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LB11850VA

Single‐phase Full‐wave Pre‐driver with Speed Control Function for Fan Motor

ON Semiconductor 

LB11850VA

4. Procedure for Calculating Constants:

<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(%)

Figure 8.

100%

1) Obtain FG signal frequency fFG (Hz) of the maximum speed of the motor.

(With FG2 pulses per rotation)

fFG (Hz) = 2 rpm / 60 .... <1>

2) Obtain the time constant which is connected to the RC pin.

(Have “DUTY” (example: 100% = 1.0, 60% = 0.6) serve as the CTL duty ratio at which the maximum speed is

to be obtained.)

R × C = DUTY / (3.3 × 1.1 × fFG) .... <2>

3) Obtain the resistance and capacitance of the capacitor.

Based on the discharge capacity of the RC pin, the capacitance of the capacitor which can be used is 0.01 to

0.015 mF. Therefore, find the appropriate resistance using equation <3> or <4> below from the result of <2>

above.

R = (R × C) / 0.01 mF .... <3>

R = (R × C) / 0.015 mF .... <4>

The temperature characteristics of the curve are determined by the temperature characteristics of the capacitor of the RC

pin. When temperature-caused fluctuations in the speed are to be minimized, use a capacitor with good temperature

characteristics.

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