NCS2002, NCV2002
http://onsemi.com
13
The non−inverting input threshold levels are set so that
the capacitor voltage oscillates between 1/3 and 2/3 of
V
CC
. This requires the resistors R
1a
, R
1b
and R
2
to be of
equal value. The following formula can be used to ap-
proximate the output frequency.
R
T
470 k
R
2
470 k
R
1b
470 k
R
1a
470 k
C
T
1.0 nF
0.9 V
f
O
= 1.5 kHz
0.67 V
CC
R
1b
470 k
V
CC
D
2
1N4148
f
O
-
+
-
+
0.9 V
f
O
+
1
1.39 R
T
C
T
V
CC
0.33 V
CC
0
Output Voltage
Timing Capacitor
Voltage
V
CC
R
2
470 k
D
1
1N4148
10 k
10 k
1.0 M
cw
R
1a
470 k
C
T
1.0 nF
0.67 V
CC
V
CC
0.33 V
CC
0
Output Voltage
Timing Capacitor
Voltage
0.67 V
CC
V
CC
0.33 V
CC
0
Output Voltage
Timing Capacitor
Voltage
The timing capacitor C
T
will charge through diode D
2
and discharge
through diode D
1
, allowing a variable duty cycle. The pulse width of the
signal can be programmed by adjusting the value of the trimpot. The ca-
pacitor voltage will oscillate between 1/3 and 2/3 of V
CC
, since all the
resistors at the non−inverting input are of equal value.
Clock−wise, Low Duty Cycle
Counter−Clock−wise, High Duty Cycle
Figure 37. 0.9 V Square Wave Oscillator
Figure 38. Variable Duty Cycle Pulse Generator
cww
