LM2907.PDF

February 1995

LM2907/LM2917 Frequency to Voltage Converter

General Description

The LM2907, LM2917 series are monolithic frequency to

voltage converters with a high gain op amp/comparator de-

signed to operate a relay, lamp, or other load when the input

frequency reaches or exceeds a selected rate. The tachom-

eter uses a charge pump technique and offers frequency

doubling for low ripple, full input protection in two versions

(LM2907-8, LM2917-8) and its output swings to ground for a

zero frequency input.

Advantages

Y Output swings to ground for zero frequency input

Y Easy to use; V OUT e f IN c V CC c R1 c C1

Y Only one RC network provides frequency doubling

Y Zener regulator on chip allows accurate and stable fre-

quency to voltage or current conversion (LM2917)

Y Frequency doubling for low ripple

Y Tachometer has built-in hysteresis with either differen-

tial input or ground referenced input

Y Built-in zener on LM2917

Y g 0.3% linearity typical

Y Ground referenced tachometer is fully protected from

damage due to swings above V CC and below ground

Applications

Y Over/under speed sensing

Y Frequency to voltage conversion (tachometer)

Y Speedometers

Y Breaker point dwell meters

Y Hand-held tachometer

Y Speed governors

Y Cruise control

Y Automotive door lock control

Y Clutch control

Y Horn control

Y Touch or sound switches

Features

Y Ground referenced tachometer input interfaces directly

with variable reluctance magnetic pickups

Y Op amp/comparator has floating transistor output

Y 50 mA sink or source to operate relays, solenoids, me-

ters, or LEDs

Block and Connection Diagrams Dual-In-Line and Small Outline Packages, Top Views

TL/H/7942±1

Order Number LM2907M-8 or LM2907N-8

See NS Package Number M08A or N08E

TL/H/7942±2

Order Number LM2917M-8 or LM2917N-8

See NS Package Number M08A or N08E

TL/H/7942±3

TL/H/7942±4

Order Number LM2907N

See NS Package Number N14A

Order Number LM2917M or LM2917N

See NS Package Number M14A or N14A

C 1995 National Semiconductor Corporation

TL/H/7942

RRD-B30M115/Printed in U. S. A.

Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales

Office/Distributors for availability and specifications.

Supply Voltage

Power Dissipation

LM2907-8, LM2917-8

1200 mW

LM2907-14, LM2917-14

1580 mW

28V

(See Note 1)

Operating Temperature Range

Supply Current (Zener Options)

25 mA

b 40 § Cto a 85 § C

Collector Voltage

28V

Storage Temperature Range

b 65 § Cto a 150 § C

Differential Input Voltage

Tachometer

28V

Soldering Information

Dual-In-Line Package

Soldering (10 seconds)

Op Amp/Comparator

28V

260 § C

Small Outline Package

Vapor Phase (60 seconds) 215 § C

Infrared (15 seconds) 220 § C

See AN-450 ``Surface Mounting Methods and Their Effect

on Product Reliability'' for other methods of soldering sur-

face mount devices.

Input Voltage Range

Tachometer LM2907-8, LM2917-8

g 28V

LM2907, LM2917

0.0V to a 28V

Op Amp/Comparator

0.0V to a 28V

Electrical Characteristics V CC e 12 V DC ,T A e 25 § C, see test circuit

Symbol

Parameter

Conditions

Min Typ Max

Units

TACHOMETER

Input Thresholds

V IN e 250 mVp-p @ 1 kHz (Note 2) g 10 g 25 g 40

Hysteresis

V IN e 250 mVp-p @ 1 kHz (Note 2)

Offset Voltage

V IN e 250 mVp-p @ 1 kHz (Note 2)

LM2907/LM2917

3.5

LM2907-8/LM2917-8

Input Bias Current

V IN e g 50 mV DC

0.1

m A

V OH

Pin 2

V IN ea 125 mV DC (Note 3)

8.3

V OL

Pin 2

V IN eb 125 mV DC (Note 3)

2.3

I 2 ,I 3 Output Current

V2 e V3 e 6.0V (Note 4)

