LB11948T

No.7947-11/13

Usage Notes

(1) Simple Formulas for Determining Resister and Capacitor Values

The formula for setting the rising time (T1) and the falling time (T2) for the RC oscillator are shown below. (Refer to

Fig. 1)

Fig. 1

Formulas

Oscillation period

T = T1+T2

(sec)

Threshold voltages V1 = ((VCC - Vset1) × 10.7k/48.7k) + Vset1

(V)

V2 = ((VCC - Vset2) × 42.7k/80.7k) + Vset2

(V)

Vset1 : VCE voltage of transistor for internal comparator hysteresis = 0.05V

Vset2 : VCE voltage of reference resistance switching transistor of oscillation circuit = 0.1V

When charging :

T1 = -C

× R × ln {(VCC - V2)/(VCC - V1)} (sec)

When discharging : T2 = -C

× Rin × ln (V1/V2) (sec)

Rin : Internal discharge resistance of the CR pin1.3k

Ω

C : External capacitor

R : External resistor

Oscillation frequency

Fc = 1/T (Hz)

The T2 fall time serves as the noise canceling time (Tn). This time is a forced-on time for the output, and the output is

not turned off even when the E pin voltage is higher than the sense voltage that has been preset by VREF.

T2

T1

V1

V2