SBVS206A November 2012 – March 2015 DRV10866
DRV10866 is a 3-phase, sensorless motor driver with integrated power MOSFETs with drive current capability up to 680-mA peak. DRV10866 is specifically designed for low noise, low external component count fan motor drive applications. DRV10866 has built-in overcurrent protection with no external current sense resistor needed. The synchronous rectification mode of operation achieves increased efficiency for motor driver applications. DRV10866 can output either FG or ½ FG to indicate motor speed with open-drain output through FGS pin selection. A 150° sensorless BEMF control scheme is implemented for a 3-phase motor. Voltage surge protection scheme prevents input VCC capacitor from over charge during motor acceleration and deceleration modes. DRV10866 has multiple built-in protection blocks including UVLO, overcurrent protection, lock protection and thermal shutdown protection.
DRV10866 can control motor speed through either the PWMIN or VCC pin. Motor speed will increase with higher PWMIN duty cycle or VCC input voltage. The curve of motor speed (RPM) vs PWMIN duty cycle or VCC input voltage is close to linear in most cases. However, motor characteristics will affect the linearity of this speed curve. DRV10866 can operate at very low VCC input voltage down to 1.65 V. The PWMIN pin is pulled up to VCC internally and frequency range can vary from 15 kHz to 50 kHz. The motor driver MOSFETs will operate at constant switching frequency 156 kHz. With this high switching frequency, DRV10866 can eliminate audible noise and reduce the ripple of VCC input voltage and current, and thus minimize EMI noise.
The FG pin outputs a 50% duty cycle of PWM waveform in the normal operation condition. The frequency of the FG signal represents the motor speed and phase information. The FG pin is an open-drain output, so an external pullup resistor is needed when connected to an external system. During the start-up, FG output will stay at high impedance until the motor speed reaches a certain level and BEMF is detected. During lock protection condition, FG output will remain high until the motor restarts and start-up process is completed. DRV10866 can output either FG or ½ FG to indicate motor status with open-drain output through FGS pin selection. When FGS is pulled to VCC, the frequency of FG output is half of that when FGS is pulled to GND. Motor speed can be calculated based on the FG frequency when FGS is pulled to GND, which equals to:
If the motor is blocked or stopped by an external force, the lock protection is triggered after lock detection time. During lock detection time, the circuit monitors the PWM and FG signals. If PWM has an input signal while the FG output is in high impedance during this period, the lock protection will be enabled and DRV10866 will stop driving the motor. After lock release time, DRV10866 will resume driving the motor again. If the lock condition still exists, DRV10866 will proceed with the next lock protection cycle until the lock condition is removed. With this lock protection, the motor and device will not get over heated or be damaged.
The DRV10866 has a unique feature to clamp the VCC voltage during lock protection and standby mode. If the lock mode condition is caused by an external force that suddenly stops the motor at a high speed, or the device goes into standby mode from a high duty cycle, either situation releases the energy in the motor winding into the input capacitor. When a small input capacitor and anti-reverse diode are used in the system design, the input voltage of the IC could rise above the absolute voltage rate of the chip. This condition either destroys the device or reduces the reliability of the device. For this reason, the DRV10866 has a voltage clamp circuit that clamps the input voltage at 5.95 V, and has a hysteresis of 150 mV. This clamp circuit is only active during the lock protection cycle or when the device enters standby mode. It is disabled during normal operation.
The DRV10866 can adjust the overcurrent point through an external resistor connected to the CS pin (pin 9) and ground. Without this external current sense resistor, the DRV10866 senses the current through the power MOSFET. Therefore, there is no power loss during the current sensing. The current sense architecture improves the overall system efficiency. Shorting the CS pin to ground disables the overcurrent protection feature. During overcurrent protection, the DRV10866 only limits the current to the motor; it does not shut down the device. The overcurrent limit can be set by the value of current sensing resistor through Equation 2.
The DRV10866 has a built-in UVLO function block. The hysteresis of UVLO threshold is 150 mV. The device will be locked out when VCC reaches 1.65 V and woke up at 1.8 V.
The DRV10866 has a built-in thermal shunt down function, which will shut down the device when the junction temperature is over 160°C and will resume operating when the junction temperature drops back to 150°C.
At start-up with motor at standstill, commutation logic starts to drive the motor in open-loop with U-phase high, V-phase low, and the W-phase shut off. During open-loop start-up phase, commutation logic advance to next state automatically as per Table 1 with duty cycle of 100% regardless of PWM input. At each state, commutation logic detects zero-crossing of back-emf at shut-off phase. Once motor reaches to sufficient speed to allow four consecutive successful back-emf zero-crossing, commutation logic switches to closed-loop operation mode as explained in next section.
In certain cases, the motor may have initial speed in forward direction when the device attempts to start-up the motor again. When this occurs, device commutation logic jumps over the open-loop start-up process and goes to closed loop directly. By re-synchronizing to the spinning motor, the user achieves the fastest possible start-up time for this initial condition.
Once open-loop acceleration phase is over, motor steady state speed is determined by applied duty-cycle at PWM input. In this mode, communication logic steps thought the six states mentioned in Table 1 and next commutation state is determined by actual back-emf zero-crossing event at shut-off phase. Each state remains for 150°. This is an advanced trapezoidal method that allows the device to drive the phases gradually to the maximum current and gradually to 0. Commutation logic also provides the required 15° angle-advance from zero-crossing events to efficiently commutate the motor.
For a given duty-cycle input, motor speed can be different depending upon the motor loading conditions. Device provides motor speed information at FG pin which can be used to achieve closed-loop speed control to get constant speed at varying load condition.
Motor can be decelerated gradually by slowly reducing the PWM duty command to avoid overvoltage at DC input. When the device is commanded to decelerate very fast or stop the motor suddenly from high speed, in order to protect the IC and the system, the DRV10866 goes into AVS protection, as explained in Voltage Surge Protection.