LMC567 Datasheet, PDF (4/7 Pages) National Semiconductor (TI) – Low Power Tone Decoder

Typical Performance Characteristics
Supply Current vs.
Operating Frequency
Bandwidth vs.
Input Signal Level
Largest Detection
Bandwidth vs. Temp.
Bandwidth as
a Function of C2
DS008670-3
Frequency Drift
with Temperature
DS008670-7
Frequency Drift
with Temperature
DS008670-8
DS008670-9
Applications Information (refer to Block
Diagram)
GENERAL
The LMC567 low power tone decoder can be operated at
supply voltages of 2V to 9V and at input frequencies ranging
from 1 Hz up to 500 kHz.
The LMC567 can be directly substituted in most LM567 ap-
plications with the following provisions:
1. Oscillator timing capacitor Ct must be halved to double
the oscillator frequency relative to the input frequency
(See OSCILLATOR TIMING COMPONENTS).
2. Filter capacitors C1 and C2 must be reduced by a factor
of 8 to maintain the same filter time constants.
3. The output current demanded of pin 8 must be limited to
the specified capability of the LMC567.
OSCILLATOR TIMING COMPONENTS
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:
DS008670-10
DS008670-11
This equation is accurate at low frequencies; however,
above 50 kHz (F_osc = 100 kHz), internal delays cause the
actual frequency to be lower than predicted.
The choice of Rt and Ct will be a tradeoff between supply
current and practical capacitor values. An additional supply
current component is introduced due to Rt being switched to
V_s every half cycle to charge Ct:
I_s due to Rt = V_s/(4Rt)
Thus the supply current can be minimized by keeping Rt as
large as possible (see supply current vs. operating fre-
quency curves). However, the desired frequency will dictate
an RtCt product such that increasing Rt will require a smaller
Ct. Below Ct = 100 pF, circuit board stray capacitances be-
gin to play a role in determining the oscillation frequency
which ultimately limits the minimum Ct.
To allow for I.C. and component value tolerances, the oscil-
lator timing components will require a trim. This is generally
accomplished by using a variable resistor as part of Rt, al-
though Ct could also be padded. The amount of initial fre-
quency variation due to the LMC567 itself is given in the
electrical specifications; the total trim range must also ac-
commodate the tolerances of Rt and Ct.
Since this will cause an input tone of half F_osc to be decoded,
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