LB11873

No.A0081-11/14

Continued from preceding page.

Pin No.

Pin

Description

Equivalent Circuit

27

VREG

Stabilized power supply output (5V output)

Connect a capacitor between this pin and ground

for power supply stabilization.

(About 0.1

μF)

29

VCC

Power supply.

Connect a capacitor (a few tens of

μF or larger)

between this pin and ground so that noise does not

enter the IC.

3, 30

31, 32

34, 36

NC

Since this pin is not connected internally to the

chip, it can be used for wiring connections.

FRAME

Connect this pin to ground.

LB11873 Overview

1. Speed control circuit

Since the LB11873 adopts PLL speed control, it provides precise, low-jitter, and stable motor operation. This PLL

circuit compares the falling edge of the CLK signal with the FG signal (edges on which the IN1 input changes from low

to high and FGS output rising edges) and controls motor operation based on the difference.

During control operation, the FG servo frequency is the same as the CLK signal frequency.

fFG (servo) = fCLK

2. Output drive circuit

This IC minimizes motor vibration and noise by changing the output current smoothly during phase switching. Since

the change (slope) imposed on the output current during phase switching uses the slope of the Hall sensor input

waveform, the changes in the output waveforms at phase switching will become too steep if the Hall sensor input

waveform slope is steep. This will reduce the noise and vibration reducing effect of this technique. Thus care is

required concerning the slope of the Hall sensor input waveform.

Low side output transistor PWM switching is used for motor speed control and the drive output is adjusted by changing

the duty. The diode between OUT and VM used for the regenerative current when the PWM is off is built into this IC.

Due to the parasitic diode between OUT and ground, if reverse control mode (torque braking) is selected for braking, an

external Schottky barrier diode must be used. Also, if there are problems when the output current is large (for example,

incorrect operation or waveform disruption during low side kickback) a Schottky barrier diode must be connected

between OUT and ground.

Note that if it is necessary to reduce IC thermal dissipation during constant-speed operation, it may be effective to insert

a Schottky barrier diode between OUT and VM. This effect occurs because the regenerative current during PWM

switching will be dissipated in the external diode instead of the IC's internal diode.

3. Current limiter circuit

The current limiter circuit limits the drive current to a current determined by the equation I = VRF/RF, where VRF =

0.5V (typical) and Rf is the current detection resistor. The limiting operation works by reducing other output on duty to

suppress the drive current.

The current limiter circuit detects the reverse recovery current due to PWM operation and, to assure that the current

limiting operation is not performed incorrectly, provides a delay of about 2µs before it operates. Since the changes in

the current levels at startup (the state where there is no counterelectromotive force from the motor) will be rapid if

either the motor coil resistance is low or if the inductance is low, there are cases where current limiter will operates at a

current level above that set due to this delay. In these cases, it will be necessary to take the amount of current increase

due to the delay into account when setting the current limit value.

27

VCC