lb11850va Sanyo Semiconductor Corporation, lb11850va Datasheet - Page 9

• Manufacturer Part Number: lb11850va
• Description: For Fan Motor Single-phase Full-wave Pre-driver With Speed Control Function
• Manufacturer: Sanyo Semiconductor Corporation

3) Additional description of operations
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.
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.
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.
signal
CTL
EDGE pulse
1 shot output
RC pin
FG
CTL
TRC(s) = 1.15RC
Speed control block
LB11850VA
CONTROL
Feed-back
LB11850VA
SIGNAL
Closed
Loop
FG
Pre-driver block
Slope due to
constant
CR time
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