LB11921T Datasheet, PDF (15/18 Pages) Sanyo Semicon Device – Monolithic Digital IC Three-Phase Brushless Motor Driver

LB11921T
7. Hall effect sensor input signals
An input amplitude of over 100mVp-p is desirable in the Hall effect sensor inputs. The closer the input waveform is
to a square wave, the lower the required input amplitude. Inversely, a higher input amplitude is required the closer
the input waveform is to a triangular wave. Also note that the input DC voltage must be set to be within the
commonmode input voltage range.
If noise on the Hall inputs is a problem, that noise must be excluded by inserting capacitors across the inputs. Those
capacitors must be located as close as possible to the input pins.
When the Hall inputs for all three phases are in the same state, all the outputs will be in the off state.
If a Hall sensor IC is used to provide the Hall inputs, those signals can be input to one side (either the + or - side) of
the Hall effect sensor signal inputs as 0 to VCC level signals if the other side is held fixed at a voltage within the
common-mode input voltage range that applies when a Hall effect sensors are used.
8. Forward/Reverse Switching
The motor rotation direction can be switched using the F/R pin. However, the following notes must be observed if the
motor direction is switched while the motor is turning.
• This IC is designed to avoid through currents when switching directions. However, increases in the motor supply
voltage (due to instantaneous return of motor current to the power supply) during direction switching may cause
problems. The values of the capacitors inserted between power and ground must be increased if this increase is
excessive.
• If the motor current after direction switching exceeds the current limit value, the PWM drive side outputs will be
turned off, but the opposite side output will be in the short-circuit braking state, and a current determined by the
motor back EMF voltage and the coil resistance will flow. Applications must be designed so that this current does
not exceed the ratings of the output transistors used. (The higher the motor speed at which the direction is
switched, the more severe this problem becomes.)
9. Brake Switching
The LB11921T provides a short-circuit brake implemented by turning the output transistors for the UH, VH, and WH
pins for all phases on. (The opposite side transistors are turned off for all phases.) Note that the current limiter does
not operate during braking. During braking, the duty is set to 100%, regardless of the motor speed. The current that
flows in the output transistors during braking is determined by the motor back EMF voltage and the coil resistance.
Applications must be designed so that this current does not exceed the ratings of the output transistors used. (The
higher the motor speed at which braking is applied, the more severe this problem becomes.)
The braking function can be applied and released with the IC in the start state. This means that motor startup and stop
control can be performed using the brake pin with the S/S pin held at the low level (the start state). If the startup time
becomes excessive, it can be reduced by controlling motor startup and stop with the brake pin rather than with the
S/S pin. (Since the IC goes to the power saving state when stopped, enough time for the VCO circuit to stabilize will
be required at the beginning of the motor start operation.)
10. Constraint Protection Circuit
The LB11921T includes an on-chip constraint protection circuit to protect the IC and the motor in motor constraint
mode. If the LD output remains high (indicating the unlocked state) for a fixed period in the start state, the PWM
output (external) transistors are turned off. This time is set by the capacitance of the capacitor attached to the CSD
pin.
The set time (in seconds) is 15.1 × C (µF)
To clear the motor constrained protection state, the application must either switch to the stop or brake state for a fixed
period (about 1ms or longer), or the power must be turned off and reapplied.
If the motor constrained protection circuit is not used, a 360kΩ resistor and a 3300pF capacitor must be connected in
parallel between the CSD pin and ground. However, in that case, the clock disconnect protection circuit described
below will no longer function. Since the CSD pin also functions as the power-on reset pin, if the CSD pin were
connected directly to ground, the IC would go to the power-on reset state and motor drive operation would remain
off. The power-on reset state is cleared when the CSD pin voltage rises above a level of about 0.25V.
11. Clock Disconnect Protection Circuit
If the clock input goes to the no input state when the IC is in the start state, this protection circuit will operate and
turn off the PWM output. If the clock is resupplied before the motor constraint protection circuit operates, the IC will
return to the drive state, but if the motor constraint protection circuit does operate, the IC must either be set
temporarily (approximately 1 ms or over) to the stop or brake state, or the power must be turned off and reapplied.
No.A0604-15/18