AND8067 Datasheet, PDF (4/8 Pages) ON Semiconductor – NL27WZ04 Dual Gate Inverter Oscillator Increases the Brightness of LEDs While Reducing Power Consumption

AND8067/D
V₄
U_1A
V₁
C₂
0.01 µF
V₂
R₂
12 kΩ
Figure 8. “ON” Time Oscillator Subcircuit
C₁
0.1 µF
V₂
U_1B
V₃
V₄
R₁
12 kΩ
Figure 9. “OFF” Time Oscillator Subcircuit
The equations are developed to predict the time it takes the
RC circuits to discharge to the threshold switching voltage
of the inverter. The threshold voltage of the inverters will be
assumed to be one–half the supply voltage, which is equal
to the average of the High–Level–Input Voltage (V_IH) and
the Low–Level Input Voltage (V_IL). The NL27WZ04
specifies V_IH as 0.7 × V_CC (minimum) and V_IL as 0.3 × V_CC
(maximum). In addition, the initial voltage or the output
“High” voltage (V_OH) of the inverter is assumed to be equal
to V_CC. The actual V_OH value is a function of the output
current and decreases as the output current increases.
The general equation for a RC circuit discharging to a
logic switching threshold voltage (V_th) with an initial
voltage (V_i) is as follows.
–t
Vth + Vi e ^{R @ C}
These assumptions result in the equation listed below that
can be solved for time (t).
Assume
Vth + 0.5 V and Vi + VOH ^ VCC
Then
ǒ Ǔ ǒ Ǔ t + * RC ln
Vth
VCC
+ * RC ln
0.5 VCC
VCC
^ 0.693 RC
LED “ON” Time
The LED’s “ON” time is controlled by the discharge time
t₁ at inverter U_1A, as shown from the equation listed below.
t1 ^ 0.693 R2C2
Substituting values into the equation yields:
t1 ^ 0.693 (12000 W)(0.01 mF) ^ 83.2 ms
LED “OFF” Time
The LED’s “OFF” time is controlled by the discharge time
t₂ at inverter U_1B, as shown from the equation listed below.
t2 ^ 0.693 R1C1
Substituting the values into the equation yields:
t2 ^ 0.693 (12000 W)(0.1 mF) ^ 832 ms
LED Oscillation Frequency
The time period (T) of the oscillator is equal to the sum of
the charge times in the first and second RC stages. Note the
propagation delay of the inverters can be ignored at the LED
circuit’s oscillation frequency of 1 kHz.
T + t1 ) t2
T + 83.2 ms ) 832 ms + 915 ms
f
+
1
T
+
1
915
ms
+
1.09
kHz
LED Duty Cycle
The duty cycle (DS) for the oscillator at V₁ is given by the
equation:
ǒ Ǔ DSV1 +
t1
t2
100%
The duty cycle of the oscillator is proportional to the ratio
of the two time constants that are set by capacitors C₁ and C₂.
The LED oscillator has a duty cycle of ten percent as shown
below.
ǒ Ǔ DSV1 +
83.2 ms
832 ms
100% + 10%
Experimental Results
The operating characteristics of the pulsed LED oscillator
circuit were compared to the DC circuit shown in Figure 10.
The DC circuit’s current limiting resistor R₅ was selected so
the current through the LED was equal to the average (RMS)
current of the oscillator circuit’s LED. A high efficiency
green GaP/GaP LED from Chicago Miniature Lamp (part
number CMD64531) was used to evaluate the circuits. The
resistor and capacitor values are listed below.
Component Values
LED Oscillator Circuit (Figure 1):
R₁ = R₂ = 12 kΩ
R₃ = 39 Ω
R₄ = 1 MΩ
C1 = 0.1 µF
C2 = 0.01 µF
DC LED Circuit (Figure 10):
R₅ = 680 Ω
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