AN4071 datasheet(22/27 Pages) STMICROELECTRONICS

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Relaxation oscillator

AN4071

22/27

Doc ID 022939 Rev 1

Relaxation oscillator

A relaxation oscillator belongs to the regenerative circuits group. One subgroup is

multivibrators which are furthermore classified as monostable, bistable and astable. The

relaxation oscillator is an astable multivibrator.

Figure 26.

Relaxation oscillator

The circuit in Figure 26 shows a representative circuit of relaxation oscillator based on the

TS3021 comparator. It uses both positive and negative feedback. Positive feedback

produces voltage hysteresis which has already been described in Section 4.2 Threshold

voltages VLOW (from low to high) and VHIGH (from high to low) on the inverting input are

given by R2, R3 and R4 resistors together with output voltage given by power supply voltage.

Considering zero voltage drop at the output see Equation 9:

Equation 9

VLOW = VCC • R2 | | R4 / (R3 + R2 | | R4) and VHIGH = VCC • R2 / (R2 + R3 | | R4)

For R2 = R3 = R4: VLOW = 1/3 • VCC and VHIGH = 2/3 • VCC

Voltage on the non-inverting input is generated by charging and discharging capacitor C1

from the comparator output via resistor R1 in the feedback:

While C1 is charging: VC1(t) = VCC - (VCC - VLOW) • e

-t/

= VCC - 2/3 • VCC • e

-t/

Capacitor C1 is charged at voltage VHIGH after time t1:

Equation 10

VHIGH = VCC - 2/3 • VCC • e

-t1/

2/3

• VCC = VCC - 2/3 VCC • e-t1/τ

-1/3

• VCC = -2/3 • VCC • e-t1/τ

1 = 2

• e-t1/τ

-t1/τ = ln (1/2)

t1 = τ • ln (2)

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