LB11852FV Datasheet, PDF (10/12 Pages) Sanyo Semicon Device – Monolithic Digital IC For Fan Motor Single-phase Full-wave Pre-driver with Speed Control Function

LB11852FV
4. Procedure for calculating the constant
〈RC pin〉
The slope shown in the speed control diagram is determined by the constant of the RC pin.
(RPM)
Motor
at maximum speed
0%
CTL Duty(%)
100%
1) Obtain the FG signal frequency fFG (Hz) at the maximum rotational speed of the motor
(with two FG pulses per rotation).
fFG (Hz) = 2 rpm/60 ^… (1)
2) Obtain the time constant of the components connected to the RC pin
(use the duty ratio (example : 100% = 1.0 or 60% = 0.6) as the CTL duty ratio for achieving the maximum rotational
speed).
R × C = Duty ratio/ (3.3 × 1.1 × fFG) ^… (2)
3) Obtain the resistance and the capacitance of the capacitor.
Based on the discharge capability of the RC pin, the capacitance of the capacitor which can be used is in the range of
0.01μF to 0.015μF.
Therefore, obtain the appropriate resistance from the result of (2) above using the formula in (3) or (4) below.
R = (R × C)/0.01μF ^… (3)
R = (R × C)/0.015μF ^… (4)
The temperature characteristics of the curve are determined by the temperature characteristics of the capacitor of the
RC pin. To minimize the variations in the rotational speed caused by temperature, a capacitor with excellent
temperature characteristics must be used.
No.A1112-10/12