CMOS CURRENT MODE
PWM CONTROLLER
1
2
3
4
5
6
7
8
TC170
V
1
DD
F
O
PIN 8
2.3V
I
+
–
2.3V 4.3V
ON-TIME
2.3V
CHARGE
R
O
PIN 10
8
9
OUTPUT DEAD TIME (T )
D
10
R
SYNC
O
1 mA
C
DISCHARGE
CURRENT
O
Figure 6 . Oscillator Circuit
Oscillator Frequency and Output Dead Time
The oscillator frequency for RO = 24kΩ and
CO = 1000pF is:
50
45
40
35
30
25
20
15
T
= +25°C
A
1.27
2800
CO
–
FO =
2
–12
[
]
ROCO
RO CO CO + 150 × 10
where: RO = Oscillator Resistor (Ω)
CO = Oscillator Capacitor (F)
FO = Oscillator Frequency (Hz)
250pF
500pF
The oscillator resistor can range from 5 kΩ to 50 kΩ.
Oscillator capacitor can range from 250 pF to 1000 pF.
Figure 7 shows typical operation for various resistance and
capacitance values.
10
5
1000pF 750pF
0
20 40 60 80 100 120 140 160 180 200
OSCILLATOR FREQUENCY (kHz)
During transitions between the two outputs, simulta-
neous conduction is prevented. Oscillator fall time controls
the output off, or dead time (Figure 6).
Dead time is approximately:
Figure 7. Oscillator Frequency vs Oscillator Resistance
2000 [CO]
TD =
where: RO = Oscillator Resistor (kΩ)
CO = Oscillator Capacitor (pF)
TD = Output Dead Time (sec)
2.3
1 –
)
(
RO
Maximum possible duty cycle is set by the dead time.
TELCOM SEMICONDUCTOR, INC.
4-125
TELCOM [ TELCOM SEMICONDUCTOR, INC ]
