AND8067/D

http://onsemi.com

2

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

current through the LED for a current sinking configuration

If +

VCC * Vf * VSwitch

R

Figure 3. Conventional Open Collector DC LED Circuit

R

LED

ON/OFF

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.

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.

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

Figure 4. Conventional Inverter Oscillator

fOscillation ^

1

2.3R1C1

(R2 + 10R1)

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

provide a DC path through the capacitor. The parallel

The NL27WZ04 dual inverter is a standard buffered