LM2917N-14 - National Semiconductor

TL/H/7942

LM2907/LM2917 Frequency to Voltage Converter

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

YOutput swings to ground for zero frequency input

YEasy to use; VOUT efIN cVCC cR1 cC1

YOnly one RC network provides frequency doubling

YZener regulator on chip allows accurate and stable fre-

quency to voltage or current conversion (LM2917)

Features

YGround referenced tachometer input interfaces directly

with variable reluctance magnetic pickups

YOp amp/comparator has floating transistor output

Y50 mA sink or source to operate relays, solenoids, me-

ters, or LEDs

YFrequency doubling for low ripple

YTachometer has built-in hysteresis with either differen-

tial input or ground referenced input

YBuilt-in zener on LM2917

Yg0.3% linearity typical

YGround referenced tachometer is fully protected from

damage due to swings above VCC and below ground

Applications

YOver/under speed sensing

YFrequency to voltage conversion (tachometer)

YSpeedometers

YBreaker point dwell meters

YHand-held tachometer

YSpeed governors

YCruise control

YAutomotive door lock control

YClutch control

YHorn control

YTouch or sound switches

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

Order Number LM2907N

See NS Package Number N14A

TL/H/7942–4

Order Number LM2917M or LM2917N

See NS Package Number M14A or N14A

C1995 National Semiconductor Corporation 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 28V

Supply Current (Zener Options) 25 mA

Collector Voltage 28V

Differential Input Voltage

Tachometer 28V

Op Amp/Comparator 28V

Input Voltage Range

Tachometer LM2907-8, LM2917-8 g28V

LM2907, LM2917 0.0V to a28V

Op Amp/Comparator 0.0V to a28V

Power Dissipation

LM2907-8, LM2917-8 1200 mW

LM2907-14, LM2917-14 1580 mW

(See Note 1)

Operating Temperature Range b40§Ctoa

85§C

Storage Temperature Range b65§Ctoa

150§C

Soldering Information

Dual-In-Line Package

Soldering (10 seconds) 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.

Electrical Characteristics VCC e12 VDC,T

Ae25§C, see test circuit

Symbol Parameter Conditions Min Typ Max Units

TACHOMETER

Input Thresholds VIN e250 mVp-p @1 kHz (Note 2) g10 g25 g40 mV

Hysteresis VIN e250 mVp-p @1 kHz (Note 2) 30 mV

Offset Voltage VIN e250 mVp-p @1 kHz (Note 2)

LM2907/LM2917 3.5 10 mV

LM2907-8/LM2917-8 5 15 mV

Input Bias Current VIN eg50 mVDC 0.1 1 mA

VOH Pin 2 VIN ea

125 mVDC (Note 3) 8.3 V

VOL Pin 2 VIN eb

125 mVDC (Note 3) 2.3 V

I2,I

3Output Current V2 eV3 e6.0V (Note 4) 140 180 240 mA

I3Leakage Current I2 e0, V3 e0 0.1 mA

K Gain Constant (Note 3) 0.9 1.0 1.1

Linearity fIN e1 kHz, 5 kHz, 10 kHz (Note 5) b1.0 0.3 a1.0 %

OP/AMP COMPARATOR

VOS VIN e6.0V 3 10 mV

IBIAS VIN e6.0V 50 500 nA

Input Common-Mode Voltage 0 VCCb1.5V V

Voltage Gain 200 V/mV

Output Sink Current VCe1.0 40 50 mA

Output Source Current VEeVCC b2.0 10 mA

Saturation Voltage ISINK e5 mA 0.1 0.5 V

ISINK e20 mA 1.0 V

ISINK e50 mA 1.0 1.5 V

Electrical Characteristics VCC e12 VDC,T

Ae25§C, see test circuit (Continued)

Symbol Parameter Conditions Min Typ Max Units

ZENER REGULATOR

Regulator Voltage RDROP e470X7.56 V

Series Resistance 10.5 15 X

Temperature Stability a1 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 aVTH b(bVTH), offset voltage is their difference. See test circuit.

Note 3: VOH is equal to */4 cVCC b1V

BE,V

OL is equal to (/4 cVCC b1V

BE therefore VOH bVOL eVCC/2. The difference, VOH bVOL, and the mirror gain,

I2/I3, 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 cC1 that R1 is such that the maximum anticipated output voltage at pin 3 can be reached with I3cR1. The

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

Note 5: Nonlinearity is defined as the deviation of VOUT (@pin 3) for fIN e5 kHz from a straight line defined by the VOUT @1 kHz and VOUT @10 kHz.

C1 e1000 pF, R1 e68k and C2 e0.22 mFd.

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 VCC up to a

maximum VCE of 28V.

The two basic configurations offered include an 8-pin device

with a

ground referenced tachometer

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

TL/H/7942–6

Tachometer Input Threshold Measurement

TL/H/7942–7

Typical Performance Characteristics

Total Supply Current Temperature

Zener Voltage vs

Output vs Temperature

Normalized Tachometer

Output vs Temperature

Normalized Tachometer

and I3vs Supply Voltage

Tachometer Currents I2

and I3vs Temperature

Tachometer Currents I2

vs Temperature

Tachometer Linearity

vs Temperature

Tachometer Linearity

Tachometer Linearity vs R1

vs Temperature

Tachometer Input Hysteresis

Characteristics

Op Amp Output Transistor

Characteristics

Op Amp Output Transistor

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 g28V, 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

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

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

capacitor is equal to VCC/2 cC1. The average amount of

current pumped into or out of the capacitor then is:

Teic(AVG) eC1 cVCC

2c(2fIN)eVCC cfIN cC1

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

VOeiccR1, and the total conversion equation becomes:

VOeVCC cfIN cC1 cR1 cK

Where K is the gain constantÐtypically 1.0.

