No. 5625-9/11

LB1872

Continued from preceding page.

Pin No.

Symbol

Pin function

Equivalent circuit

26

FG amplifier input

27

AGC

AGC amplifier frequency characteristics correction

Insert a capacitor (about 0.1 µF) between this pin and ground.

28

GND

Ground

LB1872 Functional Description

1. Speed control circuit

Since this IC adopts a PLL speed control circuit, it can provide high-precision, jitter-free, and stable motor operation.

This PLL circuit compares the phases of the CLK (external clock) rising edge and the FG Schmitt output rising edge

and uses the error output from that comparison for control.

If an internal clock system is used, the FG servo frequency is determined by the following formula. Therefore, the

motor speed can be set by setting the number of FG pulses and the crystal oscillator frequency.

N: Clock divisor (See the separately provided table.)

If an external clock (input to the N1 pin) is used, the IC controls the motor speed by holding the FG servo frequency

identical to the external clock frequency.

2. Output drive circuit

To suppress motor noise as much as possible, this IC adopts a three-phase full wave current linear drive technique.

Also, it adopts a midpoint control technique to prevent ASO destruction of the output transistors.

This IC uses short-circuit braking (lower side output) for motor deceleration during speed switching and lock pull in.

In stop mode, the output is turned off.

If a motor with a coil resistance (interphase) of 10

Ω or lower is used, diodes (rectifying) may be inserted between the

outputs and ground (for all outputs) and current limitation may also be applied during braking to prevent excessive

braking currents. Although this is disadvantageous from an ASO standpoint, since states with a large back EMF are

states with a high switching frequency and the amount of time during which loads are applied to the transistors will

be shorter, ASO problems will not be particular severe.

3. Current limiter circuit

V (typical) and Rf is the current detection resistor).