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A3986 Dual Full-Bridge MOSFET Driver with Microstepping Translator
Features and Benefits
2-wire step and direction interface Dual full-bridge gate drive for N-channel MOSFETs Operation over 12 to 50 V supply voltage range Synchronous rectification Cross-conduction protection Adjustable mixed decay Integrated sinusoidal DAC current reference Fixed off-time PWM current control
Description
The A3986 is a dual full-bridge gate driver with integrated microstepping translator suitable for driving a wide range of higher power industrial bipolar 2-phase stepper motors (typically 30 to 500 W). Motor power is provided by external N-channel power MOSFETs at supply voltages from 12 to 50 V. This device contains two sinusoidal DACs that generate the reference voltage for two separate fixed-off-time PWM current controllers. These provide current regulation for external power MOSFET full-bridges. Motor stepping is controlled by a two-wire step and direction interface, providing complete microstepping control at full-, half-, quarter-, and sixteenth-step resolutions. The fixed-off time regulator has the ability to operate in slow-, mixed-, or fast-decay modes, which results in reduced audible motor noise, increased step accuracy, and reduced power dissipation. The translator is the key to the easy implementation of this IC. Simply inputting one pulse on the STEP input drives the motor one step (full, half, quarter, or sixteenth depending on the microstep select input). There are no phase-sequence tables, high frequency control lines, or complex interfaces to program. This reduces the need for a complex microcontroller.
Package: 38 pin TSSOP (suffix LD)
Approximate size
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Typical Application Diagram
3986-DS, Rev. 2
A3986
Dual Full-Bridge MOSFET Driver with Microstepping Translator
In addition to crossover current control, internal circuit protection provides thermal shutdown with hysteresis and undervoltage lockout. Special power-up sequencing is not required. This component is supplied in an 38-pin TSSOP (package LD). The package is lead (Pb) free, with 100% matte tin leadframe plating.
Description (continued) The above-supply voltage required for the high-side N-channel MOSFETs is provided by a bootstrap capacitor. Efficiency is enhanced by using synchronous rectification and the power FETs are protected from shoot-through by integrated crossover control and programmable dead time.
Selection Guide
Part Number A3986SLDTR-T Packing Tape and reel, 4000 pieces per reel
Absolute Maximum Ratings
Characteristic Supply Voltage Logic Supply Voltage Logic Inputs and Outputs SENSEx pins Sxx pins LSSx pins GHxx pins GLxx pins Cxx pins Operating Ambient Temperature Junction Temperature Storage Temperature TA TJ(max) Tstg Range S Symbol VBB VDD Notes Rating -0.3 to 50 -0.3 to 7 -0.3 to 7 -1 to 1 -2 to 55 -2 to 5 Sxx to Sxx+15 -2 to 16 -0.3 to Sxx+15 -20 to 85 150 -55 to 150 Units V V V V V V V V V C C C
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A3986
Dual Full-Bridge MOSFET Driver with Microstepping Translator
Functional Block Diagram
+5 V
VDD VBB
VMOTOR
VREG Bandgap Regulator CREG VREG Phase 1A Bridge1 CBOOT1A REF VREF High-Side Drive VREG DAC STEP PWM Latch Blanking Mixed Decay Low-Side Drive GL1A LSS1 SENSE1 Phase 1B Low-Side Drive Phase 1 MS2 Phase 1 Control Logic High-Side Drive GH1B CBOOT1B C1B GL1B S1B RSENSE1
P P
C1A
GH1A S1A RGH1A RGH1B
RGL1A
RGL1B
DIR
MS1
Translator PFD1 Phase 2A
VMOTOR
Bridge2
C2A CBOOT2A
PFD2 Phase 2 Control Logic ENABLE
Phase 2
High-Side Drive VREG Low-Side Drive
GH2A S2A RGH2A RGH2B
GL2A LSS2 SENSE2 RGL2A RGL2B
RESET PWM Latch Blanking Mixed Decay
Phase 2B Low-Side Drive GL2B S2B GH2B CBOOT2B C2B
