AN1139 STMicroelectronics, AN1139 Datasheet - Page 18

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

AN1139 APPLICATION NOTE
4.2.2 Two Phase Brake
This mode is available only in the L6254 device. It works exactly like the Three phase brake and it is selectable
by setting (=1) the 2/3_PHASE bit (Reg#5.3). The difference, in this case is that only the phase A and B of the
low side outputs DMOS are turned on, while the phase C is kept off.
4.2.3 Reverse Brake
It is available only when power is present (PORB=1). It is asserted by setting the REV_BRAKE bit (Reg #10.5).
At this time the current in the winding of the motor is reversed respect its previous flowing, resulting a rapid de-
celeration of the motor. The system will end over to the normal three/two phase brake (according to the Reg#5.3
bit value) as soon as the time between two zero cross is reaching a predefined value.
Three different values are available by toggling the REV_BRAKE bit (Reg#10.5) according to the following table
(#3).
Table 3. Zero Crossing Time after reverse brake
Since the saturation of the Powers are required to succeed the Reverse Brake operation, the above statement
is valid if PWM Current Control mode is operating. In case of Linear Control Loop, a little trick needs to be used.
Right before setting the REV_BRAKE bit , the CPH bit (Reg#8.7) must be set (=1). This will disconnect the FLL
loop from the Current control loop bringing quickly the low side DMOS to saturation.
4.3
The current in the spindle motor is sensed by a small resistor (typ. 0.3 ) connected to the bottom of the H bridge
and ground. The voltage drop across the resistor is amplified by a sense amplifier with a gain of four. The output
voltage of the sense amplifier is sent to the Error Amplifier Stage where it is compared to the Current command
coming from the FLL circuit. The Error Amplifier circuit will drive the power stage according to the driving mode
selected. The device provides two different Current Control Loop, Linear and Pulse Width Modulation (PWM).
By changing the Reg#2.6 bit, the selection of the two modes can be done any time regardless the motor is run-
ning or not.
4.3.1 Current Limitation
During initial Start-Up, the error signal from the output of the FLL will be at compliance in order to quickly bring
the motor up to correct speed.
The maximum current flowing into the coil at the beginning (when the motor is stopped and no BEMF is present)
is given by the following equation where RdsON is the ON resistance of the power DMOS.
Icoil = Vsupply / (2*RdsON + Rmotor + Rsense )
The motor current during this condition can be safely limited to a predetermined value by setting the appropriate
bits IL0 and IL1 (Reg#8.4.5) according to the table #4.
18/64
CURRENT CONTROL
2 Toggles
3 Toggles
Reg#10.5
1 Toggle
Zero Cross Time
10ms
20ms
40ms