AN1139 STMicroelectronics, AN1139 Datasheet - Page 7

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

2.0 AUXILIARY CIRCUITS
2.1
The two power supply monitor pins, TR_5V pin #35 and TR_12V pin #40, independently monitor the VDD and
main supply VCC by means of external resistor dividers. The threshold of the comparators is set internally by
the band-gap reference (2V) and hysteresis is 100mV. The ratio of the external resistors set the rising trip point.
At Power-Up, PORB output pin (#34) remain low until the POR delay capacitor at POR_DELAY pin (#36) is
charged. It goes low whenever either monitor input goes below the threshold . If power returns to normal during
a retract sequence, the retract will finish before the POR delay can begin, which means that the PORB (pin #34)
output will remain low until the end of the retract plus the PORB delay time. For POR_delay, the charge current
is 2 A and the threshold is 3V. POR delay capacitor is calculated as follow:
•
Example with Por Delay Time=150milli seconds :
The bits required for VCM parking are not reset by PORB and they must be always written (Reg #5.3, Reg #8.2
and Reg #9.0.1.3.6). All the other bits are reset whenever PORB is low. As long as PORB is low the Spindle
outputs are in brake by means of turning on the Low Side Dmos. In case of Power Down with the Spindle pre-
viously spinning, at the falling edge of PORB, the Spindle outputs tristate for BEMF rectification and the Voice
Coil initiate the retract phase according to the time and voltage set. After the retract time, the Spindle outputs
brake. The complete Power Off Sequence is depicted in Figure #16 in chapter 5 (Voice Coil Circuit), section
5.3 (Retract).
2.2
The Thermal Warning/Shutdown circuit senses the temperature of the die. When the temperature reaches the
thermal warning threshold, the “THERMAL_WARN” bit will be reset (=0). If the temperature rises to 25 C higher
than the thermal warning threshold, then “THERMAL” bit will be reset (=0). At this time, the Spindle outputs are
tristated and Voice Coil will retract. After retract, also Voice Coil outputs will be tristated. The registers are not
reset by Thermal Shutdown. The Spindle and VCM will be tristated as long as the internal Thermal Shutdown
signal is active. To release this signal, the die temperature has to decrease and RUN bit (Reg #2.3) or VCM_EN
bit (Reg #9.5) has to be toggled.
The “THERMAL” has approximately 60 C of hysteresis, so that the “THERMAL” bit will not return to normal until
the chip has cool to 60 C below the rising “THERMAL” trip point. (“THERMAL” and “THERMAL_WARN” are
in Reg #7.0.1).
2.3
Two external pins (CP pin 23, CS pin 24) are used. A 47nF capacitor and a diode (in series) is connected be-
tween CP and CS. A storage capacitor is connected from CP to VCC (12V). Another diode is connected be-
tween VCC and the 47nF capacitor. The voltage at the CP pin is called Vboost. An internal oscillator is running
at 200KHz. This can be checked at CS pin. The Vboost Voltage is typical VCC+5.7V. It is possible to stop the
oscillator by setting the VB/DIS bit (Reg#10.7). However these are used mainly for testing purpose. The internal
oscillator is not stopped by POR condition and it is keeping running for all the retract time as long as the VCC
is greater than 2V. The current that CP pin requires is about 3mA with both Spindle and VCM in Linear mode
operation and it is about 5mA with Spindle and VCM in PWM mode operation.
SUPPLY MONITORS AND POWER-ON RESET
THERMAL WARNING / SHUTDOWN
CHARGE-PUMP
1.
------------------- -
1.5
Time
10
6
POR Delay Capacitor (nF)
Ca pacitor
=
150
----------------------- -
1.5
10
10
–
6
AN1139 APPLICATION NOTE
3
=
100 nF
.
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