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A4934 Datasheet(PDF) 6 Page - Allegro MicroSystems |
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A4934 Datasheet(HTML) 6 Page - Allegro MicroSystems |
6 / 9 page Three-Phase Sensorless Fan Driver A4934 5 Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com the commutation delay period, a CDCOM is asserted, starting the Blank timer. The Blank signal disables the BEMF detector so the comparator is not active during the commutation transients. The next zero crossing, detected on the tri-stated output, causes FCOM to go high. This triggers the Commutation Delay timer and the sequence repeats. Startup At startup, commutations are provided by an onboard oscillator. These commutations are part of the startup scheme, to step the motor to generate BEMF until legitimate BEMF zero crossings are detected and normal BEMF sensing commutation is achieved. Until an appropriate number of FCOM pulses are achieved (96), 100% PWM will be applied to the motor windings. Standby Mode Driving PWM low for 500 μs causes the IC to enter a low power standby mode. Lock Detect Valid FCOM signals must be detected to ensure the motor is not stalled. If a valid FG is not detected for 2 s, the outputs will be disabled for 5 s before an auto-restart is attempted. FG Output The FG output provides fan speed information to the system. FG is an open drain output. PWM Input The duty cycle applied to the PWM pin is translated directly to an average duty cycle applied across the motor windings to control speed. • For voltage controlled applications, where VBB controls the speed, PWM can be left open circuit. PWM is internally pulled- up to logic high level. • PWM also can be used as a control input to start and stop the motor. • For PWM applications, input frequencies in the range 15 to 30 kHz are applied directly to the motor windings. If the PWM duty cycle is very small, then the IC will apply a mini- mum pulse width of typically 6 μs. This minimum pulse width effects the minimum speed. As a result of having a minimum pulse width, the IC can startup and operate down to very short duty cycles. SLEW Input Controls the level of soft switching: SLEW Pin Connection Soft Switch Status GND Less Open More FC Input This is the logic input to set force commutation time at startup, by connection as follows: FC Pin Connection Startup Commutation Time (ms) GND 100 VBB 50 Open 200 Overcurrent Protection If needed, a sense resistor can be installed to limit current. (See Applications Information section for more details.) The current limit trip point would be set by: IOCL = 200 mV / RS . When the trip point is reached, if the threshold voltage, VOCL, is exceeded, the drivers will be disabled for 25 μs. |
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