LB11975 Datasheet, PDF (10/12 Pages) Sanyo Semicon Device – High-Speed CD-ROM Spindle Motor Driver IC

LB11975
Hall comparator
IN1
(IN1, IN2, and IN3)
waveforms
IN2
IN3
Reverse rotation is detected with this timing.
Figure 3 Reverse Rotation Timing Chart
Overview of Reverse Torque Braking
(This circuit uses a direct PWM drive technique and allows the current limiter to operate during reverse torque braking.)
In earlier direct PWM motor drivers, speed control was implemented by applying PWM to only one (either the upper or
lower) output transistor. With this type of driver, the regenerative current formed during reverse torque braking operated
as a short-circuit braking. As a result problems such as the coil current exceeding the limit value and I_Omax being
exceeded, would occur. To prevent these problems, the LB11975 switches both the upper and lower side output
transistors during reverse torque braking to suppress the generation of overcurrents due to regenerative currents when the
PWM is off and allows the optimal design of drive currents.
Supplementary Documentation
Coil current during reverse torque braking
(1) Earlier ICs, with the lower side transistor was switched and the upper side transistor used for current detection (RF)
During reverse torque braking, when the coil current increases and the limit is reached, the lower side output
transistor is turned off. At this time the regenerative current flows through the upper side transistor. The circuit path is
as follows:
Coil → upper side diode → V_CC → RF → upper side transistor → coil
During regeneration, the upper side transistor is on and the back EMF that occurs at the upper side transistor’s emitter
pin has a low potential, and since the upper side transistor is fully on at that point, the circuit functions as short-circuit
braking.
Even if the regenerative current results in the RF voltage reaching the limit voltage, since the upper side transistor
cannot be turned off, the limit circuit will not operate and a coil current in excess of I_Omax may occur.
(2) Earlier ICs, with the upper side transistor was switched and the upper side transistor used for current detection (RF)
During reverse torque braking, when the coil current increases and the limit is reached, the upper side output
transistor is turned off. At this time the regenerative current flows through the lower side transistor. The circuit path is
as follows:
Coil → lower side transistor → ground → lower side diode → coil
During regeneration, the lower side transistor is on and the back EMF that occurs at the lower side transistor’s
collector pin has a high potential, and since the lower side transistor is fully on at that point, the circuit functions as
short-circuit braking.
Since the regenerative current does not flow through the RF pin, the current limiter circuit does not operate, and a
current in excess of I_Omax may occur in the lower side transistor.
No. 6497-10/12