LM2907.PDF

LM2907 Tachometer/Speed

Switch Building Block

Applications

National Semiconductor

Application Note 162

June 1976

INTRODUCTION

Frequency to voltage converters are available in a number

of forms from a number of sources, but invariably require

significant additional components before they can be put to

use in a given situation. The LM2907, LM2917 series of

devices was developed to overcome these objections. Both

input and output interface circuitry is included on chip so

that a minimum number of additional components is re-

quired to complete the function. In keeping with the systems

building block concept, these devices provide an output

voltage which is proportional to input frequency and provide

zero output at zero frequency. In addition, the input may be

referred to ground. The devices are designed to operate

from a single supply voltage, which makes them particularly

suitable for battery operation.

PART 1ÐGENERAL OPERATION PRINCIPLES

Circuit Description

Referring to Figure1, the family of devices all include three

basic components: an input amplifier with built-in hysteresis;

a charge pump frequency to voltage converter; and a versa-

tile op amp/comparator with an uncommitted output transis-

tor. LM2917 incorporates an active zener regulator on-chip.

LM2907 deletes this option. Both versions are obtainable in

14-pin and in 8-pin dual-in-line molded packages, and to

special order in other packages.

LM2907N-8

LM2917N-8

TL/H/7451±1

TL/H/7451±2

LM2907N

LM2917N

TL/H/7451±3

TL/H/7451±4

FIGURE 1. Block Diagrams

C 1995 National Semiconductor Corporation

TL/H/7451

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

Input Hysteresis Amplifier

The equivalent schematic diagram is shown in Figure2. Q1

through Q11 comprise the input hysteresis amplifier. Q1

through Q4 comprise an input differential amplifier which, by

virtue of PNP level shifting, enables the circuit to operate

with signals referenced to ground. Q7, Q8, D4, and D5 com-

prise an active load with positive feedback. This load be-

haves as a bi-stable flip-flop which may be set or reset de-

pending upon the currents supplied from Q2 and Q3. Con-

sider the situation where Q2 and Q3 are conducting equally,

i.e. the input differential voltage is zero. Assuming Q7 to be

conducting, it will be noted that the current from Q3 will be

drawn by Q7 and Q8 will be in the ``OFF'' state. This allows

the current from Q2 to drive Q7 in parallel with D4 and a

small resistor. D4 and Q7 are identical geometry devices, so

that the resistor causes Q7 to be biased at a higher level

than D4. Thus Q7 will be able to conduct more current than

Q3 provides. In order to reverse the state of Q7 and Q8, it

will be necessary to reduce the current from Q2 below that

provided by Q3 by an amount which is established by R1. It

can be shown that this requires a differential input to Q1 and

Q4, of approximately 15mV. Since the circuit is symmetrical,

the threshold voltage to reverse the state is 15 mV in the

other direction. Thus the input amplifier has built-in hystere-

sis at g 15 mV. This provides clean switching where noise

may be present on the input signal, and allows total rejec-

tion of noise below this amplitude where there is no input

signal.

Charge Pump

The charge pump is composed of Q12 through Q32. R4, R5,

and R6 provide reference voltages equal to 1/4 and 3/4 of

supply voltage to Q12 and Q13. When Q10 turns ``ON'' or

``OFF,'' the base voltage at Q16 changes by an amount

equal to the voltage across R5, that is 1/2 V CC . A capacitor

connected between Pin 2 and ground is either charged by

Q21 or discharged by Q22 until its voltage matches that on

the base of Q16. When the voltage on Q16 base goes low,

Q16 turns ``ON,'' which results in Q18 and Q26 turning on,

which causes the current, sourced by Q19 and Q20, to be

shunted to ground. Thus Q21 is unable to charge pin 2.

Meanwhile, Q27 and Q30 are turned off permitting the

200 m A sourced by Q28 and Q29 to enter the emitters of

Q31 and Q32 respectively. The current from Q31 is mirrored

by Q22 through Q24 resulting in a 200 m A discharge current

through pin 2. The external capacitor on pin 2 is thus dis-

charged at a constant rate until it reaches the new base

voltage on Q16. The time taken for this discharge to occur is

given by:

required to return the capacitor on pin 2 to the high level

voltage is duplicated and used to charge the capacitor con-

nected to pin 3. Thus in one cycle of input the capacitor on

pin 3 gets charged twice with a charge of CV.

