LM2907/LM2917 Frequency to Voltage Converter

February 1995

LM2907/LM2917 Frequency to Voltage Converter

Y Frequency doubling for low ripple
General Description Y Tachometer has built-in hysteresis with either differen-
The LM2907, LM2917 series are monolithic frequency to tial input or ground referenced input
voltage converters with a high gain op amp/comparator de- Y Built-in zener on LM2917
signed to operate a relay, lamp, or other load when the input
Y g 0.3% linearity typical
frequency reaches or exceeds a selected rate. The tachom-
eter uses a charge pump technique and offers frequency Y Ground referenced tachometer is fully protected from
doubling for low ripple, full input protection in two versions damage due to swings above VCC and below ground
(LM2907-8, LM2917-8) and its output swings to ground for a
zero frequency input. Applications
Y Over/under speed sensing
Advantages Y Frequency to voltage conversion (tachometer)
Y Output swings to ground for zero frequency input Y Speedometers
Y Easy to use; VOUT e fIN c VCC c R1 c C1 Y Breaker point dwell meters
Y Only one RC network provides frequency doubling Y Hand-held tachometer
Y Zener regulator on chip allows accurate and stable fre- Y Speed governors
quency to voltage or current conversion (LM2917) Y Cruise control
Y Automotive door lock control
Features Y Clutch control
Y Ground referenced tachometer input interfaces directly Y Horn control
with variable reluctance magnetic pickups Y Touch or sound switches
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 TL/H/7942 – 2
Order Number LM2907M-8 or LM2907N-8 Order Number LM2917M-8 or LM2917N-8
See NS Package Number M08A or N08E See NS Package Number M08A or N08E

TL/H/7942 – 3 TL/H/7942 – 4
Order Number LM2907N Order Number LM2917M or LM2917N
See NS Package Number N14A See NS Package Number M14A or N14A

C1995 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, Power Dissipation
please contact the National Semiconductor Sales LM2907-8, LM2917-8 1200 mW
Office/Distributors for availability and specifications. LM2907-14, LM2917-14 1580 mW
Supply Voltage 28V (See Note 1)
Supply Current (Zener Options) 25 mA Operating Temperature Range b 40§ C to a 85§ C

Collector Voltage 28V Storage Temperature Range b 65§ C to a 150§ C

Differential Input Voltage Soldering Information

Tachometer 28V Dual-In-Line Package
Op Amp/Comparator 28V Soldering (10 seconds) 260§ C
Small Outline Package
Input Voltage Range
Vapor Phase (60 seconds) 215§ C
Tachometer LM2907-8, LM2917-8 g 28V
Infrared (15 seconds) 220§ C
LM2907, LM2917 0.0V to a 28V
Op Amp/Comparator 0.0V to a 28V See AN-450 ‘‘Surface Mounting Methods and Their Effect
on Product Reliability’’ for other methods of soldering sur-
face mount devices.

Electrical Characteristics VCC e 12 VDC, TA e 25§ C, see test circuit

Symbol Parameter Conditions Min Typ Max Units
TACHOMETER
Input Thresholds VIN e 250 mVp-p @ 1 kHz (Note 2) g 10 g 25 g 40 mV
Hysteresis VIN e 250 mVp-p @ 1 kHz (Note 2) 30 mV
Offset Voltage VIN e 250 mVp-p @ 1 kHz (Note 2)
LM2907/LM2917 3.5 10 mV
LM2907-8/LM2917-8 5 15 mV
Input Bias Current VIN e g 50 mVDC 0.1 1 mA
VOH Pin 2 VIN e a 125 mVDC (Note 3) 8.3 V
VOL Pin 2 VIN e b125 mVDC (Note 3) 2.3 V
I2, I3 Output Current V2 e V3 e 6.0V (Note 4) 140 180 240 mA
I3 Leakage Current I2 e 0, V3 e 0 0.1 mA
K Gain Constant (Note 3) 0.9 1.0 1.1
Linearity fIN e 1 kHz, 5 kHz, 10 kHz (Note 5) b 1.0 0.3 a 1.0 %
OP/AMP COMPARATOR
VOS VIN e 6.0V 3 10 mV
IBIAS VIN e 6.0V 50 500 nA
Input Common-Mode Voltage 0 VCCb1.5V V
Voltage Gain 200 V/mV
Output Sink Current VC e 1.0 40 50 mA
Output Source Current VE e VCC b2.0 10 mA
Saturation Voltage ISINK e 5 mA 0.1 0.5 V
ISINK e 20 mA 1.0 V
ISINK e 50 mA 1.0 1.5 V

