AN1139 STMicroelectronics, AN1139 Datasheet - Page 14

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

AN1139 APPLICATION NOTE
4.1.2.3 Inductive Sense
Since hard drive heads are not designed for back rotation, it is important to know which Spindle phase should
be energized first, in order to avoid back-rotation.
During operation, the electrical phase of the motor can be detected by sensing the BEMF induced in the unen-
ergized winding by the rotor’s magnet in motion, however, when the motor is stopped, detection of the rotor po-
sition is more difficult. For this purpose, the Inductive Sense algorithm has been developed. To get the position
of a stopped three phase permanent magnet rotor, the inductance of each of the six motor commutation phases
is sensed by applying a voltage across each combination of two motor winding and measuring the time required
for the current through the windings to reach a certain arbitrary selected value. The shortest rise time should
correspond to the lowest inductance and this commutation phase should be the one closest to the mechanical
position of the rotor.
Altought no Inductive Sense algorithm is built inside the device, it provides all the hardware functions that inter-
acting with the P can externally perform the Inductive Sense Start-Up. The ISNS bit (Reg#8.3) must first be
set (=1) in order to preset the internal circuit to sense the current in the coils. This allow the FCOM pin (#1) to
become the information signal of threshold reached.
There are four different threshold values which adjust the maximum voltage on sense resistor. The threshold
can be selected by changing the IL0 and IL1 bit in the Reg#8.4.5. The Table #2 shows the available values.
Table 2.
The current that flow into the coil is also a function of the Sense resistor connected to the Rsense pin and it is
calculated as follow:
•
Example with Rsense=0.3 and IL0/IL1/ISNS=1 : Max Coil Current = (0.3 / 0.3) = 1 A.
Figure#4 shows the flow chart of the Inductive sense Start-Up routine. A step by step procedure to build the
Inductive sense Start-Up, can be found in chapter 8 (Appendix), section 8.3 (Inductive sense start-up step by
step).
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“IL0”
4.
0
1
0
1
Spindle Inductive Sense Current Limit
V_ sen se_limit
------------------------------------------
Rsen se
“IL1”
0
0
1
1
Spindle Inductive Sense Maximum Coil Current (Amp)
“ISNS”
1
1
1
1
V_SENSE_LIMIT (+/- 15%)
.15V
.20V
.25V
.30V