RSENSE2
P
SR
DAC OSC ROSC Protection UVLO TSD VREF High-Side Drive
GND
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A3986
Dual Full-Bridge MOSFET Driver with Microstepping Translator
ELECTRICAL CHARACTERISTICS at TA = 25C, VDD = 5 V, VBB = 12 to 50V, unless noted otherwise Characteristics Supply and Reference Load Supply Voltage Range Load Supply Current Load Supply Idle Current Logic Supply Voltage Range Logic Supply Current Logic Supply Idle Current Regulator Output Bootstrap Diode Forward Voltage Gate Output Drive Turn-On Rise Time Turn-Off Fall Time Turn-On Propagation Delay Turn-Off Propagation Delay Crossover Dead Time Pull-Up On Resistance Pull-Down On Resistance Short-Circuit Current - Source1 Short-Circuit Current - Sink GHx Output Voltage GLx Output Voltage Logic Inputs Input Low Voltage Input High Voltage Input Hysteresis Input Current1 RESET Pulse Width2 Symbol VBB IBB IBBQ VDD IDD IDDQ VREG VfBOOT tr tf tp(on) tp(off) tDEAD ROSC = 10 k, CLOAD = 1000 pF ENABLE = High, outputs disabled RESET = 0 Test Conditions Min. 12 - - - 3.0 - - 11.25 0.6 80 40 - - 0.6 30 14 -140 160 VC - 0.2 VREG - 0.2 - 0.7 VDD 150 -1 0.2 Typ. - - - - - - - - 0.8 120 60 180 180 - 40 19 -110 200 - - - - 300 - - Max. 50 10 6 100 5.5 10 300 13 1 160 80 - - 1.2 55 24 -80 250 - - 0.3 VDD - - 1 1 Units V mA mA A V mA A V V ns ns ns ns s mA mA V V V V mV A s
RESET = 0 IREGInt = 30 mA IfBOOT = 10 mA CLOAD = 1000 pF, 20% to 80% CLOAD = 1000 pF, 80% to 20% ENABLE low to gate drive on ENABLE high to gate drive off ROSC = 10 k, IGH = -25 mA IGL = 25 mA
RDS(on)UP RDS(on)DN ISC(source) ISC(sink) VGHx CBOOTx fully charged VGLx VIL VIH VIHys IIN twR
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A3986
Dual Full-Bridge MOSFET Driver with Microstepping Translator
ELECTRICAL CHARACTERISTICS, continued, at TA = 25C, VDD = 5 V, VBB = 12 to 50V, unless noted otherwise Characteristics Symbol Test Conditions Min. Typ. Max. Units Current Control Blank Time tBLANK ROSC = 10 k, 1.2 1.5 1.8 s ROSC = 10 k, , SR= High 18.12 - 23.16 s Fixed Off-Time tOFF Reference Input Voltage VREF 0.8 - 2 V Internal Reference Voltage VREFInt 20 k to VDD 1.9 2.0 2.1 V Current Trip Point Error3 EITRIP VREF = 2 V - - 5 % Reference Input Current1 IREF -3 0 3 A ROSC = 10 k 3.2 4 4.8 MHz Oscillator Frequency fOSC Protection VREG Undervoltage Lockout VREGUV Decreasing VREG 7.5 8 8.5 V VREG Undervoltage Lockout VREGUVHys 100 200 - mV Hysteresis Decreasing VDD 2.45 2.7 2.95 V VDD Undervoltage Lockout VDDUV VDD Undervoltage Lockout VDDUVHys 50 100 - mV Hysteresis Temperature increasing - 165 - C Overtemperature Shut Down TTSD Overtemperature Shut Down TTSDHys Recovery = TTSD - TTSDHys - 15 - C Hysteresis Control Timing STEP Low Duration tSTEPL 1 - - s STEP High Duration tSTEPH 1 - - s Input change to STEP pulse; 200 - - ns Setup Duration tSU MS1, MS2, DIR Input change from STEP pulse; Hold Duration tH 200 - - ns MS1, MS2, DIR Wake Time Duration tWAKE 1 - - ms 1For input and output current specifications, negative current is defined as coming out of (sourcing) the specified device pin. 2A RESET pulse of this duration will reset the translator to the Home position without entering Sleep mode. 3Current Trip Point Error is the difference between actual current trip point and the target current trip point, referred to full scale (100%) current: EITRIP = 100 x (ITRIPActual - ITRIPTarget) / IFullScale %
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A3986
Dual Full-Bridge MOSFET Driver with Microstepping Translator
THERMAL CHARACTERISTICS
Characteristic Package Thermal Resistance Symbol RJA Test Conditions* 4-layer PCB, based on JEDEC standard 1-layer PCB with copper limited to solder pads Value Units 51 127 C/W C/W
*Additional thermal information available on Allegro Web site.
