Circuit Description: continued
Setting the Oscillator
The values for tc and td are determined from the
V
OSC
formulas:
OSC
RESET
VREF - Vlower
tc = RTCT ln
(
)
VREF - Vupper
Toggle
F/F Output
VREF - IdRT - Vlower
VREF - IdRT - Vupper
td = RTCT ln
EA Output
(
)
Switch
Current
V
CC
Substituting in typical values for the parameters in:
I
O
O
VREF = 5.0V, Vupper = 2.7V, Vlower = 1.0V, Id = 8.3mA,
then
tc ≈ 0.5534RTCT
V
2.3 - 0.0083 RT
td = RTCT ln
Figure 2: Timing Diagram
(
)
4.0 - 0.0083 RT
Vupper
For better accuracy RT should be ≥10kΩ.
Grounding
Vlower
High peak currents associated with capacitive loads
necessitate careful grounding techniques. Timing and
bypass capacitors should be connected close to Gnd in a
single point ground.
ton
tC
toff
td
The transistor and 5kΩ potentiometer are used to sample
the oscillator waveform and apply an adjustable ramp to
Sense.
ton = C
t
toff = C+
t
2td
Figure 3: Timing Parameters
5
CHERRY [ CHERRY SEMICONDUCTOR CORPORATION ]
