CS52845
Circuit Description:: continued
Setting the Oscillator
V
OSC
OSC
RESET
Toggle
F/F Output
EA Output
Switch
Current
V
CC
The times T
c
and T
d
can be determined as follows:
t
c
= R
T
C
T
ln
t
d
= R
T
C
T
ln
(
(
V
REF
- V
lower
V
REF
- V
upper
)
)
V
REF
- I
d
R
T
- V
lower
V
REF
- I
d
R
T
- V
upper
I
O
Substituting in typical values for the parameters in the
above formulas:
V
REF
= 5.0V, V
upper
= 2.7V, V
lower
= 1.0V, I
d
= 8.3mA,
then
t
c
≈
0.5534R
T
C
T
t
d
= R
T
C
T
ln
V
O
Figure 2: Timing Diagram
(
2.3 - 0.0083 R
T
4.0 - 0.0083 R
T
)
V
upper
For better accuracy R
T
should be
≥10kΩ.
V
lower
t
on
t
off
Grounding
High peak currents associated with capacitive loads neces-
sitate careful grounding techniques. Timing and bypass
capacitors should be connected close to Gnd in a single
point ground.
The transistor and 5kΩ potentiometer are used to sample
the oscillator waveform and apply an adjustable ramp to
Sense.
t
c
t
on
=t
c
t
off
=t
c
+2t
d
Figure 3: Duty Cycle parameters.
t
d
5
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
