AN1139 STMicroelectronics, AN1139 Datasheet - Page 22

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

AN1139 APPLICATION NOTE
Figure 8. Spindle Pwm Current Control Loop
4.3.3.1 Constant OFF Time
The timing of the single shot circuit can be programmed by an external RC network at PWM/SLEW pin (#3).
The constant OFF time can be calculated as follow:
•
Example for Toff = 10 S and Roff = 100K (Typ.):
4.3.3.2 Minimum ON Time
In a Spindle PWM current control system, the chopping in a commanded phase causes a swinging also in the
floating phase. For this reason the BEMF zero cross signal is masked most of the time. The BEMF detection is
possible by opening a small window in which the floating phase is monitored.
The window will be opened 6.4 S (based on 20MHz SYS_CLK) after the turn ON of the driver and it will be
closed when the zero cross occurs or at the turn OFF of the driver. In the case of minimum ON time less than
6.4 S, the window will be opened only at the point where the output is about to be turned OFF and then it will
be closed right away.
Since the noise caused by the turning ON of the high side driver can be detected as a false zero cross, the
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FLL
10.
0.69 R off C off
COMPARATOR
+
-
Coff
CONSTANT TOFF STAGE
R
Spindle PWM Constant OFF Time - Toff ( S)
Q
S
F/F
-Q
PWM/SLEW
PIN#3
Roff
PMOS
SWITCH
2.5V
SEQUENCER
Coff
=
G=4
10
-------------------- - 100
SENSE
OPAMP
0.69
POWER STAGE
G
CURRENT SENSE STAGE
10
–
A
6
HIGH SIDE
LOW SIDE
ISENSE
PIN#5
10
VCC
B
3
Rsense
=
VCC
PINS#13,43
RSENSE
PINS#6,9
150pF
C
.
SPINDLE
MOTOR