LB11850VA

No.A0609-9/14

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.

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.

LB11850VA

Speed control block

FG

CONTROL

SIGNAL

CTL

CTL

signal

Closed

Feed-back

Loop

Pre-driver block

FG

EDGE pulse

RC pin

1 shot output

TRC(s) = 1.15RC

Slope due to

CR time

constant