External Component Functions

Notes on LB1823 Operation and External Components

1. Speed control circuit

The LB1823 uses a speed discriminator circuit and a PLL circuit in combination for speed control. The speed

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

generates a phase error output once every FG period, also using a charge pump technique. By using a speed

discriminator circuit and a PLL circuit together, the LB1823 can suppress speed variations better than earlier systems

that only used a speed discriminator for speed control when used with motors faced with large load variations. Since

the following formula determines the FG servo frequency, the motor speed is set by the number of FG pulses and the

crystal oscillator frequency.

2. Direct PWM drive

This IC adopts a direct PWM drive technique to minimize power loss in the outputs. The output transistors are

always saturated when on, and the motor drive power is adjusted by changing the duty with which the outputs are on.

Since the upper side transistors switch the outputs, Schottky, fast recovery, or similar diodes must be inserted

between OUT and ground (diodes D1, D2 and D3). (This is because through currents will flow in the upper side

transistors at the instant they turn on if these diodes do not have a short reverse recovery time.) Ordinary rectifying

side transistors. If these transistors include parasitic diodes, through currents will flow due to their reverse recovery

time, even if Schottky diodes are used for D1, D2 and D3.

3. Current limiter circuit

capacitor is required.

No. 4263-7/11

LB1823

Part

Function

Description

C1

Power supply stabilization

Choose a value such that the voltage fluctuations due to the motor drive currents are

stabilized.

C2

IC power supply stabilization

Attach as close as possible to pins 17 and 24.

R1, C3

PWM frequency setting

Use a resistor of 30 k

Ω or larger for R1.

C4, C5, R2

External crystal oscillator circuit

See the figure on page 6.

C6, C7, R3, R4, R5

Servo constants

IC internal power supply

If the capacitance of this capacitor is too small, it will become harder to reset the internal IC

C8

stabilization

logic reliably and incorrect logic operation may occur. Attach as close as possible to pins 11

Reset pulse generation

and 24.

C9, C10, R7, R8

FG amplifier gain and frequency

characteristics settings

R10

Limit current setting

R11, R12

Hall element bias current settings

R13, R14, R15

Sink current settings

These resistors set the sink currents for the UL, VL and WL (open collector) outputs.

R16

Shunt regulator bias current

settings

guaranteed correct operation is desired.

R17, R18, R19

Pull-down resistors

These resistors speed up the motor drive output transistor off times.

R20

Start time assurance

Include R20 in applications in which the start up time is a problem.

D1, D2, D3

Regeneration current absorption

Use Schottky diodes for D1, D2 and D3.