TDA5345HT Datasheet, PDF (3/44 Pages) NXP Semiconductors – 5 V spindle & VCM driver combo

Philips Semiconductors
5 V spindle & VCM driver combo
Preliminary specification
TDA5345HT
GENERAL DESCRIPTION
The TDA5345HT is a combination of a voice coil motor and a spindle motor driver, designed for 5 V high performance
portable small form factor hard disk drives. Communications with the micro-controller take place through a 16-bit 3-wire
uni-directional serial port. Power dissipation is a major concern in portable drives, therefore each main function can be
individually put in sleep mode when it is not used, to save as much power as possible. The serial port and the power
monitor are the only functions which remain always active.
The TDA5345HT integrates a spindle driver and the commutation logic that drives a three-phase brushless, sensorless
DC motor in full wave mode. Commutations are generated from the internal back-EMF sensing circuitry from start-up to
the running mode. An internal speed loop combining FLL and PLL technics makes sure that the spindle reaches the right
speed, programmed through the serial port. The 6-bit DAC is used to limit the Start-up current by limiting the voltage on
the speed loop filter. To reduce acoustical noise and current noise on both power supply and ground, the fly-back pulse
leading edge slew-rate is controlled. 4 different slope values are programmable.
To prevent internal parasitic effects, positive and negative fly-back pulses are clamped by the output power transistors
themselves: lower NMOS transistors are turned ON to limit the negative fly-back pulses just below ground while upper
PMOS limit the positive fly-backs just above the power supply. This active limitation is still active at power down during
the VCM retract. In this way, an efficient back-EMF rectification is obtained (no diodes losses).
The VCM driver is a linear transconductance amplifier; it is a true AB class with a 8 mA quiescent current. It means that
there is absolutely no crossover distortion. An external compensation network is used to set the loop bandwidth and
ensure the loop stability. With common VCM characteristics, the bandwidth can go up to 40 kHz. To prevent internal
parasitic effects, positive and negative fly-back pulses are clamped by the output power transistors themselves.
An on-chip 12-bit DAC is used to generate the VCM amplifier input voltage. This a signed converter, with an output range
of [1.25 V ; 1.75 V] when the low gain is selected and an output range of [0.5 V ; 2.5 V] when the high gain is selected.
The all VCM transconductance works then around a 1.5 V reference (available on one pin). It is possible to add an
external notch filter between the 12-bit DAC output and the VCM loop intput. An other 6-bit DAC is used to cancel the
Vcm loop offsets. An additional Vcm back-EMF amplifier is provided to monitor the actuator speed when ramp loading.
A ramp unload circuitry is included as well. It can be activated through the serial bus (SoftRetract) or automatically
initiated in case of temperature shut down or at power-down. The ramp unload sequence is made of 3 steps : brake, slow
retract and then full power. The retract steps duration is set by means of internal programmable counters, clocked by the
spindle back-EMF. In case of power down, this sequence is followed by a spindle brake. The three spindle lower power
NMOS are switched fully ON together.
The linear 3.3 V DC-DC converter is designed to drive an external power NPN that will supply the 3.3 V chips. It can be
enabled or disabled by hardware, using the external Reg3v3On pin.
The switched capacitor -3 V regulator is designed to supply a very clean negative voltage to the PreAmp IC in the drive.
The shock sensor amplifier is intended to be connected to an external 1 axis shock sensor. The window comparator
threshold is programmable through the serial bus.
An internal circuitry provides either an analog or a digital information about the junction temperature. These two
informations can be selected through the serial bus. When the analog output is selected, the voltage is proportional to
the internal chip temperature. When the digital output is selected, it indicates that the temperature exceeds 145 °C. An
internal thermal shut down mode is initiated when the temperature is higher than 160 °C: the 3 spindle outputs are
disabled while the vcm is immediately retracted.
The power monitor circuitry monitors the 5V power supply. The Power On Reset PORN output is driven low when the
5 V supply is below 4.1 V. This threshold can be changed by an external resistor divider. Once the power supplies is
above its threshold, the Power On Reset output goes high after a delay that is set by an external capacitor. A second
output, called Power Fault (active HIGH), indicates that the 5 V power supply is below 4.2 V when high. There is no delay
between the supply crossing the threshold and the PowerFault output change.
1999 June 10
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