AND8067/D

http://onsemi.com

5

Figure 10. DC LED Circuit with Normalized Current

Equal to the Pulsed LED Oscillator of Figure 1

680

Ω

LED

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

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

Oscillation

Frequency

Measured

Oscillation

Frequency

Measured

Duty Cycle

Pulsed LED

Peak Current

Pulsed LED

Average (RMS)

Current

DC LED

Average Current

2.5 V

1.09 kHz

1.24 kHz

9.4%

9.79 mA

0.92 mA

0.98 mA

3.3 V

1.09 kHz

1.11 kHz

9.4%

21.3 mA

2.00 mA

2.06 mA

5.0 V

1.09 kHz

1.04 kHz

9.4%

46.7 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.