LB11850VA Datasheet, PDF (9/14 Pages) Sanyo Semicon Device – Monolithic Digital IC For Fan Motor Single-Phase Full-Wave Pre-Driver with Speed Control Function

LB11850VA
3) Additional description of operations
The LB11850 forms a feedback loop inside the IC so that the FG period (motor speed) corresponding to the control
voltage is established by inputting the duty pulse.
LB11850VA
CTL
signal
FG
Speed control block
CTL
Closed
Feed-back
Loop
CONTROL
SIGNAL
Pre-driver block
The operation inside the IC is as follows. Pulse signals are created from the edges of the FG signals as shown in the
figure below, and a waveform with a pulse width which is determined by the CR time constants and which uses these
edges as a reference is generated by a one-shot multivibrator.
These pulse waveforms are integrated and the duty ratio of the pre-driver output is controlled as a control voltage.
FG
EDGE pulse
RC pin
Slope due to
CR time
constant
1 shot output
TRC(s) = 1.15RC
Furthermore, by changing the pulse width as determined by the CR time constant, the VCTL versus speed slope can be
changed as shown in the speed control diagram of the previous section.
However, since the pulses used are determined by the CR time constant, the variations in CR are output as-is as the
speed control error.
No.A0609-9/14