RESET
tSTEPH tWAKE
STEP
tSTEPL
tSU
MSx, DIR
tH
Figure 1. Logic Interface Timing Diagram
Table 1. Microstep Resolution Truth Table MS2 0 0 1 1 MS1 0 1 0 1 Microstep Resolution Full Step Half Step Quarter Step Sixteenth Step
Table 2. Mixed Decay Selection Truth Table PFD2 0 0 1 1 PFD1 0 1 0 1 Percentage of Fast decay 0% ( Slow Decay) 8% (7 cycles) 26% (23 cycles) 100% ( Fast Decay)
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A3986
Dual Full-Bridge MOSFET Driver with Microstepping Translator
Table 3. Step Sequencing Settings Home microstep position at Step Angle 45; DIR = H
Full Step (#) Half Step (#) 1 1/4 Step (#) 1 1/16 Phase 2 Phase 1 Step Step Current Current Angle (#) (% ITRIP(max)) (% ITRIP(max)) () 1 2 3 4 2 5 6 7 8 1 2 3 9 10 11 12 4 13 14 15 16 3 5 17 18 19 20 6 21 22 23 24 2 4 7 25 26 27 28 8 29 30 31 32 5 9 33 0.00 9.38 18.75 29.69 37.50 46.88 56.25 64.06 70.31 76.56 82.81 87.50 92.19 95.31 98.44 100.00 100.00 100.00 98.44 95.31 92.19 87.50 82.81 76.56 70.31 64.06 56.25 46.88 37.50 29.69 18.75 9.38 0.00 100.00 100.00 98.44 95.31 92.19 87.50 82.81 76.56 70.31 64.06 56.25 46.88 37.50 29.69 18.75 9.38 0.00 -9.38 -18.75 -29.69 -37.50 -46.88 -56.25 -64.06 -70.31 -76.56 -82.81 -87.50 -92.19 -95.31 -98.44 -100.00 -100.00 0.0 5.6 11.3 16.9 22.5 28.1 33.8 39.4 45.0 50.6 56.3 61.9 67.5 73.1 78.8 84.4 90.0 95.6 101.3 106.9 112.5 118.1 123.8 129.4 135.0 140.6 146.3 151.9 157.5 163.1 168.8 174.4 180.0 1 1 16 4 8 15 14 7 13 12 3 6 11 10 Full Step (#) Half Step (#) 5 1/4 Step (#) 9 1/16 Phase 2 Phase 1 Step Step Current Current Angle (#) (% ITRIP(max)) (% ITRIP(max)) () 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 1 0.00 -9.38 -18.75 -29.69 -37.50 -46.88 -56.25 -64.06 -70.31 -76.56 -82.81 -87.50 -92.19 -95.31 -98.44 -100.00 -100.00 -100.00 -98.44 -95.31 -92.19 -87.50 -82.81 -76.56 -70.31 -64.06 -56.25 -46.88 -37.50 -29.69 -18.75 -9.38 0.00 -100.00 -100.00 -98.44 -95.31 -92.19 -87.50 -82.81 -76.56 -70.31 -64.06 -56.25 -46.88 -37.50 -29.69 -18.75 -9.38 0.00 9.38 18.75 29.69 37.50 46.88 56.25 64.06 70.31 76.56 82.81 87.50 92.19 95.31 98.44 100.00 100.00 180.0 185.6 191.3 196.9 202.5 208.1 213.8 219.4 225.0 230.6 236.3 241.9 247.5 253.1 258.8 264.4 270.0 275.6 281.3 286.9 292.5 298.1 303.8 309.4 315.0 320.6 326.3 331.9 337.5 343.1 348.8 354.4 360.0
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A3986
Dual Full-Bridge MOSFET Driver with Microstepping Translator
STEP
100 71
STEP
100 71
Slow
Slow Mixed
Slow Mixed
IOUT1B (%*)
Phase = 1, Direction = H
0 Home Microstep Position
Slow
IOUT1B (%*)
Phase = 1, Direction = H
Home Microstep Position 0
Mixed
Home Microstep Position
-71 -100 100 71
-71 -100 100 71
Slow Slow Mixed Mixed
Slow Mixed
Slow
IOUT2B (%*)
Phase = 2, Direction = H
0
IOUT2B (%*)
Phase = 2, Direction = H Slow
-71 -100 -71 -100 0
*For precise definition of output levels, refer to table 3
*For precise definition of output levels, refer to table 3
Figure 2. Decay Mode for Full-Step Increments
STEP
100 92 71 38
Figure 3. Decay Modes for Half-Step Increments
IOUT1B (%*)
Phase = 1, Direction = H
Slow
0
Mixed
Home Microstep Position
Slow
Mixed
Slow
-38 -71 -92 -100 100 92 71 38
Slow Mixed Slow Mixed Slow Mixed
