AN1139 STMicroelectronics, AN1139 Datasheet - Page 27

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

4.4.2 External Frequency Feedback
An INDEX pin (#11) is provided for external Speed Loop Feedback frequency. This pin will allow the user to
control the motor speed with a signal taken from the data read off of the disk itself. This mode is selectable by
setting (=1) the EXT/INT bit (Reg#2.7). The Figure#10 shows the configuration.
4.4.3 Speed Loop Compensation
The Frequency Comparator and Charge Pump generate and error current based on the frequency difference
between a programmed reference frequency. This error current is converted to an error voltage by the loop filter,
which programs the current delivered to the motor through the OTA loop. The Model and a Mathcad analysis
for the Spindle Speed Control Loop can be found in chapter 8 (Appendix), section 8.1.2 (Spindle Speed Control
loop).
4.5
Since no Hall Effect Sensors are required, the commutation information is derived from the BEMF voltage zero-
crossing of the undriven phase with respect to the center tap. The Bemf comparator and associated signal levels
are depicted in Figure #11. For reliable operation, the BEMF signal amplitude should be a minimum of +/- 40mV
to be properly detected. In order to provide for noise immunity, internal hysteresis (typically 15mV) is incorpo-
rated in the detection circuitry to prevent false zero crossing detection.
Figure 11. BEMF AMPLIFIER
4.6
The Figure #12, represent the internal signal timing associated with the Motor Bemf and Output stage.
BEMF DETECTION
DELAY, COMMUTATION & MASK
Sc
Sb
Sa
-
+
VBemf
<= 40mV
AN1139 APPLICATION NOTE
C T A P
OUT A
OUT B
O U T C
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