30404di.pdf

design

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Edited by Bill Travis

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Circuit provides efficient fan-speed control

John Guy, Maxim Integrated Products, Sunnyvale, CA

A S MOORE’S LAW plunges us into the

conditions, the system is susceptible to

failure when conditions deteriorate. If,

on the other hand, you select the fan to

maintain acceptable operating temper-

atures under worst-case conditions, the

fan may produce an annoying level of

sound. Controlling fan speed is the ob-

vious solution. If the system includes a

system-management bus, you can add

one of the many available sophis-

ticated ICs for controlling fan

speed. But if such a bus is unavailable,

you need a stand-alone fan-speed con-

troller ( Figure 1 ).

Power comes from the 12V supply,

and a dc/dc converter, IC 1 , steps down

the input voltage to an intermediate volt-

age for powering the fan. The transfer

function of this voltage is a function of

resistors R 1 and R 2 and thermistor RT 1 .

The thermistor is an NTC (negative-

temperature-coefficient) type, so the

output voltage increases with increasing

temperature. The output voltage is ap-

proximately 5.5V at room temperature

realm of multigigahertz processors

and PCs with gigabytes of RAM, en-

gineers face the task of removing the

heat that these state-of-the-art compo-

nents produce. Cooling such systems

poses a dilemma. If you optimize the fan

size and speed for nominal operating

OUTPUT

VOLTAGE

(V)

25 30 35 40 45 50 55 60

TEMPERATURE (˚C)

Circuit provides

efficient fan-speed control .......................... 69

Simple circuit forms

multichannel temperature monitor .......... 70

PWM controller drives LEDs

from high-voltage lines ................................ 72

Circuit forms satellite-dish

command decoder ........................................ 72

Use a microcontroller

to design a boost converter ........................ 74

Publish your Design Idea in EDN . See the

What’s Up section at www.edn.com.

Figure 2

Output voltage for the circuit in

Figure 1 varies with temperature.

C ( Figure 2 ). You can easily select the

ratio of resistors R 1 ,R 2 , and RT 1 by us-

ing a spreadsheet. Note that thermistor

manufacturers’ tables of resistance ratio

versus temperature are easier to use than

are the cumbersome equations for ther-

mistor resistance.

Because the circuit in Figure 1 does not

monitor fan speed or current, it includes

R 3 ,C 1 , and D 1 to ensure that the fan starts

turning during start-up. The time con-

12V

L 1

BEAD

0.1

16V

OSCON

0.1

DT3316

OSCON

16V

CVH

AIN

CVL

AGND

REF

PGND

SHDN

BOOT

STBY

ILIM

SYNC

IC 1

MAX1685

BEAD

0.1

MBRS130

0.1

0.01

15 pF

R 1

27k

BEAD

0.1

Figure 1

D 1

1N4148

R 3

15k

12V

RT 1

C 1

R 2

47k

100

10k

NTC

BEAD

0.1

ALUMINUM

ELECTROLYTIC

To control fan speed, thermistor RT 1 adjusts the output voltage of this dc/dc converter.

www.edn.com

MARCH 4, 2004 | EDN 69

and increases to 12V at approximately

180

design

ideas

stant of R 3 and C 1 serves that purpose by

causing IC 1 ’s output to overshoot during

the first few seconds of operation. After

the fan starts, it easily sustains rotation

at the lower operating voltages. An im-

portant criterion in selecting a dc/dc con-

verter is the ability to operate at 100%

duty cycle. IC 1 satisfies that requirement

and offers the convenience of an internal

power MOSFET. IC 1 supplies as much as

1A output current, which is enough to

drive one to four standard fans. As an

added benefit, its high efficiency helps to

minimize the heat that the circuit re-

moves.

Simple circuit forms

multichannel temperature monitor

Susan Pratt, Analog Devices, Limerick, Ireland

Y OU CAN USE an ADT7461 single-

V CC

channel temperature monitor, an

ADG708 low-voltage, low-leakage

CMOS 8-to-1 multiplexer, and three

standard 2N3906 pnp transistors to

measure the temperatures of three sepa-

rate remote thermal zones ( Figure 1 ).

Multiplexers have an inherent-resistance

on-resistance; the channel matching and

flatness of this resistance normally results

in a varying temperature offset. The

ADT7461 temperature monitor in

this system can automatically can-

cel resistances in series with the external

temperature sensors. The resulting sys-

tem is a multichannel temperature mon-

itor. The resistance is automatically can-

celled, so on-resistance flatness and

channel-to-channel variations have no

effect. Resistance associated with the pc-

board tracks and connectors is also can-

celled, thus allowing you to place the re-

mote temperature sensors some distance

from the ADT7461. The system requires

no user calibration; therefore, you can

connect the ADT7461 directly to the

multiplexer.