IOUT2B (%*)
Phase = 2, Direction = H
0
-38 -71 -92 -100
*For precise definition of output levels, refer to table 3
Figure 4. Decay Modes for Quarter-Step Increments
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Home Microstep Position
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A3986
Dual Full-Bridge MOSFET Driver with Microstepping Translator
STEP
100 96 88 83 77 71 63 56 47 38 29 20
IOUT1B (%*)
Phase = 1, Direction = H
10
Slow
0 -10 -20 -29 -38 -47 -56 -63 -71 -77 -83 -88 -96 -100 100 96 88 83 77 71 63 56 47 38 29
Mixed
Slow
Mixed
IOUT2B (%*)
Phase = 2, Direction = H
20
Slow
10
Mixed
0 -10 -20 -29 -38 -47 -56 -63 -71 -77 -83 -88 -96 -100
Home Microstep Position Slow
Mixed
Slow
*For precise definition of output levels, refer to table 3
Figure 5. Decay Modes for Sixteenth-Step Increments
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A3986
Dual Full-Bridge MOSFET Driver with Microstepping Translator
Functional Description
Basic Operation
The A3986 is a complete microstepping FET driver with built-in translator for easy operation with a minimum number of control inputs. It is designed to operate 2-phase bipolar stepper motors in full-, half-, quarter, and sixteenth-step modes. The current in each of the two external power fullbridges, all N-channel MOSFETs, is independently regulated by a fixed off-time PWM control circuit. The full-bridge current at each step is set by the value of an external current sense resistor, RSENSEX , in the ground connection to the bridge, a reference voltage, VREF, and the output of the DAC controlled by the translator. The use of PWM with N-channel MOSFETs provides the most cost-effective solution for a high efficiency motor drive. The A3986 provides all the necessary circuits to ensure that the gate-source voltage of both high-side and low-side external MOSFETs are above 10 V, and that there is no cross-conduction (shoot through) in the external bridge. Specific functions are described more fully in the following sections. be 40 times the value of the bootstrap capacitor for PWM frequencies up to 14 kHz. Above 14 kHz, the minimum recommended value can be determined from the following formula: CREG > CBOOT x 3 x fPWM , where CREG and CBOOT are in nF, and fPWM is the maximum PWM frequency, in kHz. VREG is monitored, and if the voltage becomes too low, the outputs will be disabled. REF The reference voltage, VREF, at this pin sets the maximum (100%) peak current. The REF input is internally limited to 2 V when a 20 kpull-up resistor is connected between VREF and VDD. This allows the maximum reference voltage to be set without the need for an externallygenerated voltage. An external reference voltage below the maximum can also be input on this pin. The voltage at VREF is divided by 8 to produce the DAC reference voltage level. OSC The internal FET control timing is derived from a master clock running at 4 MHz typical. A resistor, ROSC, connected from the OSC pin to GND sets the frequency (in MHz) to approximately: fOSC 100 / (6 + 1.9 x ROSC) , where ROSC, in k, is typically between 50 k and 10 k. The master oscillator period is used to derive the PWM offtime, dead time, and blanking time.
