LB11870_07 Datasheet, PDF (12/14 Pages) Sanyo Semicon Device – For Polygonal Mirror Motors Three-Phase Brushless Motor Driver

LB11870
4. Power Saving Circuit
This IC goes into a power saving state that reduces the current drain in the stop state. The power saving state is
implemented by removing the bias current from most of the circuits in the IC. However, the 5V regulator output is
provided in the power saving state.
5. Reference Clock
Care must be taken to assure that no chattering or other noise is present on the externally input clock signal.
Although the input circuit does have hysteresis, if problems do occur, the noise must be excluded with a capacitor.
If the IC is set to the start state when the reference clock signal is not present, if the rotor constraint protection circuit
is used, the motor will turn somewhat and then motor drive will be shut off. However, if the rotor constraint
protection circuit is not used, and furthermore reverse torque control mode is selected for deceleration, the motor will
be driven at ever increasing speed in the reverse direction. (This is because the rotor constraint protection circuit
oscillator signal is used for clock cutoff protection.) Applications must implement a workaround for this problem if
there is any possibility whatsoever for it to occur.
6. Notes on the PWM Frequency
The PWM frequency is determined by the value of the capacitor C (in F) connected to the PWM pin.
fPWM ≈ 1 / (43000 × C)
If a 680pF capacitor is used, the circuit will oscillate at about 34kHz. If the PWM frequency is too low, the motor
will emit switching noise, and if it is too high, the power loss in the output will be excessive. A PWM frequency in
the range 15 to 50kHz is desirable. To minimize the influence of the output on this circuit, the ground lead of this
capacitor should be connected as close as possible to the IC control system ground (the GND1 pin).
7. Hall Input Signals
Signals with an amplitude in excess of the hysteresis (42mV maximum) must be provided as the Hall input signals.
However, an amplitude of over 100mV is desirable to minimize the influence of noise. If the output waveforms are
disturbed (at phase switching) due to noise on the Hall inputs, insert capacitors across these inputs.
8. FG Input Signal
Normally, one phase of the Hall signals is input as the FG signal. If noise is a problem the input must be filtered with
either a capacitor or an RC filter circuit. Although it is also possible to remove FG signal noise by inserting a
capacitor between the FGFIL pin and ground, the IC may not be able to operate correctly if this signal is damped
excessively. If this capacitor is used, its value must be less than about 2200pF. If the location of this capacitor's
ground lead is inappropriate, it may, inversely, make noise problems even more likely to occur. Thus the ground lead
location must be chosen carefully.
9. Rotor Constraint Protection Circuit
This IC provides a rotor constraint protection circuit to protect the IC itself and the motor when the motor is
constrained. If the LD output is high (unlocked) for over a certain fixed period with the IC in the start state, the low
side transistor will be turned off. The time constant is determined by the capacitor connected to the CSD pin.
<time constant (in seconds)> ≈ 120 × C (μF)
If a 0.068μF capacitor is used, the protection time will be about 8 seconds. The set time must be selected to have an
adequate margin with respect to the motor startup time. This protection circuit will not operate during deceleration
when the clock frequency is switched. To clear the rotor constraint protection state, the IC must be set to the stopped
state or the power must be turned off and reapplied.
Since the CSD pin also functions as the initial reset pulse generation pin at startup, the logic circuit will go to the
reset state and the IC will not be able to function if this pin is connected to ground. Therefore, both a 220kΩ resistor
and a 4700pF capacitor must be inserted between this pin and ground if the rotor constraint protection circuit is not
used.
No.7256-12/14