LB11852FV

No.A1112-9/12

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.

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.

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.

CTL

Signal

CTL

FG

LB11852

Closed

Feed-Back

Loop

CONTROL

SIGNAL

Speed

control block

Pre-driver

block

One-shot

multivibrator

output

RC pin

Edge pulse

FG

TRC (sec) = 1.15RC

Slope determined

by CR time

constant