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A3932SEQTR-T Datasheet(PDF) 10 Page - Allegro MicroSystems |
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A3932SEQTR-T Datasheet(HTML) 10 Page - Allegro MicroSystems |
10 / 13 page 3932 THREE-PHASE POWER MOSFET CONTROLLER www.allegromicro.com 9 Applications Information (cont’d) Current Control. Internal fixed off-time PWM circuitry is implemented to limit load current to a desired value. When a high-side and low-side MOSFET are turned on, current will increase in the motor winding until it reaches a value given by ITRIP ≈ VREF/RS. At the trip point, the sense comparator resets the source- enable latch, turning off the high-side driver. Load inductance causes the current to recirculate (decay) for the fixed off time. The current path during recirculation is determined by the configuration of the MODE and SR inputs. An external resistor (RT) and capacitor (CT), connected in parallel from the RC terminal to AGND, are used to set the fixed off-time period (toff = RT x CT). RT should be in the range of 10 k Ω to 500 kΩ. The t off should be in the range of 10 µs to 50 µs. Larger values for t off can result in audible noise problems. Torque control can be implemented by varying the REF input voltage as long as the PWM input stays high. If direct control of the torque/current is desired by PWM input, a voltage can be applied to the REF input to set an absolute maximum current limit. PWM Blank. The capacitor (CT) also serves as the means to set the blank time duration. At the end of the PWM off cycle, a high-side gate selected by the commutation logic will turn on. At this time, large current transients can occur during the reverse recovery time (trr) of the intrinsic body diodes of the external power MOSFETs. To prevent false tripping of the current-sense comparator, the blank function disables the comparator for a time tblank = 1.9 x CT/(0.001 - [2/RT]) The user must ensure that CT is large enough to cover the current-spike duration. Braking. The A3932 will dynamically brake by forcing all low-side MOSFETs on and all high-side MOSFETs off. This will effectively short-circuit the BEMF and brake the motor. During braking, the load current can be approximated by: IBRAKE = VBEMF/RL Because the load current does not flow through the sense resistor during a dynamic brake, care must be taken to ensure that the power MOSFET’s maximum ratings are not exceeded. RESET = 1 overrides BRAKE and turns all motor bridge FETs off, coasting the motor. Low-Voltage Operation. Although VREG can be connected to VBB for 12 V systems, the VREG maximum rating of 15 V must be observed including transients. If transients cannot be adequately controlled, use VREG in the regulator mode (not connected to VBB). With VBB less than 18 V, the VREG output voltage level specification may not be met. Note that in this mode the VREG undervoltage threshold may leave the system with little headroom if VBB is less than 12 V. Driving an H Bridge. The A3932 may be used to drive an H bridge (e.g., a brush dc motor load) by hard wiring one state for the Hall inputs (e.g., H1 = H2 = 1 (HIGH), H3 = 0 (LOW)). Leave the appropriate phase driver outputs floating (in this case CC, GHC, SC, and GLC because, from the Commutation Truth Table, SC = Z). The DIR input controls the motor rotation while the PWM, MODE, and SR inputs control the motor current behavior as described in the Input Logic table. Layout. Careful consideration must be given to PCB layout when designing high-frequency, fast-switching, high-current circuits. 1) The analog ground (AGND), the power ground (PGND), and the high-current return of the external MOSFETs (the negative side of the sense resistor) should return separately to the negative side of the motor supply filtering capacitor. This will minimize the effect of switching noise on the device logic and analog reference. 2) Minimize stray inductances by using short, wide copper runs at the drain and source terminals of all power MOSFETs. This includes motor lead connections, the input power buss, and the common source of the low-side power MOSFETs. This will minimize voltages induced by fast switching of large load currents. 3) Kelvin connect the SENSE terminal PC trace to the positive side of the sense resistor. |
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