AND8067 Datasheet, PDF (5/8 Pages) ON Semiconductor – NL27WZ04 Dual Gate Inverter Oscillator Increases the Brightness of LEDs While Reducing Power Consumption

AND8067/D
V_CC
R₅
680 Ω
LED
D₂
Figure 10. DC LED Circuit with Normalized Current
Equal to the Pulsed LED Oscillator of Figure 1
The oscillation and LED current measurements are
summarized in Table 1. Figure 11 shows the PCB that was
created to verify the operation of the LED circuits. The error
in the calculated versus measure oscillation frequency is a
result of the assumptions that V_OH = V_CC and V_TH = 0.5 ×
V_CC. In addition, the tolerance of the resistors and capacitors
contributed to the frequency error. The pulsed LED is
noticeably brighter than the DC LED; however, the LED’s
light output was not quantified with a light spectrometer.
Note that the maximum average current limit of the
NL27WZ04 inverter is specified at 24 ma. The pulsed peak
current exceeds the maximum limit; however, the current
rating of the device is not exceeded because the average
current is below the 24 ma limit. Although a maximum peak
current limit is typically not specified for logic devices, a
safe peak current can be verified by measuring the case
temperature of the IC. If the temperature of the logic device
is significantly higher than the ambient (i.e. 10–20_C), the
reliability of the circuit maybe reduced. The case
temperature of the NL27WZ04 inverter of the LED
oscillator did not significantly increase.
Table 1. Experimental Results of the LED Oscillator
Calculated
Measured
Oscillation
Oscillation
Measured
V_CC
Frequency
Frequency
Duty Cycle
2.5 V
1.09 kHz
1.24 kHz
9.4%
3.3 V
1.09 kHz
1.11 kHz
9.4%
5.0 V
1.09 kHz
1.04 kHz
9.4%
Pulsed LED
Peak Current
9.79 mA
21.3 mA
46.7 mA
Pulsed LED
Average (RMS)
Current
DC LED
Average Current
0.92 mA
0.98 mA
2.00 mA
2.06 mA
4.39 mA
4.45 mA
Figure 11. LED Oscillator Evaluation PCB
BIBLIOGRAPHY
1. “Application Note 1005: Operational
Considerations for LED Lamps and Display
Devices,” Agilent Technologies, 1999.
2. “Guidelines for Designs using LEDs: How to
Enhance Display Performance without Increasing
the Drive Current,” Fairchild Semiconductor,
1999.
3. HLMP–Pxxx, HLMP–Qxxx, HLMP–6xxx and
HLMP–70xx Series Subminiature LED Lamps
Datasheet, Agilent Technologies, 2000.
4. Smith, George, “Multiplexing LED Displays:
Appnote 3,” Siemens Semiconductor.
http://onsemi.com
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