NSC LM2917N-8, LM2917MX-8, LM2917MX, LM2917M-8, LM2917M Datasheet

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 designed to operate a relay, lamp, or other load when the input frequency reaches or exceeds a selected rate. The tachometer 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 e fIN c VCC c R1 c C1

YOnly one RC network provides frequency doubling

YZener regulator on chip allows accurate and stable frequency to voltage or current conversion (LM2917)

Features

Y Ground 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, meters, or LEDs

YFrequency doubling for low ripple

YTachometer has built-in hysteresis with either differential 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	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

Converter Voltage to Frequency LM2907/LM2917

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, 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§C to a85§C
Storage Temperature Range	b65§C to a150§C
Soldering Information
Dual-In-Line Package	260§C
Soldering (10 seconds)
Small Outline Package	215§C
Vapor Phase (60 seconds)
Infrared (15 seconds)	220§C

See AN-450 ``Surface Mounting Methods and Their Effect on Product Reliability'' for other methods of soldering surface 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)	g10	g25	g40	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 g50 mVDC		0.1	1	mA
VOH	Pin 2	VIN e a125 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)	b1.0	0.3	a1.0	%
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 aVTH b (bVTH), 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 tachometer 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 tachometer 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 regulator clamps the supply such that stable frequency to voltage 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

3

Typical Performance Characteristics

	Zener Voltage vs	Normalized Tachometer
Total Supply Current	Temperature	Output vs Temperature

TL/H/7942 ± 5

4

Applications Information

The LM2907 series of tachometer circuits is designed for minimum external part count applications and maximum versatility. In order to fully exploit its features and advantages let's examine its theory of operation. The first stage of operation 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 provide 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 discharged 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 c C1. The average amount of current pumped into or out of the capacitor then is:

DQ e ic(AVG) e C1 c VCC c (2fIN) e VCC c fIN c C1
T	2

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 VO e ic c R1, and the total conversion equation becomes:

VO e VCC c fIN c C1 c R1 c K 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 current on R1, especially at low temperatures. Several considerations 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 considered 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 e	VCC	c	C1	c	#	1 b	VCC c fIN c C1		J	pk-pk
							I2
	2 C2

It appears R1 can be chosen independent of ripple, however 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 frequency is determined by VCC, C1 and I2:

fMAX e I2

C1 c VCC

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 operate the regulator. As an example, if the raw supply varies from 9V to 16V, a resistance of 470X will minimize the zener voltage variation to 160 mV. If the resistance goes under 400X or over 600X the zener variation quickly rises above 200 mV for the same input variation.

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