Datasheet A3955 (Allegro) - 10

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