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LM1815

LM1815 Adaptive Variable Reluctance Sensor Amplifier

Literature Number: SNOSBU8E

LM1815

Adaptive Variable Reluctance Sensor Amplifier

General Description

The LM1815 is an adaptive sense amplifier and default

gating circuit for motor control applications. The sense am-

plifier provides a one-shot pulse output whose leading edge

coincides with the negative-going zero crossing of a ground

referenced input signal such as from a variable reluctance

magnetic pick-up coil.

In normal operation, this timing reference signal is pro-

cessed (delayed) externally and returned to the LM1815. A

Logic input is then able to select either the timing reference

or the processed signal for transmission to the output driver

stage.

The adaptive sense amplifier operates with a positive-going

threshold which is derived by peak detecting the incoming

signal and dividing this down. Thus the input hysteresis

varies with input signal amplitude. This enables the circuit to

sense in situations where the high speed noise is greater

than the low speed signal amplitude. Minimum input signal is

150mV

P-P

Features

nAdaptive hysteresis

nSingle supply operation

nGround referenced input

nTrue zero crossing timing reference

nOperates from 2V to 12V supply voltage

nHandles inputs from 100 mV

P-P

to over 120V

P-P

with

external resistor

nCMOS compatible logic

Applications

nPosition sensing with notched wheels

nZero crossing switch

nMotor speed control

nTachometer

nEngine testing

Connection Diagram

00789301

Top View

Order Number LM1815M or LM1815N

See NS Package Number M14A or N14A

March 2005

LM1815 Adaptive Variable Reluctance Sensor Amplifier

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 12V

Power Dissipation (Note 2) 1250 mW

Operating Temperature Range −40˚C ≤T

≤+125˚C

Storage Temperature Range −65˚C ≤T

≤+150˚C

Junction Temperature +150˚C

Input Current ±30 mA

Lead Temperature

(Soldering, 10 sec.) 260˚C

Electrical Characteristics (T

= 25˚C, V

= 10V, unless otherwise specified, see Figure 1)

Parameter Conditions Min Typ Max Units

Operating Supply Voltage 2.5 10 12 V

Supply Current Pin 3 = -0.1V, Pin 9 = 2V, Pin 11 = 0.8V 3.6 6 mA

Reference Pulse Width f

= 1Hz to 2kHz, R = 150kΩ, C = 0.001µF 70 100 130 µs

Logic Input Bias Current V

= 2V, (Pin 9 and Pin 11) 5 µA

Signal Input Bias Current V

= 0V dc, (Pin 3) -200 nA

Logic Threshold (Pin 9 and Pin 11) 0.8 1.1 2.0 V

OUT

High R

=1kΩ, (Pin 10) 7.5 8.6 V

OUT

Low I

SINK

= 0.1mA, (Pin 10) 0.3 0.4 V

Output Leakage Pin 12 V

= 11V 0.01 10 µA

Saturation Voltage P12 I

= 2mA 0.2 0.4 V

Input Zero Crossing Threshold All Modes, V

SIGNAL

= 1V pk-pk -25 0 25 mV (Note 4)

Minimum Input Arming

Threshold

Mode 1, Pin 5 = Open 30 45 60 mV (Note 4)

Mode 2, Pin5=V

200 300 450 mV (Note 4)

Mode 3, Pin 5 = Gnd -25 0 25 mV (Note 4)

Adaptive Input Arming

Threshold

Mode 1, Pin 5 = Open

SIGNAL

≥230mV pk-pk (Note 3) 40 80 90 % (Note 4)

Mode 2, Pin5=V

SIGNAL

≥1.0V pk-pk (Note 3) 80 % (Note 4)

Mode 3, Pin5=Gnd

SIGNAL

≥150mV pk-pk (Note 3) 80 % (Note 4)

Note 1: “Absolute Maximum Ratings” are those values beyond which the safety of the device cannot be guaranteed. They are not meant to imply that the devices

should be operated at these limits. The table of “Electrical Characteristics” specifies conditions of device operation.

Note 2: For operation at elevated temperatures, the device must be derated based on a 150˚C maximum junction temperature and a thermal resistance of 80˚C/W

(DIP), 120˚C/W (SO-14) junction to ambient.

