AN1139 STMicroelectronics, AN1139 Datasheet - Page 17

• Manufacturer Part Number: AN1139
• Description: L6254 - L6268 - L6269 12V DISK DRIVE POWER COMBO IC
• Manufacturer: STMicroelectronics

the BEMF (looking at FCOM pin (#1)) and take action whether internal or external Start-Up is to be ini-
tiated. If the motor is spinning and enough zero cross are detected, no actions are required, the chip sequenc-
er will automatically lock on to the proper phase and bring the motor speed up to frequency lock.
4.1.4 Motor Stuck
The purpose of this feature is to protect the motor and the circuits against an over-heating due to high current
in the same phase when the motor is not moving. During Start-Up, when a phase is energized according to the
sequencer in the GO phase, the current starts to flow into the motor winding producing a torque which should
result a mechanical movement of the rotor to the next position. If for some reason the rotor does not move (not
enough torque or the motor is stuck) and the BEMF is not detected within Tstuck time, the system will shut off
the current in the Spindle tristating the outputs and the controller will enter the ‘Stuck Rotor Hold’ state. To re-
sume from this condition, RUN bit (Reg#2,3) must be toggled. Following is the formula to calculate the Tstuck
time.
•
Example with 20MHz Sys_Clk:
The above description is valid always and not only at Start-Up. This means that the Stuck rotor counter is active
all the time, also when the motor is running at steady speed. If for some reason, which it is, of coarse unlikely
to happens (unless the speed gets extremely low such as ‘Tzero_cross > Tstuck’), the BEMF is not detected
within Tstuck time, the Stuck rotor circuit will react as explained before. The Stuck Rotor information can be read
as a ‘0’ in the Reg#7.2.
4.2
The Brake is the ability to completely stop a Spindle Motor in a reasonable time. There are two different way to
stop the spindle motor, one is Commanded through the serial port and the other one is when the Power supply
is turned off causing a POR condition. The device offers three different mode to brake,Two phase brake (only
L6254), Three phase brake and Reverse brake. In a power-down condition only Two/Three phase brake are
available.
4.2.1 Three Phase Brake
This mode is selectable by re-setting (=0) the 2/3_PHASE bit (Reg#5.3). The brake function is asserted by
resetting (=0) the RUN bit (Reg#2.3) in power on condition and after retract in case of power loss. The low side
output DMOS of phase A, B and C at this time are turned on. Since the 2/3_PHASE bit (Reg#5.3) is not reset
by POR, at power on, it is undefined, therefore it is mandatory to always write it.
In a power-down condition, after retract, when spindle brake initiate, the voltage on VCC drops very rapidly be-
cause the low side DMOS are turned on shorting the motor BEMF. In order to keep the spindle sink transistors
ON in this mode, an external capacitor is used on the BRK_CAP pin (#12) to hold the gates of the sink devices
ON. This capacitor is charged up whenever VCC is high to about 5V, and it is isolated from all supplies by block-
ing diodes. Following is the formula to calculate Brake storage Capacitor where Time is the duration in seconds
of the brake time required.
•
Example with Brake Time=8 seconds :
BRAKE
6.
7.
--------------------- -
Sys_Clk
8.4
25
10
10
–
6
8
Spindle Motor Stuck Time - Tstuck (ms)
Time
Spindle Brake Capacitor ( F)
Tstuck
Brake Capcitor
=
8.4
------------------- -
20
10
10
6
6
=
=
420 ms
25
10
–
8
8
=
AN1139 APPLICATION NOTE
2 F
.
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