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TC170 Datasheet PDF : 16 Pages
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FIGURE 3-7:
OSCILLATOR CIRCUIT
VDD
1
FO
Pin 8
TC170
2.3V 4.3V
ICHARGE ≈ 2.3V
RO
8
9
10
Sync
RO
1mA
Discharge
CO
Current
2.3V
On-Time
Pin 10
Output Dead Time (TD)
3.8 Oscillator Frequency and Output
Dead Time
The oscillator frequency for RO = 24kΩ and CO =
1000pF is:
[ ] FO =
1.27 2800
ROCO – RO2CO
CO
CO + 150 x 10–12
where: RO = Oscillator Resistor (Ω)
CO = Oscillator Capacitor (F)
FO = Oscillator Frequency (Hz)
The oscillator resistor can range from 5kΩ to 50kΩ.
Oscillator capacitor can range from 250pF to 1000pF.
Figure 3-8 shows typical operation for various
resistance and capacitance values.
During transitions between the two outputs,
simultaneous conduction is prevented. Oscillator fall
time controls the output off, or dead time (Figure 3-7).
Dead time is approximately:
TD =
2000 [CO]
( ) 1 – 2.3
RO
where: RO = Oscillator Resistor (kΩ)
CO = Oscillator Capacitor (pF)
TD = Output Dead Time (sec)
Maximum possible duty cycle is set by the dead time.
FIGURE 3-8:
OSCILLATOR
FREQUENCY VS.
OSCILLATOR
RESISTANCE
50
45
40
TA = +25°C
35
30
25
20
250pF
15
10
500pF
5
1000pF 750pF
0 20 40 60 80 100 120 140 160 180 200
OSCILLATOR FREQUENCY (kHz)
© 2002 Microchip Technology Inc.
DS21395B-page 9
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