LB11852FV Datasheet, PDF (9/12 Pages) Sanyo Semicon Device – Monolithic Digital IC For Fan Motor Single-phase Full-wave Pre-driver with Speed Control Function

LB11852FV
2. Supplementary description of operations
By inputting the duty pulses, a feedback loop is formed inside the LB11852 IC to establish the FG period (rotational
speed of the motor) that corresponds to the control voltage of the pulses.
LB11852
CTL
CTL
Signal
Speed
control block
FG
Closed
Feed-Back
Loop
CONTROL
SIGNAL
Pre-driver
block
The operation inside the IC is as flows. pulse signals are created from the edges of the FG signals as shown in the
figure below, and using these signals as a reference, waveforms with a pulse width determined by the CR time constant
are generated using a one-shot multivibrator. These pulse waveforms are then integrated to control the duty ratio of the
pre-driver output as the control voltage.
FG
Edge pulse
RC pin
Slope determined
by CR time
constant
One-shot
multivibrator
output
TRC (sec) = 1.15RC
By changing the pulse width as determined by the CR time constant, the VCTL versus rotational speed slope can be
adjusted as shown in the speed control diagram in the previous section.
However, since pulses that are determined by the CR time constant are used, the CR variations are output as-is as the
speed control error.
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