AND8067 Datasheet, PDF (3/8 Pages) ON Semiconductor – NL27WZ04 Dual Gate Inverter Oscillator Increases the Brightness of LEDs While Reducing Power Consumption

AND8067/D
(i.e. Ground) output voltage. In contrast, an unbuffered
inverter such as the NL27WZU04 functions as a voltage
amplifier for a small input voltage and thus can provide a
sine wave output during the oscillation start–up period. It is
recommended that higher frequency oscillator applications,
such as a clock generation circuit, use the unbuffered
inverters.
The LED oscillator circuit shown in Figure 1 can be used
as a “Power ON” indicator. If NAND gates are used instead
of the inverters, ON/OFF control can be implemented for
applications such as status indicator lamps. This oscillator
circuit, shown in Figure 5, could be constructed using ON
Semiconductor’s One–Gate Logic family NAND devices.
The MC74VHC1G00 is the 2–input NAND and the
MC74VHC1G01 is the 2–input NAND with an open drain
output.
V_CC
ON/OFF
U₁
R₁
R₃
LED
C₂
V_CC
U₂
R₄
R₂
C₁
Figure 5. LED Oscillator Circuit with ON/OFF Control
Figure 6 shows V₁, the LED drive voltage of the output of
inverter U_1A. The input voltage V₂ to inverter U_1B is shown
in Figure 7. Note that the voltage at V₂ may ring above V_CC
and below ground for a short duration because of capacitor
C₂. The NL27WZ04 dual inverter has an absolute DC input
voltage rating of –0.5 V to 7 V. The maximum ratings are
specified at a steady state condition and the RMS value of the
high and low sides of the V₂ are within the input voltage
specification. The voltage at V₂ swings below ground;
however, the RMS value of the minimum voltage level is
equal to only approximately –50 mV.
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
–0.5
0
500
1000
1500
2000
Time (µs)
Figure 6. V₁, Output Voltage of Inverter U_1A
(V_CC = 3.3 V)
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
–0.5
–1.0
0
500
1000
1500
2000
Time (µs)
Figure 7. V₂, Input Voltage of Inverter U_1B
(V_CC = 3.3 V)
Oscillation Equations for the Dual Inverter
Oscillator
The oscillation frequency and duty cycle of the oscillator
are obtained by analyzing the oscillator as two separate
circuits. The inverter subcircuits, shown in Figures 8 and 9,
are analyzed to obtain equations for the discharge times of
the RC networks formed at each inverter. In order to simplify
the calculation R₃, R₄ and the LED will not be included in
the analysis. The error that results from neglecting these
components in the equations is small. In addition, the input
impedance of the inverter connected to the RC network can
be neglected because the input capacitance (C_IN) for the
CMOS device is specified at only 2.5 pF.
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