4-125

TELCOM SEMICONDUCTOR, INC.

7

6

5

4

3

1

2

8

CMOS CURRENT MODE

PWM CONTROLLER

TC170

Figure 6 . Oscillator Circuit

Figure 7.

Oscillator Frequency vs Oscillator Resistance

Maximum possible duty cycle is set by the dead time.

50

45

40

35

30

25

20

15

10

5

0

20

40

60

80 100 120 140 160 180 200

TA = +25°C

250pF

500pF

OSCILLATOR FREQUENCY (kHz)

750pF

1000pF

2.3V 4.3V

I

CHARGE

2.3V

RO

8

CO

9

1 mA

DISCHARGE

CURRENT

SYNC

10

VDD

2.3V

PIN 8

PIN 10

1

ON-TIME

OUTPUT DEAD TIME (TD)

+

–

RO

FO

Oscillator Frequency and Output Dead Time

–

The oscillator resistor can range from 5 k

Ω 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:

]

[

1.27

2800

O

–12

1 –

( )

2.3