Power Supplies
Two power connections are required. The motor power supply should be connected to VBB to provide the gate drive levels. Power for internal logic is provided by the VDD input. Internal logic is designed to operate from 3 to 5.5 V, allowing the use of 3.3 or 5 V external logic interface circuits. GND The ground pin is a reference voltage for internal logic and analog circuits. There is no large current flow through this pin. To avoid any noise from switching circuits, this should have an independent trace to the supply ground star point. VREG The voltage at this pin is generated by a low-drop-out linear regulator from the VBB supply. It is used to operate the low-side gate drive outputs, GLxx, and to provide the charging current for the bootstrap capacitors, CBOOTx. To limit the voltage drop when the charge current is provided, this pin should be decoupled with a ceramic capacitor, CREG, to ground. The value CREG should typically
Gate Drive
The A3986 is designed to drive external power N-channel MOSFETs. It supplies the transient currents necessary to quickly charge and discharge the external FET gate capacitance in order to reduce dissipation in the external FET during switching. The charge and discharge rate can be controlled using an external resistor , RGx, in series with the connection to the gate of the FET. Cross-conduction is prevented by the gate drive circuits which introduce a dead time, tDEAD , between switching one FET off and the complementary FET on. tDEAD is at least 3 periods of the master oscillator but can be up to 1 cycle longer to allow oscillator synchronization.
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A3986
Dual Full-Bridge MOSFET Driver with Microstepping Translator
C1A, C1B, C2A, and C2B High-side connections for the bootstrap capacitors, CBOOTx, and positive supply for highside gate drivers. The bootstrap capacitors are charged to approximately VREG when the associated output Sxx terminal is low. When the output swings high, the voltage on this terminal rises with the output to provide the boosted gate voltage needed for the high-side N-channel power MOSFETs. The bootstrap capacitor should be ceramic and have a value of 10 to 20 times the total MOSFET gate capacitance. GH1A, GH1B, GH2A, and GH2B High-side gate drive outputs for external N-channel MOSFETs. External series gate resistors can be used to control the slew rate seen at the gate, thereby controlling the di/dt and dv/dt at the motor terminals. GHxx = 1 (high) means that the upper half of the driver is turned on and will source current to the gate of the high-side MOSFET in the external motor-driving bridge. GHxx = 0 (low) means that the lower half of the driver is turned on and will sink current from the external MOSFET's gate circuit to the respective Sxx pin. S1A, S1B, S2A, and S2B Directly connected to the motor, these terminals sense the voltages switched across the load and define the negative supply for the floating high-side drivers. The discharge current from the high-side MOSFET gate capacitance flows through these connections which should have low impedance traces to the MOSFET bridge. GL1A, GL1B, GL2A, and GL2B Low-side gate drive outputs for external N-channel MOSFETs. External series gate resistors (as close as possible to the MOSFET gate) can be used to reduce the slew rate seen at the gate, thereby controlling the di/dt and dv/dt at the motor terminals. GLxx = 1 (high) means that the upper half of the driver is turned on and will source current to the gate of the low-side MOSFET in the external motor-driving bridge. GLxx = 0 (low) means that the lower half of the driver is turned on and will sink current from the gate of the external MOSFET to the LSSx pin. LSS1 and LSS2 Low-side return path for discharge of the gate capacitors, connected to the common sources of the low-side external FETs through low-impedance traces.
Motor Control
Motor speed and direction is controlled simply by two logic inputs, and the microstep level is controlled by a further two logic inputs. At power-up or reset, the translator sets the DACs and phase current polarity to the initial Home state (see figures 2 through 5 for home-state conditions), and sets the current regulator for both phases to mixed-decay mode. When a step command signal occurs on the STEP input, the translator automatically sequences the DACs to the next level (see table 3 for the current level sequence and current polarity). The microstep resolution is set by inputs MS1 and MS2 as shown in table 1. If the new DAC level is higher or equal to the previous level, then the decay mode for that full-bridge will be slow decay. If the new DAC output level is lower than the previous level, the decay mode for that full-bridge will be set by the PFD1 and PFD2 inputs. This automatic current-decay selection improves microstepping performance by reducing the distortion of the current waveform due to the motor BEMF.
STEP A low-to-high transition on the STEP input sequences the translator and advances the motor one increment. The translator controls the input to the DACs as well as the direction of current flow in each winding. The size of the increment is determined by the state of the MSx inputs. MS1 and MS2 These Microstep Select inputs are used
to select the microstepping format, per table 1. Changes to these inputs do not take effect until the next STEP input rising edge.