The ADT7461 digital

temperature monitor can

measure the temperature

of an external sensor with

D+ ALERT

TO HOST

2N3906

ADG708

ADT7461

D–

SDA

SCL

TO HOST

A2 A1 A0

Figure 1

MULTIPLEXER CONTROL

This system measures the temperatures of three remote thermal zones.

input of the ADT7461,

and each of the base-collector junctions

connects to a separate multiplexer input

(S1 to S3). You effect the connection of

the selected remote transistor to the D

The ADT7461 measures the tempera-

ture of the selected sensor without inter-

ference from the other transistors. Figure

2 shows the results of measuring the tem-

perature of three remote temperature

sensors. The sensor at address 000 is at

room temperature, the sensor at address

001 is at a low temperature, and the sen-

sor at address 010 is at a high tempera-

ture. When you select no external sensor,

the “open-circuit” flag in the ADT7461

output asserts. You can

expand the system to in-

clude as many external

temperature sensors as

you require. The limiting

factor on the number of

external sensors is the

time available to measure

all temperature sensors. If

you require two-wire seri-

al control of the multi-

plexer, you can use an

ADG728 in place of the

ADG708.

input by addressing the multiplexer,

which is digitally controlled by address

bits, A2, A1, and A0. The ADT7461 then

measures the temperature of whichever

transistor connects through the multi-

plexer.

200

ADDRESS CHANGED HERE

150

ADDRESS =010

C accuracy. The re-

mote sensor can be a sub-

strate-based or discrete

transistor and normally

connects to the D

100

TEMPERATURE

(˚C)

ADDRESS =000

ADDRESS =001

and

1 18 35 52

69 86 103 120 137 154

171 188 205 222 239 256 273 290

pins on the ADT-

7461. In addition to the

remote-sensor-measure-

ment channel, the

ADT7561 has an on-

chip sensor. The diode-

connected transistors with

–50

–100

REMOTE TEMPERATURE

LOCAL TEMPERATURE

The ADT7461 measures the temperature of the selected sensor

without interference from the other transistors with these results for ambient-,

hot-, and cold-temperature measurements.

70 EDN | MARCH 4, 2004

www.edn.com

their emitters connected together con-

nect to the D

Figure 2

design

ideas

PWM controller drives LEDs from high-voltage lines

Christophe Basso, On Semiconductor, Toulouse, France

P OWERING LEDS from a wide

L 1

10 mH

161 mA.

In this application, the line goes

as high as 380V dc. At steady state,

L 1 and V IN dictate the on-time,

whereas the reset voltage applied

to L 1 fixes the current decrease

during off-time. This reset volt-

age, V FTOTAL , equals the total LED

forward voltage plus the forward

drop of the freewheeling diode.

The total reaches approximately

12V in this example. It can obvi-

ously vary, depending on the type

of LED you want to drive, espe-

cially with white LEDs that incur

significant forward drops of ap-

proximately 3V. To help derive the

inductance value corresponding

to your needs, a few lines of alge-

bra suffice: t OFF

dc range—say, 30 to 380V—

without wasting a lot of

power in the regulating block, is

a difficult task when the

LED current needs to be

constant. Dedicated LED drivers

are available, but they usually im-

plement boost structures and are

thus inadequate for high-voltage

inputs. The NCP1200A, a high-

voltage controller from On Semi-

conductor (www.onsemi.com),

can serve as a constant-current

generator if you add a simple coil

in series with a power MOSFET.

If you insert diodes between the

coil and the MOSFET, the circuit

becomes an economical light

generator. Furthermore, there is

no need for a transformer or any kind of

external supply, because the controller di-

rectly connects to the rectified high volt-

age and thus supplies itself ( Figure 1 ).

The circuit forces a current to build up

in the L 1 coil and the LEDs until the volt-

age developed across R 3 reaches V FB /3.3V.

At this point, power switch Q 1 turns off,

and the magnetizing current keeps circu-

lating in the coil and LEDs, thanks to free-

wheeling diode D 1 . To maintain a “clean”

current in the LEDs, L 1 must be large

enough to keep the ripple to an acceptable

value and to avoid pushing the controller

to the minimum on-time (400 nsec) in

high-line conditions. Because of the poor

T RR (reverse-recovery time) of the

LEDs, you must add an external filter,

comprising R 2 and C 1 to the IC’s internal

leading-edge-blanking circuitry. R 1 sets the

3 0 TO 80V DC

D 1

1 N 4937

Figure 1

LEDS

S ERIES

V FB

IC 1

NCP1200A

Q 1

IRF820

R 1

12k

R 2

C 1

470 pF

C VCC

R 3

4.7

A high-voltage controller makes an ideal off-line LED driver.

voltage-feedback level; keeping it lower

than 3.3V prevents the NCP1200A’s inter-

nal short-circuit protection from tripping.

