AND8067 datasheet(4/8 Pages) ONSEMI

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AND8067/D

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Figure 8. “ON” Time Oscillator Subcircuit

U1A

12 k

Ω

0.01

µF

Figure 9. “OFF” Time Oscillator Subcircuit

U1B

12 k

Ω

0.1

µF

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

–t

@ C

These assumptions result in the equation listed below that

can be solved for time (t).

Assume

Vth + 0.5

and

Vi + VOH ^ VCC

Then

+* RC ln

Vth

VCC

+* RC ln

0.5

VCC

^ 0.693

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

Substituting values into the equation yields:

t1 ^ 0.693

(12000

W)(0.01 mF) ^ 83.2 ms

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.

t2 ^ 0.693

R1C1

Substituting the values into the equation yields:

t2 ^ 0.693

(12000

W)(0.1 mF) ^ 832 ms

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.

+ t1 ) t2

+ 83.2 ms ) 832 ms + 915 ms

+ 1

915

ms +

1.09 kHz

LED Duty Cycle

The duty cycle (DS) for the oscillator at V1 is given by the

equation:

DSV1 +

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 +

83.2

832

100%

+ 10%

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.

Component Values

LED Oscillator Circuit (Figure 1):

R1 = R2 = 12 kΩ

R3 = 39 Ω

R4 = 1 MΩ

C1 = 0.1

µF

C2 = 0.01

µF

DC LED Circuit (Figure 10):

R5 = 680 Ω

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