AN1139 STMicroelectronics, AN1139 Datasheet - Page 35

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

5.4
A calibration mode can be enabled through the serial port by setting VCM_CAL bit (Reg#0.7) which allows the
user to determine the offset of the Sense Amplifier. This is accomplished by enabling all of the VCM control cir-
cuits except for the output drivers. This means that offset of the Isense amplifier can be read directly on the
VCM_CAL pin (#22). The output offsets is calculated by programming zero current in the normal operation mode
(VCM_CAL bit=0) and measuring the output of the VCM_CAL. By adjusting the DAC bits, monitoring in the
mean time the VCM_CAL pin until the voltage on that pin reach the same value previously measured (when
the VCM_CAL bit was set (=1)), the DAC value obtained will represent the real zero current reference. The VR
bit (Reg #9.2) needs to be set prior to start the offset calibration. This allow to reference the calibration amplifier
to a very precise voltage (2V). The voltage on VCM_CAL pin during calibration is 2V +/- offset. Figure #17
shows the Vcm Calibration circuit.
Following is the step by step procedure to calibrate the Voice Coil offset.
1) Enable Voice Coil Circuits by setting VCM_EN bit (=1) (Reg#9.5).
2) Apply a reference voltage to the Calibration Amplifier by setting VR bit (=1) (Reg#9.2).
3) Set VCM_CAL bit (=1) (Reg#0.7). Control circuits are ON but power drivers are OFF.
4) Use an external ADC to measure the Voltage on pin VCM_CAL as Voffset.
5) Reset VCM_CAL bit (=0) (Reg#0.7).
6) Adjust DAC output through VCM DAC register (Reg#0 and Reg#1) until V(VCM_CAL) =Voffset.
7) Save the DAC value as a zero current reference.
Figure 17. Voice Coil Offset Calibration
5.5
The Voice Coil is a class AB, H bridge type power driver with all power devices internal to the device. The H
bridge consist of four NMOS power transistor built by a special low Rdson DMOS power structure. The 12 volts
power supply is connected to the H bridge via an external power NMOS device. The H bridge returns to ground
via VCM_GND pin (28).
OFFSET CALIBRATION
POWER STAGE
DAC
ERROR_IN
Pin 39
Ri
Rf
Cc1
VCC/2
VR
Cc2
ERROR_OUT
-
+
Pin 38
AMPLIFIER
Rc
ERROR
Reg#9.2
SENSE_OUT
Pin 37
Vx
VCC/2
16K
16K
A-
A+
16K
SENSE_IN+
G=4
Pin 27
+
-
PIN 26
VCM_A-
PIN 30
VCM_A+
16K
SENSE AMPLIFIER
-
+
CALIBRATION
AMPLIFIER
AN1139 APPLICATION NOTE
Rs
Rm
Lm
SENSE_IN-
Pin 22
VCM_CAL
Pin 29
Motor
35/64