In the example, the feedback voltage of

2.5V thus imposes a peak current of

I is the rip-

ple current in L 1 ,V FTOTAL is the previous-

ly described reset voltage, and V IN is the

dc input voltage. Because the circuit runs

in continuous-current mode, the sum of

on-time and off-time gives the switching

period of the 1200AP60: L 1 (

L 1 (

I/V IN ), where

L 1 (

I/V FTOTAL )

1/f S ,where f S is the switching

frequency. Extracting L 1 yields L 1

I/V IN )

I/V FTOTAL )

(1/f S )

I).

If you select a ripple current of 20 mA

peak-to-peak at 380V dc, then L 1

16.66

V IN )/(V FTOTAL

V IN )](1/

9.6 mH. From this val-

ue, you can check the minimum on-time

using the equation: t ON

11.6

9.6mH

0.02/

508 nsec, above the minimum lim-

it. Figure 2 portrays typical signals cap-

tured on the prototype supplied with low

line voltage.

Figure 2

The prototype with low line voltage pro-

vides these typical signals.

Circuit forms satellite-dish command decoder

Mark Giebler, Oakdale, MN

B Y DECODING THE COMMANDS sent

by a direct-broadcast satellite re-

ceiver that uses the DISEQC (digi-

tal-satellite-equipment-control) proto-

col, you can troubleshoot the com-

mands or simply listen in. Eutelsat Corp

(www.eutelsat.com) offers the DISEQC

protocol. The technique uses only the

coaxial cable between the receiver and

the dish to send commands for actions

such as changing the low-noise-block

frequency range or switching between

dishes for multisatellite reception. The

DISEQC protocol specifies a bit time of

1.5 msec and bit values as shown in Fig-

ure 1 , the timing diagram of bit modu-

lation on the coaxial cable. The signal’s

ac portion is a 22-Hz burst whose am-

plitude ranges from 300 to 600 mV. A

voltage-doubler circuit detects the 22-

Hz portion, producing a pulse stream in

72 EDN | MARCH 4, 2004

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2.5/3.3/4.7

and t ON

L 1 (

[(V FTOTAL

380

design

ideas

which constant-voltage pulses

having amplitudes of 0.6 to

1.2V replace the 22-Hz bursts.

Decoding this bit stream into

ASCII hex values is an ideal job

for a low-cost 8-bit microcon-

troller. Using a microcontroller

with onboard flash mem-

ory, such as the NEC Elec-

tronics

ZERO DATA BIT

ONE DATA BIT

until its value exceeds 120

and then perform a loop

while doing analog-to-dig-

ital conversions. If the loop

count reaches 24 with

ADC values greater than

120, the bit is a zero. If the

pulse has gone away, the

bit is a one. Any extra de-

lay from executing in-

structions in the loop has

little effect, because the bit windows leave

plenty of margin.

1 mSEC

0.5 mSEC 0.5 mSEC

1 mSEC

Figure 1

The DISEQC protocol specifies a bit time of 1.5 msec

and these bit values.

PD78F9418A (www.

necelam.com), eliminates the

need for external memory. The only ex-

ternal components are a few discrete de-

vices for the signal detector and the coax-

ial-cable loop-through ( Figure 2 ). You

can add an RS-232 driver if you want to

display the ASCII codes on a laptop com-

puter via HyperTerminal. You can also

use the

pulse. Set the A/D converter’s conversion

time to 28.8

V DD

22-kHz VOLTAGE-

DOUBLING DETECTOR

SATELLITE IF

LOOP-THROUGH

CONNECTORS

PD78F9418A’s onboard LCD

controller to display the codes on a ded-

icated display.

One of the

TO MICROPROCESSOR

ADC INPUT

F CONNECTOR

TO LOW-NOISE

BLOCK

PD78F9418A microcon-

troller’s 10-bit A/D converters performs

pulse detection and acts as a simple

timing device. Using a reference

voltage of 5V, the converter provides ap-

proximately 4.88 mV per step. An A/D-

converter conversion value greater than

120 counts (585 mV) represents a valid

0.01

FERRITE

BEAD

SIX TURNS

10k

0.01

Figure 2

F CONNECTOR

TO SET-TOP BOX

FERRITE

BEAD

SIX TURNS

This circuit enables the microcontroller to decode the DISEQC-protocol bit stream.

