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A3955SLB-T Datasheet(PDF) 9 Page - Allegro MicroSystems |
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A3955SLB-T Datasheet(HTML) 9 Page - Allegro MicroSystems |
9 / 13 page Full-Bridge PWM Microstepping Motor Driver A3955 9 Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com Mixed Current-Decay Mode. If VPFD is between 1.1 V and 3.1 V, the device will be in a mixed current-decay mode. Mixed-decay mode allows the user to achieve good current regulation with a minimum amount of ripple current and motor/driver losses by selecting the minimum percentage of fast decay required for their application (see also the Stepper Motor Applications section). As in fast current-decay mode, mixed-decay starts with the sink and source drivers disabled after the load current reaches ITRIP. When the voltage at the RC terminal decays to a value below VPFD, the sink drivers are re-enabled, placing the device in slow current-decay mode for the remainder of the fixed off-time (figure 2). The percentage of fast decay (PFD) is user determined by VPFD or two external resistors. PFD = 100 ln (0.6[R1+R2]/R2) where: DEP 062 PFD V CC R 2 R 1 Fixed Off-Time. The internal PWM current-control circuitry uses a one shot to control the time the driver(s) remain(s) off. The one-shot off-time, tOFF, is determined by the selection of an external resistor (RT) and capacitor (CT) connected from the RC timing terminal to ground. The off-time, over a range of values of CT = 470 pF to 1500 pF and RT = 12 kΩ to 100 kΩ, is approximated by: tOFF ≈ RTCT. When the load current is increasing, but has not yet reached the sense-current comparator threshold (ITRIP), the voltage on the RC terminal is approximately 0.6VCC. When ITRIP is reached, the PWM latch is reset by the current-sense comparator and the voltage on the RC terminal will decay until it reaches approximately 0.22VCC. The PWM latch is then set, thereby re-enabling the driver(s) and allowing load current to increase again. The PWM cycle repeats, maintaining the peak load current at the desired value. With increasing values of tOFF, switching losses will decrease, low-level load-current regulation will improve, EMI will be reduced, the PWM frequency will decrease, and ripple current will increase. A value of tOFF can be chosen for optimization of these parameters. For applications where audible noise is a concern, typical values of tOFF are chosen to be in the range of 15 to 35 μs. RC Blanking. In addition to determining the fixed off-time of the PWM control circuit, the CT component sets the comparator blanking time. This function blanks the output of the current- sense comparator when the outputs are switched by the internal current-control circuitry (or by the PHASE input, or when the device is enabled with the DAC data inputs). The comparator output is blanked to prevent false over-current detections due to reverse recovery currents of the clamp diodes, and/or switching transients related to distributed capacitance in the load. During internal PWM operation, at the end of the tOFF time, the comparator’s output is blanked and CT begins to be charged from approximately 0.22VCC by an internal current source of approximately 1 mA. The comparator output remains blanked until the voltage on CT reaches approximately 0.6VCC. The blanking time, tBLANK, can be calculated as: tBLANK = RTCT ln (RT/[RT – 3 kΩ]). When a transition of the PHASE input occurs, CT is discharged to near ground during the crossover delay time (the crossover delay time is present to prevent simultaneous conduction of the source and sink drivers). After the crossover delay, CT is charged by an internal current source of approximately 1 mA. The comparator output remains blanked until the voltage on CT reaches approximately 0.6VCC. Similarly, when the device is disabled, via the DAC data inputs, CT is discharged to near ground. When the device is re-enabled, CT is charged by an internal current source of approximately 1 mA. The comparator output remains blanked until the voltage on CT reaches approximately 0.6VCC. The blanking time, tBLANK, can be calculated as: tBLANK = RTCT ln ([RT - 1.1 kΩ]/RT - 3 kΩ). The minimum recommended value for CT is 470 pF ± 5 %. This value ensures that the blanking time is sufficient to avoid false trips of the comparator under normal operating conditions. For optimal regulation of the load current, this value for CT is recommended and the value of RT can be sized to determine tOFF. |
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