AND8067 Datasheet, PDF (1/8 Pages) ON Semiconductor – NL27WZ04 Dual Gate Inverter Oscillator Increases the Brightness of LEDs While Reducing Power Consumption

AND8067/D
NL27WZ04 Dual Gate
Inverter Oscillator
Increases the Brightness
of LEDs While Reducing
Power Consumption
Prepared by: Jim Lepkowski
Senior Applications Engineer
Mike Hoogstra
JTL Design
Christopher Young
Arizona State University
http://onsemi.com
APPLICATION NOTE
INTRODUCTION
ON Semiconductor’s new family of two–gate logic
devices offer space saving solutions to the logic designer.
The LV–CMOS two gate logic family consists of inverters,
buffers and logic gates in both the SC–88 and TSOP–6
package. These versatile devices have several features
including a wide 2.3 V to 5.5 V operating voltage range, low
quiescent power supply current and an output capable of
sinking or sourcing 24 mA.
NL27WZ04
V_{4 U1A}
V₁
C₂
0.01 µF
R₁
12 kΩ
R₃
39 Ω
R₄
1 MΩ
LED
D₁
C₁
0.1 µF
NL27WZ04
V₂
U_1B
V₃
R₂
12 kΩ
Figure 1. LED Oscillator Circuit
The versatile features of the two gate devices will be
demonstrated by using the NL27WZ04 dual inverter IC to
create the Light Emitting Diode (LED) oscillator circuit
shown in Figure 1. An oscillator can be used to increase the
brightness of an LED without increasing the system’s power
requirements. The brightness of an LED is directly
proportional to the current through the LED, which creates
a challenge for low voltage and battery powered
applications. Thus, a high peak current is required to obtain
a bright LED, while a low average current is needed to
minimize the power consumption. The LED oscillator
circuit achieves these requirements by providing a low duty
cycle waveform with a short duration “ON” time and a long
“OFF” time.
Light Emitting Diodes
LEDs are manufactured out of a variety of semiconductor
materials and are comprised of a “P” and “N” type junction,
which establishes a voltage potential across the junction.
The LED provides a light output when the diode is forward
biased, causing current to flow through the device. The
forward voltage (V_f) of the diode will be different for the
various materials and colors and ranges from approximately
1.5 V for red to 3.3 V for blue LEDs.
A pulsating LED drive circuit can enhance the light output
of an LED by using a peak current of a much higher level
than sustainable under direct drive conditions [1][2]. A high
peak current pulse of short duration with a “OFF” period
between pulses allows time for the LED’s junction to cool
down. High drive currents can result in a degradation of the
light output and the life expectancy (time to half light output)
of an LED. However, the reduction in the life of a pulsed
LED is minimal if the peak current is below the maximum
current limit specified for the device.
Why Are Pulsed LEDs Brighter Than DC LEDs?
There are two main reasons why LEDs are brighter when
pulsed. First, the human eye functions as both a peak
detector and an integrator; therefore, the eye perceives a
pulsed LED’s brightness somewhere between the peak and
the average brightness [4]. Thus, an LED driven by a high
intensity low duty cycle light looks brighter in a pulsed
circuit compared to a DC drive circuit that is equal to the
average of the pulsed signal.
The second factor controlling the improved brightness is
shown in the relative efficiency versus peak current curves
of an LED. Figure 2 shows the efficiency curves for the
© Semiconductor Components Industries, LLC, 2001
1
October, 2001 – Rev. 0
Publication Order Number:
AND8067/D