2
 Electrical Characteristics VCC e 12 VDC, TA e 25§ C, see test circuit (Continued)
Symbol Parameter Conditions Min Typ Max Units
ZENER REGULATOR
Regulator Voltage RDROP e 470X 7.56 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 a VTH b ( b VTH), offset voltage is their difference. See test circuit.
Note 3: VOH is equal to */4 c VCC b 1 VBE, VOL is equal to (/4 c VCC b 1 VBE therefore VOH b VOL e VCC/2. The difference, VOH b VOL, 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 c C1 that R1 is such that the maximum anticipated output voltage at pin 3 can be reached with I3 c R1. The
maximum value for R1 is limited by the output resistance of pin 3 which is greater than 10 MX typically.
Note 5: Nonlinearity is defined as the deviation of VOUT ( @ pin 3) for fIN e 5 kHz from a straight line defined by the VOUT @ 1 kHz and VOUT @ 10 kHz.
C1 e 1000 pF, R1 e 68k and C2 e 0.22 mFd.

General Description (Continued)

The op amp/comparator is fully compatible with the ta- The more versatile configurations provide differential ta-
chometer and has a floating transistor as its output. This chometer input and uncommitted op amp inputs. With this
feature allows either a ground or supply referred load of up version the tachometer input may be floated and the op
to 50 mA. The collector may be taken above VCC up to a amp becomes suitable for active filter conditioning of the
maximum VCE of 28V. tachometer output.
The two basic configurations offered include an 8-pin device Both of these configurations are available with an active
with a ground referenced tachometer input and an internal shunt regulator connected across the power leads. The reg-
connection between the tachometer output and the op amp ulator clamps the supply such that stable frequency to volt-
non-inverting input. This version is well suited for single age and frequency to current operations are possible with
speed or frequency switching or fully buffered frequency to any supply voltage and a suitable resistor.
voltage conversion applications.

Test Circuit and Waveform

Tachometer Input Threshold Measurement

TL/H/7942 – 7

TL/H/7942 – 6

3
Typical Performance Characteristics
Zener Voltage vs Normalized Tachometer
Total Supply Current Temperature Output vs Temperature

Normalized Tachometer Tachometer Currents I2 Tachometer Currents I2

Output vs Temperature and I3 vs Supply Voltage and I3 vs Temperature

Tachometer Linearity Tachometer Linearity

vs Temperature vs Temperature Tachometer Linearity vs R1

Tachometer Input Hysteresis Op Amp Output Transistor Op Amp Output Transistor

vs Temperature Characteristics Characteristics

TL/H/7942 – 5

4
Applications Information
The LM2907 series of tachometer circuits is designed for The size of C2 is dependent only on the amount of ripple
minimum external part count applications and maximum ver- voltage allowable and the required response time.
satility. In order to fully exploit its features and advantages
CHOOSING R1 AND C1
let’s examine its theory of operation. The first stage of oper-
ation is a differential amplifier driving a positive feedback There are some limitations on the choice of R1 and C1
flip-flop circuit. The input threshold voltage is the amount of which should be considered for optimum performance. The
differential input voltage at which the output of this stage timing capacitor also provides internal compensation for the
changes state. Two options (LM2907-8, LM2917-8) have charge pump and should be kept larger than 500 pF for very
one input internally grounded so that an input signal must accurate operation. Smaller values can cause an error cur-
swing above and below ground and exceed the input rent on R1, especially at low temperatures. Several consid-
thresholds to produce an output. This is offered specifically erations must be met when choosing R1. The output current
for magnetic variable reluctance pickups which typically pro- at pin 3 is internally fixed and therefore VO/R1 must be less
vide a single-ended ac output. This single input is also fully than or equal to this value. If R1 is too large, it can become
protected against voltage swings to g 28V, which are easily a significant fraction of the output impedance at pin 3 which
attained with these types of pickups. degrades linearity. Also output ripple voltage must be con-
sidered and the size of C2 is affected by R1. An expression
The differential input options (LM2907, LM2917) give the
that describes the ripple content on pin 3 for a single R1C2
user the option of setting his own input switching level and
combination is:
still have the hysteresis around that level for excellent noise

