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A3951SW Datasheet(PDF) 6 Page - Allegro MicroSystems |
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A3951SW Datasheet(HTML) 6 Page - Allegro MicroSystems |
6 / 12 page 3951 FULL-BRIDGE PWM MOTOR DRIVER 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 6 Choosing a small value for R S essentially disables the current limiting during braking. Therefore, care should be taken to ensure that the motor’s current does not exceed the absolute maximum ratings of the device. The braking current can be measured by using an oscilloscope with a current probe connected to one of the motor’s leads. CAUTION: Because the kinetic energy stored in the motor and load inertia is being converted into current, which charges the V BB supply bulk capacitance (power supply output and decoupling capacitance), care must be taken to ensure the capacitance is sufficient to absorb the energy without exceeding the voltage rating of any devices connected to the motor supply. RC Fixed Off Time The internal PWM current control circuitry uses a one shot to control the time the driver remains off. The one shot time, t off (fixed off time), is determined by the selection of an external resistor (R T) and capacitor (CT) connected in parallel from the RC terminal to ground. The fixed off time, over a range of values of C T = 820 pF to 1500 pF and RT = 12 k Ω to 100 kΩ, is approximated by t off = RT•CT. When the PWM latch is reset by the current compara- tor, the voltage on the RC terminal will begin to decay from approximately 3 volts. When the voltage on the RC terminal reaches approximately 1.1 volts, the PWM latch is set, thereby re-enabling the driver. RC Blanking In addition to determining the fixed off time of the PWM control circuit, the C T component sets the compara- tor blanking time. This function blanks the output of the comparator when the outputs are switched by the internal current control circuitry (or by the PHASE, BRAKE, or ENABLE 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 off time, the comparator’s output is blanked and C T begins to be charged from approximately 1.1 V by an internal current source of approximately 1 mA. The comparator output remains blanked until the voltage on C T reaches approxi- mately 3.0 volts. Similarly, when a transition of the PHASE input occurs, C T 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, C T is charged by an internal current source of approximately 1 mA. The comparator output remains blanked until the voltage on C T reaches approximately 3.0 volts. Similarly, when the device is disabled via the ENABLE input, C T is discharged to near ground. When the device is re-enabled, C T is charged by the internal current source. The comparator output remains blanked until the voltage on C T reaches approximately 3.0 V. For most applications, the minimum recommended value is C T = 820 pF ±5 %. This value ensures that the blanking time is sufficient to avoid false trips of the com- parator under normal operating conditions. For optimal regulation of the load current, the above value for C T is recommended and the value of R T can be sized to deter- mine t off. For more information regarding load current regulation, see below. LOAD CURRENT REGULATION WITH THE INTERNAL PWM CURRENT-CONTROL CIRCUITRY During operation, the A3951S– have a lower limit to the range of PWM current control. This directly relates to the limitations imposed by the V REF input (2.0 V, minimum). Applications requiring a broader or full range ( ≈0% to 100%) should utilize the A3952S–, which are recom- mended for the improvements they bring to new designs. LOAD CURRENT REGULATION WITH EXTERNAL PWM OF THE PHASE OR ENABLE INPUTS The PHASE or ENABLE inputs can be pulse-width modulated to regulate load current. Typical propagation delays from the PHASE and ENABLE inputs to transitions of the power outputs are specified in the electrical charac- teristics table. If the normal PWM current control is used, then the comparator blanking function is active during phase and enable transitions. This eliminates false tripping of the over-current comparator caused by switch- ing transients (see “RC Blanking” above). |
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