20 SEC/DIV

FREQUENCY (Hz)

Step response at the OUTR output of IC 1 in Figure 1, which drives

a WM-R57A piezoelectric speaker, shows that IC 1 remains stable

with the speaker.

Figure 3

Testing the circuit yields this

THD

N versus frequency for the Figure 1 circuit.

86 EDN | OCTOBER 16, 2003

www.edn.com

L OW-PROFILE PIEZOELECTRIC speak-

design

ideas

Code provides adjustable differential drive for robots

Alton Harkcom, EGO North America, Newnan, GA

ing the motor-drive

commands to a robot

can give you control finesse

during competitions. Mov-

ing a joystick hard right, for

example, might have differ-

ent effects, depending on

the robot’s speed and di-

rection. Software running

on an inexpensive micro-

controller (in this case, an

NEC (www.nec.com) PD7

8F9814) manages this con-

trol by calculating

separate drive and

adjust vectors and then

combining the vectors and

calculating the appropriate

power ratio for two tread

motors ( Figure 1 ). This

demonstration system uses

a simple treaded toy vehicle

to show how the ratio-drive

concept works and requires

no sensors in the vehicle. The microcon-

troller controls the tread motors with sim-

ple forward/reverse signals based on the

calculated motor speeds and directions.

Listing 1, which you can download

from the Web version of this Design Idea

at www.edn.com, shows the main routine

for this application. It begins by initial-

izing all the signals and then calibrating

them with the joystick at the idle position

(MID) and each extreme (LO and HI).

The code uses several sets of variables.

Drive variables (DriveRaw, DriveHI,

DriveMID, DriveLO, and Drive) specify

@ DRIVE

@ RATIO

* CALIBRATE

DRIVE-

VECTOR

CALCULATION

DRIVE VECTOR

*ENGAGE

COMBINED

VECTOR

CALCULATION

COMBINED VECTOR

MOTOR-

RATIO

CALCULATION

~ LEFT SPEED

# LEFT DIRECTION (2)

~ RIGHT SPEED

# RIGHT DIRECTION (2)

Figure 1

ADJUST VECTOR

NOTES:

*=INTERRUPT.

@=ADC INPUT.

#=DIGITAL INPUT OR OUTPUT.

~=PWM OUTPUT.

&=COMMUNICATION DATA.

$=DATA-TABLE VALUES.

(0 TO 9)=NUMBER OF CONNECTIONS.

@ADJUST

ADJUSTMENT-

VECTOR

CALCULATION

An NEC microcontroller calculates the appropriate power ratios for two tread motors in a robot.

the combined drive speed for the motors.

Ratio variables (RatioRaw, RatioHI, Ra-

tioMID, RatioLO, and Ratio) specify the

ratio of right-to-left motor balance. Ad-

just variables (AdjustRaw, AdjustHI, Ad-

justLO, and Adjust) specify the adjust-

ment value to the ratio value. The

adjustment reduces the ratio value to

better control left/right balance at par-

ticular speeds. Range-conversion vari-

ables (RangedUpper, RangedLower, and

Ranged) rerange the ADC inputs to the

desired 0 to 100% values for the motor

speed and direction. After the code cal-

culates the Left and Right motor vectors,

another routine uses these vectors to

drive the motors. If Right is greater than

1, for example, the drive routine enables

a RightForward PWM signal and disables

the reverse signal. For experimentation

purposes, you can use a 1A quad half-H

bridge to route the speed and direction

signals to the motors. In actual competi-

tions, you need a heavier duty motor con-

troller because the motor-stall currents

can exceed 1A.

Microcontroller produces analog output

Abel Raynus, Armatron International, Melrose, MA

voltage levels to control its speed: 0V

to stop the motor, 5V to run it at max-

imum speed, and some voltages between

these extremes to run it slower. When you

use such a motor in a system under mi-

crocontroller supervision, the microcon-

troller should generate all these voltages.

But a microcontroller is a digital device,

and it usually has no analog output. Sev-

eral methods are available to overcome

this deficiency. For example, you could use

a DAC, a digitally programmable poten-

tiometer, or some analog switches con-

nected to resistor dividers. However, when

you need only a few intermediate voltage

levels, it would be more attractive to find

a method that uses microcontroller soft-

ware. This Design Idea exploits the fact

that you can program a microcontroller’s

I/O pins as either input or output. When

you program a pin as output, you set its

voltage level to high (5V) or low (0V).

When you program a pin as input, it has

88 EDN | OCTOBER 16, 2003

www.edn.com

I NTELLIGENTLY modify-

A BRUSHLESS DC MOTOR needs several

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: