CC

c(AVG)

IN

CC

IN

V

Q

= i

= C1×

× (2 f ) = V

× f × C1

T

2

'

13

LM2907-N, LM2917-N

www.ti.com

SNAS555D – JUNE 2000 – REVISED DECEMBER 2016

Product Folder Links: LM2907-N LM2917-N

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Copyright © 2000–2016, Texas Instruments Incorporated

10 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component

specification, and TI does not warrant its accuracy or completeness. TI’s customers are

responsible for determining suitability of components for their purposes. Customers should

validate and test their design implementation to confirm system functionality.

10.1 Application Information

The LM2907 series of tachometer circuits is designed for minimum external part count applications and

maximum versatility. To fully exploit its features and advantages, first 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 (8-pin

LM2907 and LM2917) 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 ±28 V, 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 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 × C1.

The average amount of current pumped into or out of the capacitor is shown in Equation 4.

(4)

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 = ic × R1, and the total conversion formula

becomes Equation 5.

VO = VCC × fIN × C1 × R1 × K

where

•

K is the gain constant (typically 1)

(5)

The size of C2 is dependent only on the amount of ripple voltage allowable and the required response time.