AN1139 STMicroelectronics, AN1139 Datasheet - Page 23

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

minimum ON time has to be chosen large enough to cover those spike. In the other hand has not to be too
large otherwise the motor can not slow down fast when required or it may run at higher undesired speed.
For this reason four different ON times value are available through the serial port, MIN1 and MIN2 bits
(Reg#3.1.2), Those timing are not dependent from the external SYS_CLK signal but are internally generated.
The table #5 shows the timing available.
Table 5. Spindle PWM Minimum ON Time
4.3.3.3 Slew Rate Control
In the PWM current control mode, two different slew rate control are present, the first one during commutation
and the second one during chopping. Both are addressable through the serial port. The COMSLEW bit
(Reg#11.3) will performs slew rate at commutation with ‘0’ for 30V/ S and ‘1’ for 2V/ S. The SSLEW bit
(Reg#8.0) will perform slew rate during chopping with ‘0’ for 10V/ S and ‘1’ for 20V/ S.
4.3.3.4 PWM Design Methodology
The bandwidth of the PWM loop was optimized to reject unwanted switching noise while providing sufficient re-
sponse, commensurate with the switching speed of the output drivers. At higher frequencies the switching loss-
es inherent in the drivers start to negate any of the power dissipation savings gained with PWM operation. At
lower frequencies, less that 20KHz, the switching of the coil may result in the undesirable acoustic range. A
good compromised PWM frequency can be assumed around:
The time that the current flowing into an inductor needs to reach a certain value is equal to a time that it needs
to get from that value to zero. This time is depending from the Inductor (Lm) and the resistor (Rm) values. In
the case of a running motor the BEMF generated by the motor itself at that speed needs to be taken in consid-
eration. Due to the fact that BEMF is opposing to the power supply, the voltage across the motor will be less
(Vmotor = Vpower-Bemf). For this reason the charging and discharging of the inductor will result with different
voltage and so different duration. The ratio to have a good regulation margin will be about:
The device architecture has been designed to work in constant OFF time mode, this means that the system is
using the frequency (changing only the ON time) to adjust the speed. The ratio TON / TOFF provide the motor
with the required amount of current to spin at desired speed.
Assuming that 40KHz chopping frequency is used, at steady speed, the period will be equal to 25 S. For the
previous assumption (2), the TOFF needs to be 30% of 25 S so will be about 7.5 S. In this case the TON will
be equal to 17.5 S (TON=Period -TOFF). If this Ratio can not be achieved, most probably the motor is not
been designed to run at that speed (BEMF provided is too high).
4.4
The rotational position of the motor is inferred from the BEMF waveform generated by the floating coil. The chip
uses the instant of a particular zero-crossing and the period between successive zero crossings to dictate the
(1) Chopping Frequency: 30-40KHz.
(2) Ton / Toff Ratio: TON = 70% - TOFF = 30%
SPEED CONTROL (FLL)
“MIN1”
0
0
1
1
“MIN2”
0
1
0
1
Minimum ON Time
AN1139 APPLICATION NOTE
12.0 S
5.21 S
5.9 S
1.4 S
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