Note 3: Tested per Figure 1, VSIGNAL is a Sine Wave; FSIGNAL is 1000Hz.

Note 4: The Min/Typ Max limits are relative to the positive voltage peak seen at VIN Pin 3.

LM1815

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Typical Performance Characteristics

Mode 1 Minimum Arming Threshold

vs Temperature

Mode 2 Minimum Arming Threshold

vs Temperature

00789305 00789306

Mode 3 Minimum Arming Threshold

vs Temperature

Mode 1 Minimum Arming Threshold

vs V

00789307 00789308

Mode 2 Minimum Arming Threshold

vs V

Pin3V

vs V

SIGNAL

00789309 00789310

LM1815

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Typical Performance Characteristics (Continued)

Pin3V

vs V

SIGNAL

= 10kΩ

Pin3V

vs V

SIGNAL

= 20kΩ

00789311 00789312

Pin3V

vs V

SIGNAL

= 50kΩ

Pin 3 Bias Current

vs Temperature

00789313 00789314

Peak Detector Charge Current

vs Temperature

Peak Detector Charge Current

vs V

00789315 00789316

LM1815

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Typical Performance Characteristics (Continued)

Peak Detector Voltage

vs Pin 3 V

, Mode 1

Peak Detector Voltage

vs Pin 3 V

, Mode 2

00789317 00789318

Peak Detector Voltage

vs Pin 3 V

, Mode 3

00789319

LM1815

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Truth Table

Signal

Input

Pin 3

Timing

Pin 14

Input

Select

Pin 11

Timing

Input

Pin 9

Gated

Output

Pin 10

±Pulses RC L X Pulses = RC

XXHH H

XXHL L

±Pulses L L L Zero

Crossing

00789302

FIGURE 1. LM1815 Adaptive Sense Amplifier

LM1815

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Schematic Diagram

00789304

LM1815

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Application Hints

INPUT VOLTAGE CLAMP

The signal input voltage at pin 3 is internally clamped. Cur-

rent limit for the Input pin is provided by an external resistor

which should be selected to allow a peak current of ±3mAin

normal operation. Positive inputs are clamped by a 1kΩ

resistor and series diode (see R4 and Q12 in the internal

schematic diagram), while an active clamp limits pin 3 to

typically 350mV below Ground for negative inputs (see R2,

R3, Q10, and Q11 in the internal schematic diagram). Thus

for input signal transitions that are more than 350mV below

Ground, the input pin current (up to 3mA) will be pulled from

the V+ supply. If the V+ pin is not adequately bypassed the

resulting voltage ripple at the V+ pin will disrupt normal

device operation. Likewise, for input signal transitions that

are more than 500mV above Ground, the input pin current

will be dumped to Ground through device pin 2. Slight shifts

in the Ground potential at device pin 2, due to poor ground-

ing techniques relative to the input signal ground, can cause

unreliable operation. As always, adequate device grounding,

and V+ bypassing, needs to be considered across the entire

input voltage and frequency range for the intended applica-

tion.

INPUT CURRENT LIMITING

As stated earlier, current limiting for the Input pin is provided

by a user supplied external resistor. For purposes of select-

ing the appropriate resistor value the Input pin should be

considered to be a zero ohm connection to ground. For

applications where the input voltage signal is not symmetri-

cal with relationship to Ground the worst case voltage peak

should be used.

Minimum Rext = [(Vin peak)/3mA]

In the application example shown in figure 1 (Rext = 18kΩ)

the recommended maximum input signal voltage is ±54V

(i.e. 108Vp-p).

OPERATION OF ZERO CROSSING DETECTOR

The LM1815 is designed to operate as a zero crossing

detector, triggering an internal one shot on the negative-

going edge of the input signal. Unlike other zero crossing

detectors, the LM1815 cannot be triggered until the input

signal has crossed an "arming" threshold on the positive-

going portion of the waveform. The arming circuit is reset

when the chip is triggered, and subsequent zero crossings

are ignored until the arming threshold is exceeded again.

This threshold varies depending on the connection at pin 5.

