LB11922_08 Datasheet, PDF (12/15 Pages) Sanyo Semicon Device – For OA Products Three-Phase Brushless Motor Driver

LB11922
LB11922 Description
1. Speed Control Circuit
This IC implements speed control using the combination of a speed discriminator circuit and a PLL circuit. The speed
discriminator circuit outputs (This counts a single FG period.) an error signal once every two FG periods. The PLL
circuit outputs an error signal once every one FG Period. As compared to the earlier technique in which only a speed
discriminator circuit was used, the combination of a speed discriminator and a PLL circuit allows variations in motor
speed to be better suppressed when a motor that has large load variations is used. The FG servo frequency (fFG) is
controlled to have the following relationship with the crystal oscillator frequency (fOSC).
fFG = fOSC ÷ (16 × <number of counts>)
N
High or open
Low
Number of counts
512
1024
Therefore it is possible to implement half-speed control without switching the clock frequency by using combinations
of the N1 = high, N2 = low state and other setting states.
2. Reference Clock
This IC supports the use of either of the following methods for providing the speed control reference clock.
(1) Crystal oscillator
Use a circuit consisting of a crystal and capacitors such as the one shown below to implement a crystal oscillator.
XI
XO
C3
C1
C2
C1 : Used to prevent oscillation at upper harmonic frequencies.
C2 : Used for stabilization and to prevent oscillation at upper harmonic frequencies.
C3 : Used for oscillator coupling.
Oscillator frequency (MHz)
3 to 5
C1 (pF)
39
C2 (pF)
10
C3 (pF)
47
(Values provided for reference purposes)
5 to 8
10
10
47
8 to 10
5
10
22
This circuit and these component values are only provided for reference purposes. When implementing a crystal
oscillator in an application, it is necessary to consult the manufacturer of the crystal to verify that problems will not
occur due to interactions between stray capacitances due to wiring in the PCB and the crystal.
Notes :
The capacitor C1 is effective at lowering negative resistance values at high frequencies, but care is required to assure
that it does not excessively reduce the negative resistance at the fundamental frequency.
Since this crystal oscillator circuit is a high-frequency circuit, it can be easily influenced by stray capacitances on the
PCB. To minimize stray capacitances, keep connections between external components as short as possible and use
narrower line widths in the PCB patter.
The C1 and C2 ground lines must be as short as possible, and must be connected to the IC's ground pin (pin 23, GND).
If the PCB lines are excessively long, the oscillator circuit may be influenced by fluctuations in the ground line voltage
when, for example, the motor is overloaded, and the oscillator frequency may change. The C1 and C2 ground lines can
be made shorter by using the NC pins next to the XI and XO pins for the C1 and C2 ground, and connecting those pins
across the back of the IC to the IC GND pin.
No.7497-12/15