This section describes the LB1921 and the external components used.

1. Speed Control Circuit

This IC uses the combination of a speed discriminator circuit and a PLL circuit for speed control. The speed

discriminator circuit outputs an error signal once every two FG periods using a charge pump technique. The PLL

circuit outputs a phase error signal once every FG period, also using a charge pump technique. As compared to earlier

schemes that only used a speed discriminator, the combination of a PLL circuit with a speed discriminator provides

improved speed variation suppression when using a motor with large load variations. Since the following formula

determines the FG servo frequency, the motor speed must be set using the number of FG pulses and the crystal

oscillator frequency.

2. Direct PWM Drive

This IC adopts direct PWM drive to minimize power loss in the output. The output transistors are always saturated

when on and the motor drive power is adjusted by changing the output on duty. Since output switching is performed

that a through current will flow at the instant the lower side transistor turns on if these diodes do not have a short

reverse recovery time.) Normal rectifying diodes can be used for the diodes between OUT and ground.

3. Current Control Circuit

The current control circuit applies current control at a current determined by the relation I = 0.5/Rf, i.e. peak current

limitation. The control operation reduces the output on duty and thus suppresses the current. No phase compensation

capacitor is required.

4. Speed Lock Range

The speed lock range is ±6.25% of the fixed speed and the LD pin goes low when the motor is in the lock range.

(This pin is an open-collector output.) When the motor speed goes out of the lock range, the LB1921 changes the

motor drive output on duty according to the speed error to control the motor speed to be within the lock range.

5. PWM Frequency

The PWM frequency is determined by the resistor and capacitor (R3 and C6) connected to the CR pin.

• When R3 is connected to the 4-V fixed voltage supply

• When R3 is connected to the 7-V fixed voltage supply

R3 should not be any smaller than 30 k

Ω. A PWM frequency of about 15 kHz is desirable. If the PWM frequency is

too low the motor will vibrate at the PWM frequency when the motor is restrained causing disturbing audible noise.

Inversely, switching loss increases if the PWM frequency is too high.

6. Ground Leading

GND1 (pin 22) is the ground for circuits other than the output block.

GND2 (pin 11) is the ground for the output block (emitters of the sink transistors).

D4, D5 and D6 are connected to GND2. All other external components are connected to GND1. The GND1 and

GND2 lines are connected to a single ground point at the connector. Since GND2 carries large currents, it should be

kept as short as possible.

7. Output Parasitic Effects

Parasitic effects occur when the output pin voltage falls –0.7 V (this value decreases as the temperature increases)

than 1 V. When parasitism occurs, initially speed control is lost intermittently, but if the amount of parasitism

increases the output transistors can be destroyed. Schottky diodes with a small Vf are used for D1, D2 and D3 to

problem. Although normal rectifying diodes can be used for D4, D5 and D6, the printed circuit board pattern must be

kept as short as possible (as recommended in item 6) to prevent parasitism from occurring.

No. 5439-7/10

LB1921