Circuit Description: continued
Setting the Oscillator
The times Tc and Td can be determined as follows:
V
OSC
VREF - Vlower
tc = RTCT ln
OSC
RESET
(
)
VREF - Vupper
Toggle
F/F Output
VREF - IdRT - Vlower
VREF - IdRT - Vupper
td = RTCT ln
(
)
EA Output
Switch
Current
Substituting in typical values for the parameters in the
above formulas:
V
CC
VREF = 5.0V, Vupper = 2.7V, Vlower = 1.0V, Id = 8.3mA,
then
I
O
O
tc ≈ 0.5534RTCT
V
2.3 - 0.0083 RT
td = RTCT ln
(
)
4.0 - 0.0083 RT
Figure 2: Timing Diagram
For better accuracy RT should be ≥10kΩ.
V
V
upper
Grounding
lower
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.
t
t
off
on
t
d
t
C
The transistor and 5kΩ potentiometer are used to sample
the oscillator waveform and apply an adjustable ramp to
Sense.
t
t
t
on = C
t
2t
d
off = C+
Figure 3: Duty Cycle parameters.
5
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
