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A3951 Datasheet(PDF) 5 Page - Allegro MicroSystems |
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A3951 Datasheet(HTML) 5 Page - Allegro MicroSystems |
5 / 12 page 3951 FULL-BRIDGE PWM MOTOR DRIVER 5 www.allegromicro.com FUNCTIONAL DESCRIPTION INTERNAL PWM CURRENT CONTROL DURING FORWARD AND REVERSE OPERATION The A3951SB/SW contain a fixed off-time pulse-width modulated (PWM) current-control circuit that can be used to limit the load current to a desired value. The value of the current limiting (I TRIP) is set by the selection of an external current sensing resistor (R S) and reference input voltage (V REF). The internal circuitry compares the voltage across the external sense resistor to one tenth the voltage on the REF input terminal, resulting in a function approximated by I TRIP = VREF/(10•RS). In forward or reverse mode the current-control cir- cuitry limits the load current. When the load current reaches I TRIP, the comparator resets a latch to turn off the selected sink driver. The load inductance causes the current to recirculate through the source driver and flyback diode (two-quadrant operation or slow decay). See figure 1. Figure 1 — Load-Current Paths The user selects an external resistor (R T) and capaci- tor (C T) to determine the time period (toff = RT•CT) during which the drivers remain disabled (see “RC Fixed OFF Time” below). At the end of the R TCT interval, the drivers are re-enabled allowing the load current to increase again. The PWM cycle repeats, maintaining the load current at the desired value (see figure 2). Dwg. EP-006-9 R S BB V DRIVE CURRENT RECIRCULATION ENABLE LOAD CURRENT RC I TRIP Dwg. WP-015-3 Figure 2 — Load-Current Waveform INTERNAL PWM CURRENT CONTROL DURING BRAKE MODE OPERATION The brake circuit turns off both source drivers and turns on both sink drivers. For dc motor applications, this has the effect of shorting the motor’s back-EMF voltage, resulting in current flow that brakes the motor dynamically. However, if the back-EMF voltage is large and there is no PWM current limiting, then the load current can increase to a value that approaches a locked rotor condition. To limit the current, when the I TRIP level is reached, the PWM circuit disables the conducting sink driver. The energy stored in the motor’s inductance is then discharged into the load supply causing the motor current to decay. As in the case of forward/reverse operation, the drivers are re-enabled after a time given by t off = RT•CT (see “RC Fixed Off Time” below). Depending on the back-EMF voltage (proportional to the motor’s decreasing speed), the load current again may increase to I TRIP. If so, the PWM cycle will repeat, limiting the load current to the desired value. Brake Operation During braking, the peak current limit defaults inter- nally to a value approximated by I TRIP = 1.5 V/RS. In this mode, the value of R S determines the ITRIP value independent of V REF. This is useful in applications with differing run and brake currents and no practical method of varying V REF. |
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