Use a microcontroller to design a boost converter

Ross Fosler, Microchip Technology, Chandler, AZ

switchers, have traditionally re-

ceived their control signals from a

dedicated circuit. However, a recent trend

is to integrate simple switching-power-

supply building blocks into generic de-

vices, such as microcontrollers. An excel-

lent example of this concept is a

microcontroller that combines digital

and analog circuitry and makes it easier

to build simple power supplies. The pro-

gramming capability of a microcon-

troller is an added benefit in power-sup-

ply designs, especially when you want to

experiment with the supplies. Figure 1 il-

lustrates a simple boost-converter design

using a microcontroller; the basic boost

topology in Figure 1 is a type of flyback

circuit. The basic concept is

easy to understand. When the

MOSFET, Q, turns on, the cur-

rent flowing through the in-

ductor, L, begins to ramp up

linearly ( Figure 2 ), resulting in

energy storage in the inductor.

The MOSFET turns off before

the inductor saturates. At this

time, the inductor releases its

energy to the storage capaci-

tor, C, and the load.

You can design a simple boost con-

verter with the following conditions:

V IN

9V, V OUT

18V, R LOAD

mV, where F is the switching frequency,

62.5 kHz,

70%, and

V DROP

V DROP is the out-

put ripple voltage. You can calculate the

on-time, current, ramp-down time, and

the total period in terms of inductance:

is the efficiency, and

V IN

V OUT

PWM

R VF1

V VF

CONTROL

Figure 1

V IG

R VF2

R IF

This circuit block represents the basic topology of a boost

converter.

Then, you calculate the peak current

through the inductor and the inductance

value:

74 EDN | MARCH 4, 2004

www.edn.com

sec and wait to detect a

pulse edge by reading the A/D converter

B OOST CONVERTERS, like other

design

ideas

gram. You may be able

to design an adaptive

power-control system by

adjusting the phase and

gain to meet the desired

needs of a system.

Firmware placement

within the control loop

is not the only possibili-

ty; you could use a com-

bination of firmware

and hardware to moni-

tor the system. Be-

cause the analog

information is visible

and the analog functions are controllable

within the PIC16C782 device, you can

monitor an active system for perform-

ance and function. In essence, the system

can have self-diagnostic capabilities to

check stability, load, input and output

conditions, or anything else a system may

require. You can also obtain Information

t ON

t R

t D

I PEAK

SWITCH CURRENT

Finally, you calculate the capacitance

based on the ripple voltage:

DIODE CURRENT

F ca-

pacitor. The difference in the inductor

value is absorbed in the dead time, as is

the power loss.

The control circuit can take many

forms, especially if you choose a device

such as the PIC16C782 microcontroller.

This device integrates a built-in analog

peripheral set, diverse analog visibility,

and a mixed-signal PWM block. The

control circuit in Figure 3 demonstrates

how the analog and

pulse-width modulation

is contained within

the PIC16C782, with

the exception of the FET

driver. This control cir-

cuit combines analog

current control and

firmware voltage control.

The interesting part is the

firmware, which is direct-

ly in the voltage-feedback path of the

control loop. Through firmware, you can

alter the dynamics of the control loop by

changing the functions within the pro-

H inductor and a 220-

Figure 2

These curves show the switch and diode

currents in the circuit of Figure 1.

change the functions without changing

hardware. This approach eases experi-

mentation; you simply changing

firmware rather than spending hours in

the lab adding or changing parts.

Figures 4 and 5 are oscilloscope pho-

tos from a working example of the boost

converter implementing the basic topol-

ogy in Figure 1 and the

control block in Figure 3 .

The peak current in the in-

ductor is 0.3 mV

Figure 3

1.5A ( Figure 4 ). The on-

time is approximately 5.9

0.2

V I

CURRENT

CONTROL

TC4427

TO POWER

SWITCH

PSMC

DAC

ADC

VOLTAGE

CONTROL

FIRMWARE

V V

sec. The output voltage is

18V into a 72

load ( Fig-

ure 5 ). The efficiency is ap-

proximately 90%. These

boost-converter design and

control ideas are just a few of the many

possible ones using a PIC16C782 de-

vice.

A microcontroller contains all the elements necessary for boost-converter

control.

about the system, through a serial port or

some other means, by routing the data to

a terminal or computer display. Even bet-

ter, the firmware allows the design to

A working example of the boost converter

implementing the basic topology of Figure 1

shows the duty cycle (top) and the current-ramp-down waveform (bot-

tom) for the circuit in Figure 1.

Figure 5

The example shows the duty cycle (top) and

the output voltage (bottom) of the circuit in

Figure 1.

76 EDN | MARCH 4, 2004

www.edn.com

Note that the design is slightly altered

to use readily available components, by

using a 33-

Figure 4

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