Thus the total charge pumped into the capacitor on pin 3

per cycle is:

Q e 2 CV

(2)

Q e CV CC

A resistor connected between pin 3 and ground causes a

discharge of the capacitor on pin 3, where the total charge

drained per cycle of input signal is equal to:

(3)

V3 # T

where V3 e the average voltage on pin 3

T e period of input signal

R e resistor connected to pin 3

In equilibrium Q e Q1

i.e., CV CC e V3 # T

Q1 e

(4)

and

V3 e V CC # RC

(5)

V3 e V CC # R # C # f

where f e input frequency

Op Amp/Comparator

Again referring toFigure2, the op amp/comparator includes

Q35 through Q45. A PNP input stage again provides input

common-mode voltages down to zero, and if pin 8 is con-

nected to V CC and the output taken from pin 5, the circuit

behaves as a conventional, unity-gain-compensated opera-

tional amplifier. However, by allowing alternate connections

of Q45 the circuit may be used as a comparator in which

loads to either V CC or ground may be switched. Q45 is ca-

pable of sinking 50 mA. Input bias current is typically 50 nA,

and voltage gain is typically 200 V/mV. Unity gain slew rate

is 0.2 V/ m s. When operated as a comparator Q45 emitter

will switch at the slew rate, or the collector of Q45 will

switch at that rate multiplied by the voltage gain of Q45,

which is user selectable.

Active Zener Regulator

The optional active zener regulator is also shown in Figure

2. D8 provides the voltage reference in conjunction with

Q33. As the supply voltage rises, D8 conducts and the base

voltage on Q33 starts to rise. When Q33 has sufficient base

voltage to be turned ``ON,'' it in turn causes Q34 to conduct

current from the power source. This reduces the current

available for D8 and the negative feedback loop is thereby

completed. The reference voltage is therefore the zener

voltage on D8 plus the emitter base voltage of Q33. This

results in a low temperature coefficient voltage.

Input Levels and Protection

In 8-pin versions of the LM2907, LM2917, the non-inverting

input of the op amp/comparator is connected to the output

of the charge pump. Also, one input to the input hysteresis

amplifier is connected to ground. The other input (pin 1) is

then protected from transients by, first a 10k X series resis-

(6)

t e CV

(1)

where C e capacitor on pin 2

V e change in voltage on Q16 base

I e current in Q22

During this time, Q32 sources an identical current into pin 3.

A capacitor connected to pin 3 will thus be charged by the

same current for the same amount of time as pin 2. When

the base voltage on Q16 goes high, Q18 and Q26 are

turned off while Q27 and Q30 are turned ``ON.'' In these

conditions, Q21 and Q25 provide the currents to charge the

capacitors on pins 2 and 3 respectively. Thus the charge

Now, since V e V CC /2

then

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

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

TL/H/7451±5

Note: Pin numbers refer to 14-pin package.

FIGURE 2. Equivalent Schematic Diagram

tor, R3 (Figure 2) which is located in a floating isolation

pocket, and secondly by clamp diode D1. Since the voltage

swing on the base of Q1 is thus restricted, the only restric-

tion on the allowable voltage on pin 1 is the breakdown

voltage of the 10 k X resistor. This allows input swings to

g 28V. In 14-pin versions the link to D1 is opened in order to

allow the base of Q1 to be biased at some higher voltage.

Q5 clamps the negative swing on the base of Q1 to about

300 mV. This prevents substrate injection in the region of

Q1 which might otherwise cause false switching or errone-

ous discharge of one of the timing capacitors.

The differential input options (LM2907-14, LM2917-14), give

the user the option of setting his own input switching level

and still having the hysteresis around that level for excellent

noise rejection in any application.

HOW TO USE IT

Basic f to V Converter

The operation of the LM2907, LM2917 series is best under-

stood by observing the basic converter shown inFigure3. In

this configuration, a frequency signal is applied to the input

of the charge pump at pin 1. The voltage appearing at pin 2

will swing between two values which are approximately 1/4

(V CC ) b V BE and 3/4 (V CC ) b V BE . The voltage at pin 3 will

have a value equal to V CC # f IN # C1 # R1 # K, where K is

the gain constant (normally 1.0).

The emitter output (pin 4) is connected to the inverting input

of the op amp so that pin 4 will follow pin 3 and provide a

low impedance output voltage proportional to input frequen-

cy. The linearity of this voltage is typically better than 0.3%

of full scale.

