INPUT

1

F

Hz

2.8 RtCt

#

OSC

1

F

Hz

1.4 RtCt

#

VCO

Phase

Detector

Amplitude

Detector

÷2

÷2

1

2

3

4

8

7

6

5

LMC567

SNOSBY1C – JUNE 1999 – REVISED DECEMBER 2015

www.ti.com

9 Detailed Description

9.1 Overview

The LMC567C is a low-power, general-purpose tone decoder with similar functionality to the industry standard

LM567. The device requires external components set up the internal oscillator to run at twice the input frequency

and determine the required filter constants. Internal VCO and Phase detector form a Phase-locked loop which

locks to an input signal frequency that is established by external timing components. When PLL is locked, a

switch to ground is activated in the output of the device.

9.2 Functional Block Diagram

9.3 Feature Description

9.3.1 Oscillator

The voltage-controlled oscillator (VCO) on the LMC567 must be set up to run at twice the frequency of the input

signal tone to be decoded. The center frequency of the VCO is set by timing resistor Rt and timing capacitor Ct

connected to pins 5 and 6 of the IC. The center frequency as a function of Rt and Ct is given by Equation 4:

(4)

(5)

actual frequency to be lower than predicted.

The choice of Rt and Ct is a tradeoff between supply current and practical capacitor values. An additional supply

(6)

Thus the supply current can be minimized by keeping Rt as large as possible (see Figure 1). However, the

desired frequency dictates an RtCt product such that increasing Rt requires a smaller Ct. Below

Ct = 100 pF, circuit board stray capacitances begin to play a role in determining the oscillation frequency which

ultimately limits the minimum Ct.

To allow for IC and component value tolerances, the oscillator timing components requires a trim. This is

generally accomplished by using a variable resistor as part of Rt, although Ct could also be padded. The amount

of initial frequency variation due to the LMC567 itself is given in the Electrical Characteristics; the total trim range

must also accommodate the tolerances of Rt and Ct.

8

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