Three different modes of operation are possible:

MODE 1, PIN 5 OPEN

The adaptive mode is selected by leaving device pin 5 open

circuit. For input signals of less than ±135mV (i.e. 270

mVp-p) and greater than typically ±75mV (i.e. 150mVp-p),

the input arming threshold is typically at 45mV. Under these

conditions the input signal must first cross the 45mV thresh-

old in the positive direction to arm the zero crossing detector,

and then cross zero in the negative direction to trigger it.

If the signal is less than 30mV peak (minimum rating in

Electrical Characteristics), the one shot is guaranteed to not

trigger.

Input signals of greater than ±230mV (i.e. 460 mVp-p) will

cause the arming threshold to track at 80% of the peak input

voltage. A peak detector capacitor at device pin 7 stores a

value relative to the positive input peaks to establish the

arming threshold. Input signals must exceed this threshold in

the positive direction to arm the zero crossing detector,

which can then be triggered by a negative-going zero cross-

ing.

The peak detector tracks rapidly as the input signal ampli-

tude increases, and decays by virtue of the resistor con-

nected externally at pin 7 track decreases in the input signal.

If the input signal amplitude falls faster than the voltage

stored on the peak detector capacitor there may be a loss of

output signal until the capacitor voltage has decayed to an

appropriate level.

Note that since the input voltage is clamped, the waveform

observed at pin 3 is not identical to the waveform observed

at the variable reluctance sensor. Similarly, the voltage

stored at pin 7 is not identical to the peak voltage appearing

at pin 3.

00789303

FIGURE 2. LM1815 Oscillograms

LM1815

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Application Hints (Continued)

MODE 2, PIN 5 CONNECTED TO V+

The input arming threshold is fixed at 200mV minimum when

device pin 5 is connected to the positive supply. The chip has

no output for signals of less than ±200 mV (i.e. 400mVp-p)

and triggers on the next negative-going zero crossing when

the arming threshold is has been exceeded.

MODE 3, PIN 5 GROUNDED

With pin 5 grounded, the input arming threshold is set to 0V,

±25mV maximum. Positive-going zero crossings arm the

chip, and the next negative-going zero crossing triggers it.

This is the very basic form of zero-crossing detection.

ONE SHOT TIMING

The one shot timing is set by a resistor and capacitor con-

nected to pin 14. The recommended maximum resistor value

is 150kohms. The capacitor value can be changed as

needed, as long as the capacitor type does not present any

signfigant leakage that would adversely affect the RC time

constant.

The output pulse width is:

pulse width = 0.673xRxC (1)

For a given One Shot pulse width, the recommended maxi-

mum input signal frequency is:

Fin(max) = 1/(1.346xRxC) (2)

In the application example shown in figure 1 (R=150kohms,

C=0.001µF) the recommended maximum input frequency

will typically be 5kHz. Operating with input frequencies

above the recommended Fin (max) value may result in

unreliable performance of the One Shot circuitry. For those

applications where the One Shot circuit is not required,

device pin 14 can be tied directly to Ground.

LOGIC INPUTS

In some systems it is necessary to externally generate

pulses, such as during stall conditions when the variable

reluctance sensor has no output. External pulse inputs at pin

9 are gated through to pin 10 when Input Select (pin 11) is

pulled high. Pin 12 is a direct output for the one shot and is

unaffected by the status of pin 11.

Input/output pins 9, 11, 10, and 12 are all CMOS logic

compatible. In addition, pins 9, 11, and 12 are TTL compat-

ible. Pin 10 is not guaranteed to drive a TTL load.

Pins 1, 4, 6 and 13 have no internal connections and can be

grounded.

LM1815

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Physical Dimensions inches (millimeters)

unless otherwise noted

14-Lead Small Outline Circuit (M)

Order Number LM1815M

NS Package Number M14A

LM1815

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Physical Dimensions inches (millimeters) unless otherwise noted (Continued)

Molded Dual-In-Line Package (N)

Order Number LM1815N

NS Package Number N14A

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.

For the most current product information visit us at www.national.com.

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2. A critical component is any component of a life support

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LM1815 Adaptive Variable Reluctance Sensor Amplifier

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