DIR This Direction input determines the direction of rotation
of the motor. When low, the direction is "clockwise" and "counterclockwise" when high. A change on this input does not take effect until the next STEP rising edge.
Internal PWM Current Control
Each full-bridge is independently controlled by a fixed offtime PWM current control circuit that limits the load current in the phase to a desired value, ITRIP. Initially, a diagonal pair
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A3986
Dual Full-Bridge MOSFET Driver with Microstepping Translator
of source and sink MOSFETs are enabled and current flows through the motor winding and the current sense resistor, RSENSEx. When the voltage across RSENSEx equals the DAC output voltage, the current sense comparator resets the PWM latch, which turns off the source MOSFET (slow decay mode) or the sink and source MOSFETs (fast decay mode). The maximum value of current limiting is set by the selection of RSENSE and the voltage at the REF input, with a transconductance function approximated by: ITRIP(max) = VREF / (8 x RSENSE ) The DAC, controlled by the translator, reduces the reference voltage, VREF , in precise steps to produce the required sinusoidal reference levels for the current sense comparator. This limits the phase current trip level, ITRIP, to a portion of the maximum current level, ITRIP(max), defined by: ITRIP = (% ITRIP(max) / 100) x ITRIP(max) See table 3 for % ITRIP(max) at each step.
ENABLE This input simply turns off all the power MOSFETs. When set at logic high, the outputs are disabled. When set at logic low, the internal control enables the outputs as required. Inputs to the translator (STEP, DIR, MS1, and MS2) and the internal sequencing logic are all active independent of the ENABLE input state.
RESET An active-low control input used to minimize power consumption when not in use. This disables much of the internal circuitry, including the output MOSFETs and internal regulator. When set at logic high, allows normal operation and start-up of the device in the home position. When coming out of sleep mode, wait 1 ms before issuing a STEP command, to allow the internal regulator to stabilize. The outputs can also be reset to the home position without entering sleep mode. To do so, pulse the RESET input low, with a pulse width between twR(min) and twR(max). Mixed Decay Operation
Mixed decay is a technique that provides greater control of phase currents while the current is decreasing. When a stepper motor is driven at high speed, the back EMF from the motor will lag behind the driving current. If a passive current decay mode, such as slow decay, is used in the current control scheme, then the motor back EMF can cause the phase current to rise out of control. Mixed decay eliminates this effect by putting the full-bridge initially into fast decay, and then switching to slow decay after some time. Because fast decay is an active (driven) decay mode, this portion of the current decay cycle will ensure that the current remains in control. Using fast decay for the full current decay time (off-time) would result in a large ripple current, but switching to slow decay once the current is in control will reduce the ripple current value. The portion of the off-time that the full-bridge has to remain in fast decay will depend on the characteristics and the speed of the motor. When the phase current is rising, the motor back EMF will not affect the current control and slow decay may be used to minimize the phase current ripple. The A3986 automatically switches between slow decay, when the current is rising, and mixed decay, when the current is falling. The portion of the off-time that the full-bridge remains in fast decay is defined by the PFD1 and PFD2 inputs.
Fixed Off-Time The internal PWM current control
circuitry uses the master oscillator to control the length of time the power MOSFETs remain off. The off-time, tOFF , is nominally 87 cycles of the master oscillator (21.75 s at 4 MHz), but may be up to 1 cycle longer to synchronize with the master oscillator.
Blanking This function blanks the output of the current
sense comparator when the outputs are switched by the internal current control. The comparator output is blanked to prevent false overcurrent detection due to reverse recovery currents of the clamp diodes and switching transients related to the capacitance of the load. The blank time, tBLANK , is 6 cycles of the master oscillator (1.5 s at 4 MHz). Because the tBLANK follows after the end of tOFF, no synchronization error occurs.
Dead Time To prevent cross-conduction (shoot through)
in the power full-bridge, a dead time is introduced between switching one MOSFET off and switching the complementary MOSFET on. The dead time, tDEAD, is 3 cycles of the master oscillator (750 ns at 4MHz), but may be up to 1 cycle longer to synchronize with the master oscillator.
