DRV11873 Datasheet, PDF (7/16 Pages) Texas Instruments – 12-V, 3-PHASE, SENSORLESS BLDC MOTOR DRIVER

DRV11873
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DETAILED DEVICE DESCRIPTION
SLWS237 – NOVEMBER 2012
DRV11873 is a three phase, sensorless motor driver with integrated power MOSFETs with drive current
capability up to 1.5-A continuous and 2-A peak. It is specifically designed for low noise and low external
component count fan motor drive applications. DRV11873 has built in over current protection with no external
current sense resistor needed. The synchronous rectification mode of operation achieves increased efficiency for
motor driver applications. DRV11873 outputs FG and RD to indicate motor status with open drain output. A 150°
sensorless BEMF control scheme was implemented for a three phase motor. DRV11873 can fit a wide range of
fan motors with the FS pin selection function. Voltage surge protection scheme prevents the input V_CC capacitor
from over charge during motor braking mode. DRV11873 has multiple built in protection blocks including UVLO,
over current protection, lock protection and thermal shut down protection.
SPEED CONTROL
DRV11873 can control motor speed through either the PWM_IN or V_CC pin. Motor speed will increase with higher
PWM_IN duty cycle or V_CC input voltage. The curve of motor speed (RPM) vs PWM_IN duty cycle or V_CC input
voltage is close to linear in most cases. However, motor characteristics will affect the linearity of this speed
curve. DRV11873 can operate at low V_CC input voltage down to 4.1 V. The PWM_IN pin is pulled up to V5
internally and the frequency range can vary from 15 kHz to 100 kHz. The motor driver MOSFETs will operate at
constant switching frequency 125 kHz when the FS pin is pulled high and 62.5 kHz when the FS pin is pulled
low. With this high switching frequency, DRV11873 can eliminate audible noise and reduce the ripple of V_CC
input voltage and current.
FREQUENCY GENERATOR
FG output is a 50% duty square wave output in the normal operation condition. Its frequency represents the
motor speed and phase information. FG pin is an open drian output. An external pull up resistor is needed to
connect external system. During the start up FG output will stay at high impedance until the motor speed reaches
certain level and BEMF is detected. If FG is not used, this pin can be left for floating. FG pin can be tie to either
5 V or V_CC through a pull up resistor. Normally the pull up resistor value can be 100 kΩ or higher. During lock
protection condition, FG output will keep high until the lockout protection is dismissed and restart completed. A
current limit function has been built in for FG pin. It will prevent FG open drain MOSFET get damaged in case
V_CC or 5 V directly connected to FG pin by accident. To calculate RPM based on FG frequency, please refer to
below equation.
RPM =
(FG ? 60)
pole pairs
(1)
Where FG is in hertz (Hz).
FS SETTING
DRV11873 can fit a wide range of fan motors by setting the FS pin. For high speed fan motors with low motor
winding resistance and low inductance, the FS pin should be pulled high. For low speed fan motors with high
motor winding resistance and high inductance, the FS pin should be pulled low. Through FS pin selection,
DRV11873 can be used for wide applications from low speed refrigerator cooling fans to high speed server
cooling fans. FS status can only be set during device power up.
LOCK PROTECTION AND RD OUTPUT
If the motor is blocked or stopped by the external force, the lock protection will be triggered after detection time.
During lock detection time, the circuit monitors FG signal. If the FG output is high during lock detection time, the
lock protection will stop driving the motor. After lock release time, DRV11873 will resume driving the motor. If the
lock condition is still there, DRV11873 will proceed with 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. A different FS
setting will determine a different lock detection and lock release time.
The RD pin is an open drain output which can be tied to either V5 or V_CC through a pull-up resistor. Normally the
pull-up resistor value can be 100 kΩ or higher. During the lock protection condition, RD output will keep high until
the lock protection is dismissed and restart completed. A current limit function has been built in for the RD pin
which prevents the open drain MOSFET from damage in case V_CC or V5 directly connects to the RD pin.
Copyright © 2012, Texas Instruments Incorporated
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