AND8067 Datasheet, PDF (2/8 Pages) ON Semiconductor – NL27WZ04 Dual Gate Inverter Oscillator Increases the Brightness of LEDs While Reducing Power Consumption

AND8067/D
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.
Figure 2. LED Efficiency – Pulsed vs. DC Operation
LED Drive Techniques
DC Method
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 (I_f) through the diode. The
current through the LED for a current sinking configuration
is calculated using V_CC, V_f, R, and the voltage drop across
the driver (V_Switch) as shown below.
If
+
VCC
*
Vf *
R
VSwitch
V_CC
R
I_f
LED
ON/OFF
Figure 3. Conventional Open Collector DC LED Circuit
AC Method
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.
ǒ Ǔ If +
VOH * Vf
R
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.
ǒ Ǔ If +
VCC * Vf * VSwitch
R
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.
U_1A
R₂
U_1B
R₁
C₁
fOscillation
^
1
2.3R1C1
(R2 + 10R1)
Figure 4. Conventional Inverter Oscillator
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. V_cc) or a “low”
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