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A3986
Dual Full-Bridge MOSFET Driver with Microstepping Translator
PFD1 and PFD2 The Percent Fast Decay pins are used
to select the portion of fast decay, according to table 2, to be used when mixed decay is enabled. Mixed decay is enabled when a STEP input signal commands an output current that is lower than for the previous step. In mixed decay mode, as the trip point is reached, the A3986 goes into fast decay mode until the specified number of master oscillator cycles has completed. After this fast decay portion, the A3986 switches to slow decay mode for the remainder of the fixed off-time, tOFF. Using PFD1 and PFD 2 to select 0% fast decay will effectively maintain the full-bridge in slow decay at all times. This option can be used to keep the phase current ripple to a minimum when the motor is stationary or stepping at very low rates. Selecting 100% fast decay will provide the fastest current control when the current is falling and can help when the motor is being driven at very high step rates.
out the body diodes with the low RDS(ON) of the MOSFET. This lowers power dissipation significantly and eliminates the need for additional Schottky diodes. Synchronous rectification can be set to either active mode or disabled mode. * Active Mode When the SR pin input is logic low, active mode is enabled and synchronous rectification will occur. This mode prevents reversal of the load current by turning off synchronous rectification when a zero current level is detected. This prevents the motor winding from conducting in the reverse direction. * Disabled Mode When the SR pin input is logic high, synchronous rectification is disabled. This mode is typically used when external diodes are required to transfer power dissipation from the power MOSFETs to external, usually Schottky, diodes. Shutdown Operation In the event of an overtemperature fault, or an undervoltage fault on VREG, the MOSFETs are disabled until the fault condition is removed. At power-up, and in the event of low voltage at VDD, the under voltage lockout (UVLO) circuit disables the MOSFETs until the voltage at VDD reaches the minimum level. Once VDD is above the minimum level, the translator is reset to the home state, and the MOSFETs are reenabled.
SR Input used to set synchronous rectification mode. When a PWM off-cycle is triggered, load current recirculates according to the decay mode selected by the control logic. The synchronous rectification feature turns on the appropriate MOSFETs during the current decay and effectively shorts
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A3986
Dual Full-Bridge MOSFET Driver with Microstepping Translator
Applications Information
Current Sensing
To minimize inaccuracies in sensing the IPEAK current level caused by ground-trace IR drops, the sense resistor, RSENSEx, should have an independent return to the supply ground star point. For low-value sense resistors, the IR drops in the sense resistor PCB traces can be significant and should be taken into account. The use of sockets should be avoided as they can introduce variation in RSENSEx due to their contact resistance.
3. The GND pin should be connected by an independent low impedance trace to the supply common at a single point. 4. Check the peak voltage excursion of the transients on the LSS pin with reference to the GND pin using a close grounded (tip and barrel) probe. If the voltage at LSS exceeds the absolute maximum specified in this datasheet, add additional clamping, capacitance, or both between the LSS pin and the AGND pin. Other layout recommendations: 1. Gate charge drive paths and gate discharge return paths may carry transient current pulses. Therefore, the traces from GHxx, GLxx, Sxx, and LSSx should be as short as possible to reduce the inductance of the circuit trace. 2. Provide an independent connection from each LSS pin to the common point of each power bridge. It is not recommended to connect LSS directly to the GND pin. The LSS connection should not be used for the SENSE connection. 3. Minimize stray inductance by using short, wide copper runs at the drain and source terminals of all power FETs. This includes motor lead connections, the input power bus, and the common source of the low-side power FETs. This will minimize voltages induced by fast switching of large load currents. 4. Consider the use of small (100 nF) ceramic decoupling capacitors across the source and drain of the power FETs to limit fast transient voltage spikes caused by trace inductance. The above are only recommendations. Each application is different and may encounter different sensitivities. Each design should be tested at the maximum current, to ensure any parasitic effects are eliminated.
Thermal Protection
All drivers are turned off when the junction temperature reaches 165C typical. This is intended only to protect the A3986 from failures due to excessive junction temperatures. Thermal protection will not protect the A3986 from continuous short circuits. Thermal shutdown has a hysteresis of approximately 15C.
Circuit Layout
Because this is a switch-mode application, where rapid current changes are present, care must be taken during layout of the application PCB. The following points are provided as guidance for layout. Following all guidelines will not always be possible. However, each point should be carefully considered as part of any layout procedure. Ground connection layout recommendations: 1. Decoupling capacitors for the supply pins VBB, VREG, and VDD should be connected independently close to the GND pin and not to any ground plane. The decoupling capacitors should also be connected as close as possible to the corresponding supply pin. 2. The oscillator timing resistor ROSC should be connected to the GND pin. It should not be connected to any ground plane, supply common, or the power ground.
Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com
14
A3986
Dual Full-Bridge MOSFET Driver with Microstepping Translator
Pin-out Diagram
C2A 1 GH2A 2 S2A 3 GL2A 4 NC 5 VREG 6 VBB 7 GL1A 8 S1A 9 GH1A 10 C1A 11 C1B 12 GH1B 13 S1B 14 GL1B 15 LSS1 16 SENSE1 17 SR 18 ENABLE 19 Translator Control Logic
38 C2B 37 GH2B 36 S2B 35 GL2B 34 NC 33 LSS2 32 SENSE2 31 PFD1 30 DIR 29 MS2 28 MS1 27 PFD2 26 STEP 25 VDD 24 NC 23 OSC 22 RESET 21 REF 20 GND
Terminal List Table Number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38
Name C2A GH2A S2A GL2A NC VREG VBB GL1A S1A GH1A C1A C1B GH1B S1B GL1B LSS1 SENSE1 SR ENABLE GND REF RESET OSC NC VDD STEP PFD2 MS1 MS2 DIR PFD1 SENSE2 LSS2 NC GL2B S2B GH2B C2B
Description Phase 2 bootstrap capacitor drive A connection Phase 2 high-side gate drive A Phase 2 motor connection A Phase 2 low-side gate drive A No internal connection Regulator decoupling capacitor connection Motor supply voltage Phase 1 low-side gate drive A Phase 1 motor connection A Phase 1 high-side gate drive A Phase 1 bootstrap capacitor drive A connection Phase 1 bootstrap capacitor drive B connection Phase 1 high-side gate drive B Phase 1 motor connection B Phase 1 low-side gate drive B Phase 1 low-side source connection Phase 1 bridge current sense input Synchronous rectification enable Output enable Ground Reference voltage Reset input Oscillator input, ROSC resistor connection No internal connection Logic supply voltage Step input Percent Fast Decay input 2 Microstep Select input 1 Microstep Select input 2 Direction input Percent Fast Decay input 1 Phase 2 bridge current sense input Phase 2 low-side source connection No internal connection Phase 2 low-side gate drive B Phase 2 motor connection B Phase 2 high-side gate drive B Phase 2 bootstrap capacitor drive B connection
Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com
15
A3986
Dual Full-Bridge MOSFET Driver with Microstepping Translator
LD Package, 38-Pin TSSOP
1.60 9.70 0.10 38 +0.06 0.15 -0.05 4 38 0.50 0.30
4.40 0.10 A
6.40 0.20
6.00
12 0.25 SEATING PLANE GAUGE PLANE 12
B
PCB Layout Reference View
38X 0.10 C 0.22 0.05 0.50
SEATING PLANE
C
All dimensions nominal, not for tooling use (reference JEDEC MO-153 BD-1) Dimensions in millimeters A Terminal #1 mark area
B
1.20 MAX 0.10 0.05
Reference pad layout (reference IPC SOP50P640X110-38M) All pads a minimum of 0.20 mm from all adjacent pads; adjust as necessary to meet application process requirements and PCB layout tolerances
Copyright (c)2005-2009, Allegro MicroSystems, Inc. The products described here are manufactured under one or more U.S. patents or U.S. patents pending. Allegro MicroSystems, Inc. reserves the right to make, from time to time, such departures from the detail specifications as may be required to permit improvements in the performance, reliability, or manufacturability of its products. Before placing an order, the user is cautioned to verify that the information being relied upon is current. Allegro's products are not to be used in life support devices or systems, if a failure of an Allegro product can reasonably be expected to cause the failure of that life support device or system, or to affect the safety or effectiveness of that device or system. The information included herein is believed to be accurate and reliable. However, Allegro MicroSystems, Inc. assumes no responsibility for its use; nor for any infringement of patents or other rights of third parties which may result from its use. For the latest version of this document, visit our website: www.allegromicro.com
Allegro MicroSystems, Inc. 115 Northeast Cutoff Worcester, Massachusetts 01615-0036 U.S.A. 1.508.853.5000; www.allegromicro.com
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