AN1139 STMicroelectronics, AN1139 Datasheet - Page 31

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

5.0
5.1
A complete Voice Coil control circuit plus 14 bit DAC and DMOS full bridge drivers are integrated in the chip.
The class AB power stage which do not require snubber networks for load compensation, yet maintaining high
accuracy on gain and offset parameter, allows a well controlled quiescent current, therefore maximum precision
in the positioning of the head actuator. The reference voltage for the system is VCC/2 to allow symmetrical use
of the available power supply. The DAC command input to the actuator is a voltage centered around VCC/2 that
swings ±1V.
5.2
The current in the actuator is sensed by a small resistor (typ. 250m ) in series to the coil. The voltage drop
across the resistor is amplified by a differential amplifier with a gain of four. The output voltage of this amplifier
is proportional to the current in the coil. This voltage and the DAC voltage summed at the input of the error
amplifier, represent the difference between the desired current and the actual motor current. The output of the
error amplifier is the signal that will drive the power stage according to the driving mode selected. Two dif-
ferent modes are available, Linear mode and Phase Shift Modulation
Mode (PSM). By changing the Reg#0.6 bit, the selection of the two modes can be done any time regardless
the Voice Coil motor is moving or not.
The Model and a Mathcad analysis for the VCM Current Control Loop can be found in chapter 8 (Appendix),
section 8.1.3 (Voice Coil Current Control loop).
5.2.1 Linear Control Loop
The Linear control loop is selected by a ‘0’ in the Reg#0.6 bit. The complete system is described in Figure #13.
Figure 13. Voice Coil Linear Control Loop
The loop consists of a 14 bit DAC, error amplifier referenced to VCC/2, fixed gain (A=4) differential input current
sense amplifier and positive and negative power output amplifiers (referenced to VCC/2). The output of the cur-
rent sense amplifier (which is referenced to VCC/2) and the output of the DAC are summed with external resis-
tors at the input of the error amplifier. The error amplifier allows the bandwidth of the VCM to be compensated
with an external RC filter and drives the positive and negative power amplifiers. The transconductance of the
DAC
VOICE COIL CIRCUITS
DESCRIPTION
CURRENT CONTROL
ERROR_IN
Pin 39
Ri
Rf
Cc1
Cc2
-
+
AMPLIFIER
Rc
ERROR
VCC/2
ERROR_OUT
Pin 38
Vx
2K
-
+
16K
A-
SENSE_OUT
Pin 37
SENSE AMPLIFIER
PIN 26
VCM_A-
G=4
Rm
-
+
Motor
SENSE_IN-
Pin 29
Lm
AN1139 APPLICATION NOTE
Rs
Pin 30
VCM_A+
SENSE_IN+
Pin 27
14K
A+
+
-
2K
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