Data Sheet G14309EJ2V0DS00

21

µµµµPC1909

4.3 Oscillator Settings

The oscillator circuit is shown in Figure 4-4 below.

Figure 4-4.

µµµµPC1909 Oscillator Circuit

GND

3

ICT

Tr1

Tr3

Tr2

2·ICT

VTH(H)

VREF

14

RT

RT

15

CT

CT

16

VTH(L)

SQ

R

Comparator H

Flip-flop

Comparator L

A timing resistor (RT) is connected between the RT pin and GND, and a timing capacitor (CT) is connected between the

CT pin and GND. The charge/discharge current of CT is determined by RT. The oscillator operates as follows.

<1>

If ICT is taken as the current that flows through Tr1, the current the flows through Tr3 is set as 2 x ICT. Because

the flip-flop (Q) outputs a high level at startup, Tr2 is off, and CT is charged with ICT.

<2>

When the CT voltage reaches VTH(H) = 3.5 V TYP., the output of comparator H is inverted, and Tr2 is turned on.

Due to the discharging of the current set by 2 x ICT, the current flowing through CT is (ICT – 2 x ICT) = – ICT, so

CT is discharged by ICT.

<3>

If the CT voltage drops to VTH(L) = 1.5 V TYP., the output of comparator L is inverted, the flip-flop is reset, and

CT is recharged because Tr2 is off.

<4>

<2> and <3> are repeated, generating a triangle wave with an amplitude of 1.5 to 3.5 V.

The oscillation frequency can be approximated from the following equation.

fOSC

[kHz]

0.8251

× CT [pF] × (RT [kΩ] + 0.8) + 320

1

× 106