Choosing R1, C1 and C2

There are some limitations on the choice of R1, C1 and C2

(Figure3)which should be considered for optimum perform-

ance. C1 also provides internal compensation for the

charge pump and should be kept larger than 100 pF. Small-

er values can cause an error current on R1, especially at

low temperatures. Three considerations must be met when

choosing R1.

First, the output current at pin 3 is internally fixed and there-

fore V3 max, divided by R1, must be less than or equal to

this value.

Second, if R1 is too large, it can become a significant frac-

tion of the output impedance at pin 3 which degrades linear-

ity. Finally, ripple voltage must be considered, and the size

of C2 is affected by R1. An expression that describes the

ripple content on pin 3 for a single R1, C2 combination is:

V RIPPLE e

V CC

# C1

1 b

V CC # f IN # C1

I 2

p-p

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 frequency increases as the size of C2 increases, so a

compromise between ripple, response time, and linearity

must be cosen carefully. R1 should be selected according

to the following relationship:

C is selected according to:

V3 Full Scale

R1 # V CC # f FULL SCALE

Next decide on the maximum ripple which can be accepted

and plug into the following equation to determine C2:

C2 e V CC

C1 e

The kind of capacitor used for timing capacitor C1 will deter-

mine the accuracy of the unit over the temperature range.

Figure15 illustrates the tachometer output as a function of

temperature for the two devices. Note that the LM2907 op-

erating from a fixed external supply has a negative tempera-

ture coefficient which enables the device to be used with

capacitors which have a positive temperature coefficient

and thus obtain overall stabililty. In the case of the LM2917

the internal zener supply voltage has a positive coefficient

which causes the overall tachometer output to have a very

low temperature coefficient and requires that the capacitor

temperature coefficient be balanced by the temperature co-

efficient of R1.

Using Zener Regulated Options (LM2917)

For those applications where an output voltage or current

must be obtained independently of the supply voltage varia-

tions, the LM2917 is offered. The reference typically has an

11 X source resistance. In choosing a dropping resistor from

the unregulated supply to the device note that the tachome-

ter and op amp circuitry alone require about 3 mA at the

voltage level provided by the zener. At low supply voltages,

V RIPPLE

1 b V 3

R 1 I 2

V3 max

I 3MIN

where V3 max is the full scale output voltage required

1 3MIN is determined from the data sheet (150 m A)

TL/H/7451±6

FIGURE 3. Basic f to V Converter

. . . R1 t

there must be some current flowing in the resistor above the

3 mA circuit current to operate the regulator. As an exam-

ple, if the raw supply varies from 9V to 16V, a resistance of

470 X will minimize these zener voltage variations to 160

mV. If the resistor goes under 400 X or over 600 X the zener

variation quickly rises above 200 mV for the same input vari-

ation. Take care also that the power dissipation of the IC is

not exceeded at higher supply voltages. Figure 4 shows

suitable dropping resistor values.

Input Interface Circuits

The ground referenced input capability of the LM2907-8 al-

lows direct coupling to transformer inputs, or variable reluc-

tance pickups. Figure 5(a) illustrates this connection. In

many cases, the frequency signal must be obtained from

another circuit whose output may not go below ground. This

may be remedied by using ac coupling to the input of the

LM2907 as illustrated in Figure5(b). This approach is very

suitable for use with phototransistors for optical pickups.

Noisy signal sources may be coupled as shown in Figure

5(c). The signal is bandpass filtered. This can be used, for

example, for tachometers operating from breakerpoints on a

conventional Kettering ignition system. Remember that the

minimum input signal required by the LM2907 is only 30

mVp-p, but this signal must be able to swing at least 15 mV

on either side of the inverting input. The maximum signal

which can be applied to the LM2907 input, is g 28V. The

input bias current is a typically 100 nA. A path to ground

must be provided for this current through the source or by

other means as illustrated. With 14-pin package versions of

LM2907, LM2917, it is possible to bias the inverting input to

the tachometer as illustrated inFigure5(d).This enables the

circuit to operate with input signals that do not go to ground,

but are referenced at higher voltages. Alternatively, this

method increases the noise immunity where large signal

TL/H/7451±7

FIGURE 4. Zener Regular Bias Resistor Range

TL/H/7451±8

(a) Ground Referenced Inputs

TL/H/7451±9

(b) AC Coupled Input

TL/H/7451±10

Reduces Noise

TL/H/7451±11

TL/H/7451±12

(e) High Common-Mode Rejection Input Circuit

FIGURE 5. Tachometer Input Configurations

(d) Above Ground Sensing

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