AN1139 STMicroelectronics, AN1139 Datasheet - Page 21

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

AN1139 APPLICATION NOTE
4.3.2.2 Slew Rate Control
A 3-phase motor appears as an inductive load to the power supply. The power supply sees a disturbance when
one motor phase turns OFF and another turns ON because the DMOS turn-off time is much shorter than the
L/R rise time. Furthermore, the turn-off of the DMOS can even cause current recirculation back into the supply.
However, the need for a snubber circuit can be eliminated by controlling the turn-OFF time of the DMOS. The
rate at which the upper and lower drivers turn ON and OFF is programmable via an external resistor, Rslew,
connected to the PWM/SLEW pin (#3). This resistor define an internal current source that is utilized to limit the
voltage slew rate at the outputs during transition, thus minimizing the load change that the power supply sees.
To insure proper operation, the range of the resistor value indicated should not be exceeded.
If a relatively low value of Rslew is selected, the resultant fast Slew Rate will result in increasing commutation
cross-over current, higher EMI and large amount of commutation current.
Higher values of Rslew result of course in slow Slew Rate at the outputs which is, under most conditions, the
desired case since the problems associated with fast rates are reduced. The additional advantage is lower
acoustical noise. Problems can occur though if the Slew Rate for a given application is too slow. The problem
manifest itself as the motor begins to spin-up. At lower RPMs, the Bemf of the motor is relatively small resulting
in higher amounts of commutation current. The excessively long Slew Rate may exceed the mask period and
the commutation spike can be detected as a zero cross.
With a high value of Rslew, also the Power Dissipation needs to be considered. Following is the formula to cal-
culate the intended Slew Rate Resistor in Linear mode.
4
3 10
10
•
9.
------------------------------ -
Spindle Output Slew Rate Resistror - Rslew ( )
Slew Rate
4
3 10
10
Example for Slew Rate = 6V/ S :
Rslew
50K
=
------------------------------ -
=
6
4.3.3 Pwm Current Control
The PWM mode of operation for the Spindle driver is activated by a ”1” on the Spindle Control register Reg
#2.6 bit.
In the switch mode current control operation, only the high side drivers are pulse width modulated to control the
current in the motor. The low side drivers are fully switched ON or OFF according to their commutation se-
quence timing. The PWM control of the high side drivers is achieved by a constant TOFF single shot circuit.
Constant TOFF control operates on the principle of monitoring the motor current and comparing it to a reference
or control level (FLL). When the motor current reaches this commanded level, the output drivers turn OFF and
remain OFF for a constant time. After this time the drivers turn back ON to repeat the cycle. Referring to Figure
#8, the current in the motor windings is monitored via the voltage dropped in the sensing resistor, Rsense. This
voltage is multiplied by a factor of four in the Current Sense Amplifier and sent to the negative input of a com-
parator. The control voltage (FLL), is applied to the positive input of the comparator. When the output of the
Current Sense Amplifier reaches a level that is equal to the commanded level, the output of the comparator
switches low resetting the Q output of the F/F.
This causes the upper driver to turn OFF and through -Q (which will go high) turn off the PMOS switch allowing
the capacitor (Coff) to discharge through Roff, initiating then the Constant TOFF time.
When the voltage on Coff reaches 1.2V, the F/F will be set again causing the upper drive to turn ON and the
Coff capacitor itself to be quickly recharged.
The Figure #8 shows the Current Control Loop in PWM mode.
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