#1 J pk-pk
rejection in any application. Of course in order to allow the VCC C1 V c fIN c C1
VRIPPLE e c c b CC
inputs to attain common-mode voltages above ground, input 2 C2 I2
protection is removed and neither input should be taken It appears R1 can be chosen independent of ripple, howev-
outside the limits of the supply voltage being used. It is very er response time, or the time it takes VOUT to stabilize at a
important that an input not go below ground without some new voltage increases as the size of C2 increases, so a
resistance in its lead to limit the current that will then flow in compromise between ripple, response time, and linearity
the epi-substrate diode. must be chosen carefully.
Following the input stage is the charge pump where the As a final consideration, the maximum attainable input fre-
input frequency is converted to a dc voltage. To do this quency is determined by VCC, C1 and I2:
requires one timing capacitor, one output resistor, and an
I2
integrating or filter capacitor. When the input stage changes fMAX e
state (due to a suitable zero crossing or differential voltage C1 c VCC
on the input) the timing capacitor is either charged or dis- USING ZENER REGULATED OPTIONS (LM2917)
charged linearly between two voltages whose difference is
For those applications where an output voltage or current
VCC/2. Then in one half cycle of the input frequency or a
must be obtained independent of supply voltage variations,
time equal to 1/2 fIN the change in charge on the timing
the LM2917 is offered. The most important consideration in
capacitor is equal to VCC/2 c C1. The average amount of
choosing a dropping resistor from the unregulated supply to
current pumped into or out of the capacitor then is:
the device is that the tachometer and op amp circuitry alone
DQ V require about 3 mA at the voltage level provided by the
e ic(AVG) e C1 c CC c (2fIN) e VCC c fIN c C1
T 2 zener. At low supply voltages there must be some current
The output circuit mirrors this current very accurately into flowing in the resistor above the 3 mA circuit current to op-
the load resistor R1, connected to ground, such that if the erate the regulator. As an example, if the raw supply varies
pulses of current are integrated with a filter capacitor, then from 9V to 16V, a resistance of 470X will minimize the ze-
VO e ic c R1, and the total conversion equation becomes: ner voltage variation to 160 mV. If the resistance goes un-
der 400X or over 600X the zener variation quickly rises
VO e VCC c fIN c C1 c R1 c K
above 200 mV for the same input variation.
Where K is the gain constantÐtypically 1.0.

Typical Applications
Minimum Component Tachometer

TL/H/7942 – 8

5
Typical Applications (Continued)
1
‘‘Speed Switch’’ Load is Energized When fIN t
2RC

TL/H/7942 – 9

Zener Regulated Frequency to Voltage Converter

TL/H/7942 – 10

Breaker Point Dwell Meter

TL/H/7942 – 11

6
Typical Applications (Continued)
Voltage Driven Meter Indicating Engine RPM
VO e 6V @ 400 Hz or 6000 ERPM (8 Cylinder Engine)

TL/H/7942 – 12

Current Driven Meter Indicating Engine RPM

IO e 10 mA @ 300 Hz or 6000 ERPM (6 Cylinder Engine)

TL/H/7942 – 13

Capacitance Meter
VOUT e 1V – 10V for CX e 0.01 to 0.1 mFd
(R e 111k)

TL/H/7942 – 14

7
Typical Applications (Continued)
Two-Wire Remote Speed Switch

TL/H/7942 – 15

100 Cycle Delay Switch

VCC c C1
V3 steps up in voltage by the amount
C2
for each complete input cycle (2 zero crossings)
TL/H/7942 – 16
Example:
If C2 e 200 C1 after 100 consecutive input cycles.
V3 e 1/2 VCC

8
Typical Applications (Continued)
Variable Reluctance Magnetic Pickup Buffer Circuits
Precision two-shot output frequency
equals twice input frequency.
VCC C1
Pulse width e .
2 I2
Pulse height e VZENER

TL/H/7942 – 39
TL/H/7942 – 17

Finger Touch or Contact Switch

TL/H/7942 – 19

TL/H/7942 – 18

Flashing LED Indicates Overspeed

Flashing begins when fIN t 100 Hz.

Flash rate increases with input frequency
increase beyond trip point.

TL/H/7942 – 20

9
Typical Applications (Continued)
Frequency to Voltage Converter with 2 Pole Butterworth Filter to Reduce Ripple

0.707
fPOLE e
2qRC
2.57
uRESPONSE e
2qfPOLE

TL/H/7942 – 21

Overspeed Latch

Output latches when TL/H/7942 – 23

R2 1
fIN e
R1 a R2 RC
Reset by removing VCC.

TL/H/7942–22

10
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

11
Typical Applications (Continued)
Changing the Output Voltage for an Input Frequency of Zero

TL/H/7942 – 30

TL/H/7942 – 29

Changing Tachometer Gain Curve or Clamping the Minimum Output Voltage

TL/H/7942 – 32

TL/H/7942–31

12
Anti-Skid Circuit Functions
‘‘Select-Low’’ Circuit

TL/H/7942 – 34
VOUT is proportional to the lower of the
two input wheel speeds.

TL/H/7942 – 33

‘‘Select-High’’ Circuit

TL/H/7942 – 36
VOUT is proportional to the higher of
the two input wheel speeds.

TL/H/7942 – 35

‘‘Select-Average’’ Circuit

TL/H/7942 – 37

13
 Equivalent Schematic Diagram

14
TL/H/7942 – 38
*This connection made on LM2907-8 and LM2917-8 only.
**This connection made on LM2917 and LM2917-8 only.
15
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

16
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

17
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

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