AND8067/D NL27WZ04 Dual Gate Inverter Oscillator Increases the Brightness of LEDs While Reducing Power Consumption APPLICATION NOTE Prepared by: Jim Lepkowski Senior Applications Engineer Mike Hoogstra JTL Design Christopher Young Arizona State University 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. C2 0.01 µF NL27WZ04 V4 U1A V1 R1 12 kΩ R3 39 Ω R4 1 MΩ http://onsemi.com V2 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 (Vf) 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. NL27WZ04 V3 U1B R2 12 kΩ LED D1 C1 0.1 µF Figure 1. LED Oscillator Circuit 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 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 Semiconductor Components Industries, LLC, 2001 October, 2001 – Rev. 0 1 Publication Order Number: AND8067/D AND8067/D AC Method Agilent Technologies HLMP subminiature LED lamps [3]. For example, the pulsed emerald green LED will have a light output approximately 30% brighter then the equivalent DC drive circuit at a peak pulsed current of 30 mA. Note that the pulsed circuit does not always produce a brighter LED. The pulsed emerald green LED has a brighter light output at peak currents greater than 10 mA; however, the DC circuit produces a brighter LED for peak currents less than 10 mA. The second method to drive LEDs uses a pulsating square wave voltage. The suggested frequency and duty cycle varies for different LEDs; however, the typical frequency used is 1 kHz with a 10 to 30% duty cycle. Pulsing LEDs is the standard method used with multiplexed displays when a single driver circuit is interfaced to multiple LEDs. The current through a pulsed current sourcing driver such as the oscillator circuit shown in Figure 1 is calculated as shown below. If VOHR Vf Duty Cycle (current sourcing driver) The equation for a current sinking AC driver is similar to the DC method, except that the duty cycle is used to reduce the current consumption. If VCC VfR VSwitch Duty Cycle (current sinking driver) Dual Gate Inverter Oscillator Circuit The LED oscillator circuit, shown in Figure 1 is derived from the conventional two–inverter oscillator shown in Figure 4. The conventional oscillator is often denoted as an astable multivibrator and has a duty cycle of approximately 50%. In contrast, the LED oscillator circuit has two RC time constants so that both the duty cycle and frequency can be adjusted. R2 and C2 control the “ON” time of the LED pulse, while R1 and C1 control the “OFF” time. Figure 2. LED Efficiency – Pulsed vs. DC Operation LED Drive Techniques DC Method U1A Single LEDs are often driven using either a high side or low side switch. The conventional LED interface circuit consists of an open collector/drain driver to sink the LED current as shown in Figure 3. The brightness of the LED is proportional to the current (If) through the diode. The current through the LED for a current sinking configuration is calculated using VCC, Vf, R, and the voltage drop across the driver (VSwitch) as shown below. R2 U1B R1 C1 1 fOscillation (R2 10R1) 2.3R1C1 V Vf VSwitch If CC R Figure 4. Conventional Inverter Oscillator VCC The LED oscillator with the NL27WZ04 duel gate inverter and the given RC values is stable and does not have the oscillation start–up problem that often occurs with the conventional two inverter oscillator. In order to ensure oscillation at power–up, R4 was added in parallel with C2 to provide a DC path through the capacitor. The parallel impedance combination of R4 and C2 is effectively equal to the impedance of C2 at the oscillation frequency; therefore, R4 does not effect the oscillation frequency. The NL27WZ04 dual inverter is a standard buffered inverter that produces either a “high” (i.e. Vcc) or a “low” R If LED ON/OFF Figure 3. Conventional Open Collector DC LED Circuit http://onsemi.com 2 AND8067/D 3.5 (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. 3.0 Voltage (V) 2.5 2.0 1.5 1.0 0.5 0 –0.5 0 500 1000 Time (µs) 1500 2000 Figure 6. V1, Output Voltage of Inverter U1A (VCC = 3.3 V) VCC 3.5 3.0 R3 2.5 2.0 ON/OFF C2 VCC U1 R1 R4 Voltage (V) LED U2 R2 1.5 1.0 0.5 0 –0.5 –1.0 0 C1 500 1000 Time (µs) 1500 2000 Figure 7. V2, Input Voltage of Inverter U1B (VCC = 3.3 V) Figure 5. LED Oscillator Circuit with ON/OFF Control Figure 6 shows V1, the LED drive voltage of the output of inverter U1A. The input voltage V2 to inverter U1B is shown in Figure 7. Note that the voltage at V2 may ring above VCC 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. 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 (CIN) for the CMOS device is specified at only 2.5 pF. http://onsemi.com 3 AND8067/D C2 0.01 µF V4 V1 U1A LED “OFF” Time The LED’s “OFF” time is controlled by the discharge time t2 at inverter U1B, as shown from the equation listed below. V2 R2 12 kΩ t2 0.693 R1C1 Substituting the values into the equation yields: t2 0.693 (12000 )(0.1 F) 832 s Figure 8. “ON” Time Oscillator Subcircuit 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. C1 0.1 µF V2 U1B V3 V4 R1 12 kΩ T t1 t2 T 83.2 s 832 s 915 s f 1 1 1.09 kHz 915 s T Figure 9. “OFF” Time Oscillator Subcircuit LED Duty Cycle The duty cycle (DS) for the oscillator at V1 is given by the equation: 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 (VIH) and the Low–Level Input Voltage (VIL). The NL27WZ04 specifies VIH as 0.7 × VCC (minimum) and VIL as 0.3 × VCC (maximum). In addition, the initial voltage or the output “High” voltage (VOH) of the inverter is assumed to be equal to VCC. The actual VOH 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 (Vth) with an initial voltage (Vi) is as follows. Vth Vi DSV1 tt12 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. DSV1 s 83.2 100% 10% 832 s 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. –t eR 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 th RC ln0.5 VCC VVCC VCC Component Values t RC ln LED Oscillator Circuit (Figure 1): 0.693 RC R1 = R2 = 12 kΩ R3 = 39 Ω R4 = 1 MΩ C1 = 0.1 µF C2 = 0.01 µF LED “ON” Time The LED’s “ON” time is controlled by the discharge time t1 at inverter U1A, as shown from the equation listed below. t1 0.693 R2C2 DC LED Circuit (Figure 10): R5 = 680 Ω Substituting values into the equation yields: t1 0.693 (12000 )(0.01 F) 83.2 s http://onsemi.com 4 AND8067/D result of the assumptions that VOH = VCC and VTH = 0.5 × VCC. 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. 10–20C), the reliability of the circuit maybe reduced. The case temperature of the NL27WZ04 inverter of the LED oscillator did not significantly increase. VCC 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 Table 1. Experimental Results of the LED Oscillator VCC 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. http://onsemi.com 5 AND8067/D Notes http://onsemi.com 6 AND8067/D Notes http://onsemi.com 7 AND8067/D ON Semiconductor and are trademarks of Semiconductor Components Industries, LLC (SCILLC). SCILLC reserves the right to make changes without further notice to any products herein. SCILLC makes no warranty, representation or guarantee regarding the suitability of its products for any particular purpose, nor does SCILLC assume any liability arising out of the application or use of any product or circuit, and specifically disclaims any and all liability, including without limitation special, consequential or incidental damages. “Typical” parameters which may be provided in SCILLC data sheets and/or specifications can and do vary in different applications and actual performance may vary over time. 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