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 VO/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:

VRIPPLE eVCC

2cC1

C2 c#1bVCC cfIN cC1

I2Jpk-pk

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

er response time, or the time it takes VOUT 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 VCC, C1 and I2:

fMAX eI2

C1 cVCC

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 470Xwill minimize the ze-

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

der 400Xor over 600Xthe zener variation quickly rises

above 200 mV for the same input variation.

Typical Applications

Minimum Component Tachometer

TL/H/7942–8

Typical Applications (Continued)

‘‘Speed Switch’’ Load is Energized When fIN t1

2RC

TL/H/7942–9

Zener Regulated Frequency to Voltage Converter

TL/H/7942–10

Breaker Point Dwell Meter

TL/H/7942–11

Typical Applications (Continued)

Voltage Driven Meter Indicating Engine RPM

VOe6V @400 Hz or 6000 ERPM (8 Cylinder Engine)

TL/H/7942–12

Current Driven Meter Indicating Engine RPM

IOe10 mA @300 Hz or 6000 ERPM (6 Cylinder Engine)

TL/H/7942–13

Capacitance Meter

VOUT e1V– 10V for CXe0.01 to 0.1 mFd

(R e111k)

TL/H/7942–14

Typical Applications (Continued)

Two-Wire Remote Speed Switch

TL/H/7942–15

100 Cycle Delay Switch

V3 steps up in voltage by the amount VCC cC1

for each complete input cycle (2 zero crossings) TL/H/7942–16

Example:

If C2 e200 C1 after 100 consecutive input cycles.

V3 e1/2 VCC

Typical Applications (Continued)

Variable Reluctance Magnetic Pickup Buffer Circuits

Precision two-shot output frequency

equals twice input frequency.

Pulse width eVCC

I2 .

Pulse height eVZENER

TL/H/7942–39

TL/H/7942–17

Finger Touch or Contact Switch

TL/H/7942–18

TL/H/7942–19

Flashing LED Indicates Overspeed

Flashing begins when fIN t100 Hz.

Flash rate increases with input frequency

increase beyond trip point.

TL/H/7942–20

Typical Applications (Continued)

Frequency to Voltage Converter with 2 Pole Butterworth Filter to Reduce Ripple

fPOLE e0.707

2qRC

uRESPONSE e2.57

2qfPOLE

TL/H/7942–21

Overspeed Latch

TL/H/7942–22

Output latches when TL/H/7942–23

fIN eR2

R1 aR2

Reset by removing VCC.

Typical Applications (Continued)

Some Frequency Switch Applications May Require Hysteresis in the

Comparator Function Which can be Implemented in Several Ways:

TL/H/7942–24

TL/H/7942–25 TL/H/7942–26

TL/H/7942–27 TL/H/7942–28

Typical Applications (Continued)

Changing the Output Voltage for an Input Frequency of Zero

TL/H/7942–29

TL/H/7942–30

Changing Tachometer Gain Curve or Clamping the Minimum Output Voltage

TL/H/7942–31

TL/H/7942–32

Anti-Skid Circuit Functions

‘‘Select-Low’’ Circuit

TL/H/7942–33

TL/H/7942–34

VOUT is proportional to the lower of the

two input wheel speeds.

‘‘Select-High’’ Circuit

TL/H/7942–35

TL/H/7942–36

VOUT is proportional to the higher of

the two input wheel speeds.

‘‘Select-Average’’ Circuit

TL/H/7942–37

Equivalent Schematic Diagram

TL/H/7942–38

*This connection made on LM2907-8 and LM2917-8 only.

**This connection made on LM2917 and LM2917-8 only.

Physical Dimensions inches (millimeters)

8-Lead (0.150×Wide) Molded Small Outline Package, JEDEC

Order Number LM2907M-8 or LM2917M-8

NS Package Number M08A

Physical Dimensions inches (millimeters) (Continued)

Molded SO Package (M)

Order Number LM2917M

NS Package Number M14A

Molded Dual-In-Line Package (N)

Order Number LM2907N-8 or LM2917N-8

NS Package Number N08E

LM2907/LM2917 Frequency to Voltage Converter

Physical Dimensions inches (millimeters) (Continued)

Molded Dual-In-Line Package (N)

Order Number LM2907N or LM2917N

NS Package Number N14A

LIFE SUPPORT POLICY

NATIONAL’S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT

DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF NATIONAL

SEMICONDUCTOR CORPORATION. As used herein:

1. Life support devices or systems are devices or 2. A critical component is any component of a life

systems which, (a) are intended for surgical implant support device or system whose failure to perform can

into the body, or (b) support or sustain life, and whose be reasonably expected to cause the failure of the life

failure to perform, when properly used in accordance support device or system, or to affect its safety or

with instructions for use provided in the labeling, can effectiveness.

be reasonably expected to result in a significant injury

to the user.

National Semiconductor National Semiconductor National Semiconductor National Semiconductor

Corporation Europe Hong Kong Ltd. Japan Ltd.

1111 West Bardin Road Fax: (

49) 0-180-530 85 86 13th Floor, Straight Block, Tel: 81-043-299-2309

Arlington, TX 76017 Email: cnjwge

tevm2.nsc.com Ocean Centre, 5 Canton Rd. Fax: 81-043-299-2408

Tel: 1(800) 272-9959 Deutsch Tel: (

49) 0-180-530 85 85 Tsimshatsui, Kowloon

Fax: 1(800) 737-7018 English Tel: (

49) 0-180-532 78 32 Hong Kong

Fran3ais Tel: (

49) 0-180-532 93 58 Tel: (852) 2737-1600

Italiano Tel: (

49) 0-180-534 16 80 Fax: (852) 2736-9960

National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.