140

180

240

m A

I 3

Leakage Current

I2 e 0, V3 e 0

0.1

m A

Gain Constant

(Note 3)

0.9

1.0

1.1

Linearity

f IN e 1 kHz, 5 kHz, 10 kHz (Note 5) b 1.0 0.3 a 1.0 %

OP/AMP COMPARATOR

V OS

V IN e 6.0V

I BIAS

V IN e 6.0V

500

Input Common-Mode Voltage

V CC b 1.5V

Voltage Gain

200

V/mV

Output Sink Current

V C e 1.0

Output Source Current

V E e V CC b 2.0

Saturation Voltage

I SINK e 5 mA

0.1

0.5

I SINK e 20 mA

1.0

I SINK e 50 mA

1.0

1.5

Electrical Characteristics V CC e 12 V DC ,T A e 25 § C, see test circuit (Continued)

Symbol

Conditions

Min

Typ

Max

Units

ZENER REGULATOR

Regulator Voltage R DROP e 470 X 7.56 V

Series Resistance 10.5 15 X

Temperature Stability a 1 mV/ § C

TOTAL SUPPLY CURRENT 3.8 6 mA

Note 1: For operation in ambient temperatures above 25 § C, the device must be derated based on a 150 § C maximum junction temperature and a thermal resistance

of 101 § C/W junction to ambient for LM2907-8 and LM2917-8, and 79 § C/W junction to ambient for LM2907-14 and LM2917-14.

Note 2: Hysteresis is the sum a V TH b ( b V TH ), offset voltage is their difference. See test circuit.

Note 3: V OH is equal to

General Description (Continued)

The op amp/comparator is fully compatible with the ta-

chometer and has a floating transistor as its output. This

feature allows either a ground or supply referred load of up

to 50 mA. The collector may be taken above V CC up to a

maximum V CE of 28V.

The two basic configurations offered include an 8-pin device

with a groundreferencedtachometer input and an internal

connection between the tachometer output and the op amp

non-inverting input. This version is well suited for single

speed or frequency switching or fully buffered frequency to

voltage conversion applications.

The more versatile configurations provide differential ta-

chometer input and uncommitted op amp inputs. With this

version the tachometer input may be floated and the op

amp becomes suitable for active filter conditioning of the

tachometer output.

Both of these configurations are available with an active

shunt regulator connected across the power leads. The reg-

ulator clamps the supply such that stable frequency to volt-

age and frequency to current operations are possible with

any supply voltage and a suitable resistor.

Test Circuit and Waveform

Tachometer Input Threshold Measurement

TL/H/7942±7

TL/H/7942±6

Parameter

(/4 c V CC b 1V BE therefore V OH b V OL e V CC /2. The difference, V OH b V OL , and the mirror gain,

I 2 /I 3 , are the two factors that cause the tachometer gain constant to vary from 1.0.

Note 4: Be sure when choosing the time constant R1 c C1 that R1 is such that the maximum anticipated output voltage at pin 3 can be reached with I 3 c R1. The

maximum value for R1 is limited by the output resistance of pin 3 which is greater than 10 M X typically.

Note 5: Nonlinearity is defined as the deviation of V OUT ( @ pin 3) for f IN e 5 kHz from a straight line defined by the V OUT @ 1 kHz and V OUT @ 10 kHz.

C1 e 1000 pF, R1 e 68k and C2 e 0.22 mFd.

