AN1139 STMicroelectronics, AN1139 Datasheet - Page 20

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

AN1139 APPLICATION NOTE
The output current is controlled in a Linear fashion via a transconductance loop. Referring to Figure #6, the
sourcing FET (High Side Dmos) of one phase is forced into full conduction, while the sinking FET (Low Side
Dmos) of an appropriate phase operate as a transconductance element. During a run condition, the current in
the sinking FET, is monitored by a RSENSE resistor. The resulting voltage that appears across the resistor is
amplified by a factor of four by the sense amplifier opamp and is sent to the Error Amplifier Stage where it is
compared to the Current command coming from the FLL circuit. The Error Amplifier circuit along with the Buffer,
provides sufficient drive to the sinking FET in order to maintain the motor speed at the proper level as command-
ed by the Speed Controller (FLL).
4.3.2.1 Transconductance Loop Stability
The RC network connected to the Compensation pin (SPIN_COMP pin#44 - Figure #6) provides a single pole/
zero compensation scheme. The pole/zero locations are adjusted such that a few dB (typ. 20dB) remains in the
transconductance loop at frequencies higher than the zero. The inductive characteristic of the load provides the
pole necessary for loop stability. Thus the loop bandwidth is actually limited by the motor itself.
Figure 7. Spindle Linear Current Loop Bode Diagram
The Bode plot in Figure #7, depicts the normal way to achieve stability in the loop. The pole and the zero are
used to set a gain of 20dB at a higher frequency and the pole of the motor cuts the gain to achieve stability.
Loop instability may be caused by two factor:
1) The motor pole is too close to zero. The zero is not able to decrement the shift of the phase, and when the
2) The motor capacitance itself can interfere with the loop, creating double poles. If the gain at higher frequen-
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effect of the motor pole is present, the phase shift may reach 180 and the loop will oscillate. To rectify this
situation the pole/zero must be shifted at lower frequencies by increasing the compensation capacitor (Cc).
cies is sufficiently high, the double pole slope of 40dB/decade can cause the phase shift to reach 180 , re-
sulting in oscillation. By leaving the pole unchanged and increasing the zero, the stable response can be
achieved. The Model and a Mathcad analysis for the Spindle Current Control Loop can be found in chapter
8 (Appendix), section 8.1.1 (Spindle Current Control loop).
20dB
Av
RoCc
RcCc
Ro = OTA output impedance
Frequency
Motor Pole