CMOS CURRENT MODE
PWM CONTROLLER
TC170
1
VDD
PIN 8
1
FO
2
3
2.3V 4.3V
I
CHARGE
2.3V
RO
+
–
2.3V
PIN 10
ON-TIME
10
SYNC
DISCHARGE
CURRENT
1 mA
8
9
RO
OUTPUT DEAD TIME (TD)
CO
4
Figure 6 . Oscillator Circuit
Oscillator Frequency and Output Dead Time
The oscillator frequency for R
O
= 24kΩ and
C
O
= 1000pF is:
OSCILLATOR RESISTANCE (kΩ)
50
45
40
35
30
25
20
15
10
5
0
500pF
1000pF 750pF
20 40 60 80 100 120 140 160 180 200
OSCILLATOR FREQUENCY (kHz)
250pF
TA = +25°C
F
O
=
[
1.27
2800
–
R
O
C
O
R
O2
C
O
]
C
O
C
O
+ 150
×
10
–12
5
6
7
where: R
O
= Oscillator Resistor (Ω)
C
O
= Oscillator Capacitor (F)
F
O
= Oscillator Frequency (Hz)
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.
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:
T
D
=
2000 [C
O
]
1–
Figure 7.
Oscillator Frequency vs Oscillator Resistance
( )
2.3
R
O
where: R
O
= Oscillator Resistor (kΩ)
C
O
= Oscillator Capacitor (pF)
T
D
= Output Dead Time (sec)
Maximum possible duty cycle is set by the dead time.
8
TELCOM SEMICONDUCTOR, INC.
4-125
TELCOM [ TELCOM SEMICONDUCTOR, INC ]
