AN1139 STMicroelectronics, AN1139 Datasheet - Page 30

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

AN1139 APPLICATION NOTE
4.9
POWER STAGE
The Spindle Power driver consist of a 3 Phase H bridge power transistor. Both the low and the high side drivers
are NMOS transistor. The drivers are built by a special low Rdson DMOS structure. The 12 volts power supply
is connected to the H bridge via an external power NMOS device. The H bridge returns to ground through the
sink pins and via an external resistor. An on chip Boost voltage generator is being used to switch the high side
drivers and the external isolation NMOS as explained in chapter 2 (Auxiliary circuits), section 2.3 (Charge
Pump).
4.10 EXTERNAL ISOFET
An external NMOS device is used to connect the Power Supply to the Spindle and VCM high side Dmos. The
meaning of this device is to isolate the Spindle and VCM power stage from the power supply during retract at
Power Off. This will insure the Bemf voltage available from the Spindle motor to stay inside the chip and thus
providing the necessary voltage to retract the Voice Coil. Without this device, the Bemf voltage will be dis-
charged very fast through the power supply. The external Isofet is turned on upon the POR signal goes HIGH
and is turned off when the POR signal goes low. The high voltage (~17V) to apply to the gate necessary to
turn it on, it is provided by the charge pump circuit and it is available on SW1 pin (33). If no particular care is
needed on the voltage drop of the external ISOFET, it can be substituted with a normal Schottky diode.
4.11 EXTERNAL BEMF PROCESSING OPTION
The external Bemf processing is available by re-setting (=0) the Reg#2.1 bit. At this time the FCOM signal has
to be monitored by the external P as a zero crossing reference. Upon the transition of the FCOM signal, a se-
quencer commutation needs to be programmed after a delay calculated by the P itself. The Delay has to be
calculated as a percentage of the previous period (typical 30 electrical degree equal to half of the previous pe-
riod). The Mask time like in internal Bemf processing is also calculated as a percentage of the previous period,
but it is also performed by the high state of the commutation signal INCRE_SEQ (Reg#2.0 bit). When the
INCRE_SEQ bit is set (=1) the bemf is masked. The actual mask time signal is a combination of the two. The
Figure #19 in the chapter 6 (Application and Tools), section 6.1 (application configurations) depicts this feature.
4.12 EXTERNAL SPEED CONTROL OPTION
The external Speed Control is available through the FLL_FILTER pin (21). By removing the components at-
tached to this pin, the control of the input of the spindle OTA can be reached by an external analog voltage
signal (see Figure #10). The feedback frequency for the external FLL/PLL can be taken from the FCOM pin or
from the data read off of the disk itself. During Start-up, the OTA input is internally forced high until the Speed
reach the value set by the internal FLL counters, then the control is ended over to the charge pump leaving
the FLL_FILTER pin at high impedance. For this reason if the external speed control is chosen, the FLL regis-
ters has to be set to a lower speed than the target one. Another way to perform the external speed control is
through the serial port. This feature can be accomplished by turning ON and OFF the CPH and CPL bits
(Reg#8.6.7, see Figure #10). In this case the external filter components are to be connected. The Figure #20 in
the chapter 6 (Application & Tools), section 6.1 (application configurations) depicts this feature.
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