*/4 c V CC b 1V BE ,V OL is equal to

Typical Performance Characteristics

Zener Voltage vs

Normalized Tachometer

Total Supply Current

Temperature

Output vs Temperature

Normalized Tachometer

Tachometer Currents I 2

Output vs Temperature

and I 3 vs Supply Voltage

and I 3 vs Temperature

Tachometer Linearity

vs Temperature

Tachometer Linearity vs R1

Tachometer Input Hysteresis

Op Amp Output Transistor

vs Temperature

Characteristics

TL/H/7942±5

Applications Information

The LM2907 series of tachometer circuits is designed for

minimum external part count applications and maximum ver-

satility. In order to fully exploit its features and advantages

let's examine its theory of operation. The first stage of oper-

ation is a differential amplifier driving a positive feedback

flip-flop circuit. The input threshold voltage is the amount of

differential input voltage at which the output of this stage

changes state. Two options (LM2907-8, LM2917-8) have

one input internally grounded so that an input signal must

swing above and below ground and exceed the input

thresholds to produce an output. This is offered specifically

for magnetic variable reluctance pickups which typically pro-

vide a single-ended ac output. This single input is also fully

protected against voltage swings to g 28V, which are easily

attained with these types of pickups.

The differential input options (LM2907, LM2917) give the

user the option of setting his own input switching level and

still have the hysteresis around that level for excellent noise

rejection in any application. Of course in order to allow the

inputs to attain common-mode voltages above ground, input

protection is removed and neither input should be taken

outside the limits of the supply voltage being used. It is very

important that an input not go below ground without some

resistance in its lead to limit the current that will then flow in

the epi-substrate diode.

Following the input stage is the charge pump where the

input frequency is converted to a dc voltage. To do this

requires one timing capacitor, one output resistor, and an

integrating or filter capacitor. When the input stage changes

state (due to a suitable zero crossing or differential voltage

on the input) the timing capacitor is either charged or dis-

charged linearly between two voltages whose difference is

V CC /2. Then in one half cycle of the input frequency or a

time equal to 1/2 f IN the change in charge on the timing

capacitor is equal to V CC /2 c C1. The average amount of

current pumped into or out of the capacitor then is:

D Q

T e i c(AVG) e C1 c

The size of C2 is dependent only on the amount of ripple

voltage allowable and the required response time.

CHOOSING R1 AND C1

There are some limitations on the choice of R1 and C1

which should be considered for optimum performance. The

timing capacitor also provides internal compensation for the

charge pump and should be kept larger than 500 pF for very

accurate operation. Smaller values can cause an error cur-

rent on R1, especially at low temperatures. Several consid-

erations must be met when choosing R1. The output current

at pin 3 is internally fixed and therefore V O /R1 must be less

than or equal to this value. If R1 is too large, it can become

a significant fraction of the output impedance at pin 3 which

degrades linearity. Also output ripple voltage must be con-

sidered and the size of C2 is affected by R1. An expression

that describes the ripple content on pin 3 for a single R1C2

combination is:

V RIPPLE e

V CC

C2 c

1 b

V CC c f IN c C1

I 2

pk-pk

It appears R1 can be chosen independent of ripple, howev-

er response time, or the time it takes V OUT to stabilize at a

new voltage increases as the size of C2 increases, so a

compromise between ripple, response time, and linearity

must be chosen carefully.

As a final consideration, the maximum attainable input fre-

quency is determined by V CC , C1 and I 2 :

f MAX e I 2

C1 c V CC

USING ZENER REGULATED OPTIONS (LM2917)

For those applications where an output voltage or current

must be obtained independent of supply voltage variations,

the LM2917 is offered. The most important consideration in

choosing a dropping resistor from the unregulated supply to

the device is that the tachometer and op amp circuitry alone

require about 3 mA at the voltage level provided by the

zener. At low supply voltages there must be some current

flowing in the resistor above the 3 mA circuit current to op-

erate the regulator. As an example, if the raw supply varies

from 9V to 16V, a resistance of 470 X will minimize the ze-

ner voltage variation to 160 mV. If the resistance goes un-

der 400 X or over 600 X the zener variation quickly rises

above 200 mV for the same input variation.

V CC

2 c (2f IN ) e V CC c f IN c C1

The output circuit mirrors this current very accurately into

the load resistor R1, connected to ground, such that if the

pulses of current are integrated with a filter capacitor, then

V O e i c c R1, and the total conversion equation becomes:

V O e V CC c f IN c C1 c R1 c K

Where K is the gain constantÐtypically 1.0.

Typical Applications

Minimum Component Tachometer

TL/H/7942±8

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: