Data Sheet 29319.44† 3957 FULL-BRIDGE PWM MICROSTEPPING MOTOR DRIVER A3957SLB NO CONNECT 1 PFD 2 REF 3 NO CONNECT 4 RC 5 GROUND 6 GROUND 24 NO CONNECT 23 LOAD SUPPLY 22 OUTB 21 NO CONNECT 20 D0 19 GROUND 7 18 GROUND D3 8 17 SENSE LOGIC SUPPLY 9 PHASE D2 NO CONNECT NC NC VBB NC NC LOGIC VCC NC 16 10 15 11 NC 14 12 NC 13 NO CONNECT OUTA NO CONNECT D1 Dwg. PP-056-4 ABSOLUTE MAXIMUM RATINGS Load Supply Voltage, VBB . . . . . . . . . 50 V Output Current, IOUT (Continuous) . . . . . . . . . . . . . ±1.5 A* Logic Supply Voltage, VCC . . . . . . . 7.0 V Logic/Reference Input Voltage Range, VIN . . . . . . . . . . -0.3 V to V CC + 0.3 V Sense Voltage, VS . . . . . . . . . . . . . . . 1.0 V Package Power Dissipation (TA = 25°C), PD A3957SA . . . . . . . . . . . . . . . . 2.08 W† A3957SLB . . . . . . . . . . . . . . . 2.23 W† Operating Temperature Range, TA . . . . . . . . . . . . . . . -20˚C to +85˚C Junction Temperature, TJ . . . . . . . . +150˚C Storage Temperature Range, TS . . . . . . . . . . . . . . . -55˚C to +150˚C * Output current rating may be limited by duty cycle, ambient temperature, and heat sinking. Under any set of conditions, do not exceed the specified current rating or a junction temperature of 150°C. † Per SEMI G42-88 Specification, Thermal Test Board Standardization for Measuring Junctionto-Ambient Thermal Resistance of Semiconductor Packages.. The A3957SA and A3957SLB are designed for driving one winding of a bipolar stepper motor in a microstepping mode. The outputs are rated for continuous output currents to ±1.5 A and operating voltages to 50 V. Internal pulse-width modulated (PWM) current control combined with an internal four-bit nonlinear digital-to-analog converter allows the motor current to be controlled in full-, half-, quarter-, eighth-, or sixteenth-step (microstepping) modes. Nonlinear increments minimize the number of control lines necessary for microstepping. Microstepping provides for increased step resolution, and reduces torque variations and resonance problems at low speed. Internal circuitry determines whether the PWM current-control circuitry operates in a slow (recirculating) current-decay mode, fast (regenerative) current-decay mode, or in a mixed current-decay mode in which the off time is divided into a period of fast current decay with the remainder of the fixed off time spent in slow current decay. The combination of user-selectable current-sensing resistor and reference voltage, digitally selected output current ratio; and slow, fast, or mixed current-decay modes provides users with a broad, variable range of motor control. Internal circuit protection includes thermal shutdown with hysteresis, transient-suppression diodes, and crossover current protection. Special power-up sequencing is not required. The A3957S— is supplied in a choice of two power packages; a 16-pin dual-in-line plastic package (suffix ‘A’), and a 24-lead plastic SOIC with copper heat-sink tabs (suffix ‘LB’). The power tab is at ground potential and needs no electrical isolation. FEATURES ■ ■ ■ ■ ■ ■ ■ ■ ■ ±1.5 A Continuous Output Current 50 V Output Voltage Rating Internal PWM Current Control 4-Bit Non-Linear DAC for 16-Bit Microstepping Satlington™ Sink Drivers Fast, Mixed Fast/Slow, and Slow Current-Decay Modes Internal Transient-Suppression Diodes Internal Thermal-Shutdown Circuitry Crossover-Current and UVLO Protection Always order by complete part number: Part Number RθJA RθJC RθJT A3957SA 16-pin DIP Package 60°C/W 38°C/W — A3957SLB 24-lead batwing SOIC 56°C/W — 6°C/W 3957 FULL-BRIDGE PWM MICROSTEPPING MOTOR DRIVER 9 VCC PHASE 15 LOAD SUPPLY OUTB OUTA LOGIC SUPPLY FUNCTIONAL BLOCK DIAGRAM (A3957SLB shown) 22 23 10 VBB GROUND 6 7 UVLO & TSD 18 19 MIXED-DECAY COMPARATOR CURRENT-SENSE COMPARATOR + – SENSE 17 + – R Q S D/A RS + – RT 3 8 11 13 20 D0 V TH D1 RC 5 D2 VCC DISABLE ÷3 BLANKING D3 2 BLANKING GATE REF PFD PWM LATCH CT A3957SA Dwg. FP-042-1 16 NO CONNECT 15 SENSE 14 OUTA 13 D1 5 12 D2 REF 6 11 PHASE RC 7 10 LOGIC SUPPLY GROUND 8 9 D3 GROUND 1 D0 2 OUTB 3 LOAD SUPPLY 4 PFD NC LOGIC VBB VCC Dwg. PP-056-3 For the ‘A’ package, pins 1 and 8 must be externally connected together. 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 W Copyright © 1998 Allegro MicroSystems, Inc. 3957 FULL-BRIDGE PWM MICROSTEPPING MOTOR DRIVER Table 1 — PHASE Truth Table PHASE OUTA OUTB H L H L L H Table 2 — PFD Truth Table VPFD Description ≥3.5 V Slow Current-Decay Mode 1.2 V to 2.9 V Mixed Current-Decay Mode ≤0.8 V Fast Current-Decay Mode Table 3 — DAC Truth Table DAC Data Current D3 D2 D1 D0 Ratio, % VREF/VS H H H H 100 3.00 H H H L 95.7 3.13 H H L H 91.3 3.29 H H L L 87.0 3.45 H L H H 82.6 3.64 H L H L 78.3 3.83 H L L H 73.9 4.07 H L L L 69.6 4.31 L H H H 60.9 4.93 L H H L 52.2 5.74 L H L H 43.5 6.90 L H L L 34.8 8.62 L L H H 26.1 11.49 L L H L 17.4 17.24 L L L X All Outputs Disabled where VS = ITRIP • RS . See Applications section. 3957 FULL-BRIDGE PWM MICROSTEPPING MOTOR DRIVER ELECTRICAL CHARACTERISTICS at TA = 25˚C, VBB = 5 V to 50 V, V CC = 4.5 V to 5.5 V (unless otherwise noted.) Limits Characteristic Symbol Test Conditions Min. Typ. Max. Units V CC — 50 V Power Outputs Load Supply Voltage Range VBB Operating, IOUT = ±1.5 A, L = 3 mH Output Leakage Current ICEX VOUT = V BB — <1.0 50 µA VOUT = 0 V — <-1.0 -50 µA VS = 1.0 V: Source Driver, I OUT = -0.85 A Source Driver, IOUT = -1.5 A Sink Driver, IOUT = 0.85 A Sink Driver, IOUT = 1.5 A — — — — 1.1 1.4 0.5 1.2 1.2 1.5 0.7 1.5 V V V V Output Saturation Voltage (Forward or Reverse Mode) VCE(SAT) Sense Current Offset ISO IS - IOUT, IOUT = 850 mA, VS = 0 V, VCC = 5 V 20 30 40 mA Clamp Diode Forward Volt. (Sink or Source) VF IF = 0.85 A — 1.2 1.4 V IF = 1.5 A — 1.5 1.7 V — 2.0 4.0 mA D0 = D1 = D2 = D3 = 0.8 V — 1.0 50 µA Motor Supply Current (No Load) IBB(ON) IBB(OFF) Control Circuitry Logic Supply Voltage Range VCC Operating 4.5 5.0 5.5 V Reference Voltage Range V REF Operating 0.5 — 2.5 V 3.35 3.70 4.05 V 0.25 0.40 0.55 V — 42 50 mA — 14 17 mA VIN(1) 2.0 — — V VIN(0) — — 0.8 V VCC = 0 → 5 V UVLO Enable Threshold UVLO Hysteresis Logic Supply Current ICC(ON) ICC(OFF) Logic Input Voltage Logic Input Current D0 = D1 = D2 = D3 = 0.8 V IIN(1) VIN = 2.0 V — <1.0 20 µA IIN(0) VIN = 0.8 V — <-2.0 -200 µA Continued next page… 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 3957 FULL-BRIDGE PWM MICROSTEPPING MOTOR DRIVER ELECTRICAL CHARACTERISTICS at TA = 25˚C, VBB = 5 V to 50 V, V CC = 4.5 V to 5.5 V (unless otherwise noted.) Limits Characteristic Symbol Test Conditions Min. Typ. Max. Units Slow Current-Decay Mode 3.5 — — V Mixed Current-Decay Mode 1.2 — 2.9 V Fast Current-Decay Mode — — 0.8 V Control Circuitry (continued) Mixed-Decay Comparator Trip Points V PFD Mixed-Decay Comparator Input Offset Voltage VIO(PFD) — 0 ±20 mV Mixed-Decay Comparator Hysteresis ∆V IO(PFD) 5.0 25 55 mV VREF = 0 V to 2.5 V — — ±5.0 µA at trip, D0 = D1 = D2 = D3 = 2 V — 3.0 — — 1.0 V < VREF ≤ 2.5 V — — ±3.0 % 0.5 V < VREF ≤ 1.0 V — — ±4.0 % Reference Input Current IREF Reference Divider Ratio VREF /VS Digital-to-Analog Converter Accuracy* — Current-Sense Comparator Input Offset Voltage* V IO(S) VREF = 0 V — ±16 — mV Step Reference Current Ratio SRCR D0 = D1 = D2 = D3 = 0.8 V D1 = 2 V, D 0 = D2 = D3 = 0.8 V D0 = D1 = 2 V, D2 = D3 = 0.8 V D2 = 2 V, D 0 = D1 = D3 = 0.8 V D0 = D2 = 2 V, D1 = D3 = 0.8 V D1 = D2 = 2 V, D0 = D3 = 0.8 V D0 = D1 = D2 = 2 V, D 3 = 0.8 V D3 = 2 V, D 0 = D1 = D2 = 0.8 V D0 = D3 = 2 V, D1 = D2 = 0.8 V D1 = D3 = 2 V, D0 = D2 = 0.8 V D0 = D1 = D3 = 2 V, D 2 = 0.8 V D2 = D3 = 2 V, D0 = D1 = 0.8 V D0 = D2 = D3 = 2 V, D 1 = 0.8 V D1 = D2 = D3 = 2 V, D 0 = 0.8 V D0 = D1 = D2 = D3 = 2 V — — — — — — — — — — — — — — — 0 17.4 26.1 34.8 43.5 52.2 60.9 69.6 73.9 78.3 82.6 87.0 91.3 95.7 100 — — — — — — — — — — — — — — — % % % % % % % % % % % % % % % Thermal Shutdown Temp. TJ — 165 — °C Thermal Shutdown Hyst. ∆T J — 15 — °C * The total error for the VREF/VS function is the sum of the D/A error and the current-sense comparator input offset voltage. Continued next page… 3957 FULL-BRIDGE PWM MICROSTEPPING MOTOR DRIVER ELECTRICAL CHARACTERISTICS at TA = 25˚C, VBB = 5 V to 50 V, V CC = 4.5 V to 5.5 V (unless otherwise noted.) Limits Characteristic Symbol Test Conditions Min. Typ. Max. Units CT = 470 pF, RT = 43 kΩ 18.2 20.2 22.3 µs Current-Sense Comparator Trip to Source OFF, IOUT = 100 mA — 1.0 1.5 µs Current-Sense Comparator Trip to Source OFF, IOUT = 1.5 A — 1.4 2.5 µs IRC Charge ON to Source ON, IOUT = 100 mA — 0.4 0.7 µs IRC Charge ON to Source ON, IOUT = 1.5 A — 0.55 0.85 µs VCC = 5.0 V, RT ≥ 43 kΩ, CT = 470 pF IOUT = 100 mA 1.0 1.6 2.2 µs 1 kΩ Load to 25 V 0.3 1.5 3.0 µs AC Timing PWM RC Fixed Off-time PWM Turn-Off Time PWM Turn-On Time PWM Minimum On Time Crossover Dead Time tOFF RC tPWM(OFF) tPWM(ON) tON(min) tCODT 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 3957 FULL-BRIDGE PWM MICROSTEPPING MOTOR DRIVER Typical Operating Characteristics Satlington™ Sink Driver Saturation Voltage Source Driver Saturation Voltage 1.5 OUTPUT SATURATION VOLTAGE IN VOLTS OUTPUT SATURATION VOLTAGE IN VOLTS 1.5 1.25 1.0 0.75 SOURCE DRIVER 0.5 TJ = +25°C TJ = +70°C TJ = +85°C TJ = +125°C 0.25 0 SINK DRIVER 1.25 TJ = +25°C TJ = +70°C TJ = +85°C TJ = +125°C 1.0 0.75 0.5 0.25 0 0 0.25 0.5 0.75 1.0 1.25 0 1.5 0.25 OUTPUT CURRENT IN AMPERES 0.5 0.75 1.0 Dwg. GP-064-2 Dwg. GP-064-3 Flyback Diode Forward Voltage 1.5 1.5 1.25 1.25 FORWARD VOLTAGE IN VOLTS FORWARD VOLTAGE IN VOLTS Clamp Diode Forward Voltage 1.0 0.75 SINK DIODE 0.5 TJ = +25°C TJ = +70°C TJ = +85°C TJ = +125°C 0.25 1.5 1.25 OUTPUT CURRENT IN AMPERES 0 1.0 0.75 FLYBACK DIODE 0.5 TJ = +25°C TJ = +70°C TJ = +85°C TJ = +125°C 0.25 0 0 0.25 0.5 0.75 1.0 1.25 1.5 FORWARD CURRENT IN AMPERES 0 0.25 0.5 0.75 1.0 1.25 1.5 FORWARD CURRENT IN AMPERES Dwg. GD-003-1 Dwg. GD-003-2 3957 FULL-BRIDGE PWM MICROSTEPPING MOTOR DRIVER Terminal Functions A3957SA Pin A3957SLB Lead Terminal Name – 1 NC No internal connection. 5 2 PFD (Percent Fast Decay) The analog input used to set the current-decay mode. 6 3 REF (VREF) The voltage at this input (along with the value of RS and the states of DAC inputs D0, D1, and D2) set the peak output current. – 4 NC No internal connection. 7 5 RC The parallel combination of external resistor RT and capacitor CT set the off time for the PWM current regulator. CT also sets the blanking time. 8* 6-7 GROUND 9 8 D3 10 9 LOGIC SUPPLY 11 10 PHASE 12 11 D2 (DATA2) One of four control bits for the internal digital-to-analog converter. – 12 NC No internal connection. 13 13 D1 (DATA1) One of four control bits for the internal digital-to-analog converter. – 14 NC No internal connection. 14 15 OUTA – 16 NC 15 17 SENSE 16 – NC 1* 18-19 GROUND 2 20 D0 (DATA0) One of four (LSB) control bits for the internal digital-to-analog converter. – 21 NC No internal connection. 3 22 OUTB 4 23 LOAD SUPPLY – 24 NC Description Return for the logic supply (VCC) and load supply (VBB); the reference for all voltage measurements. (DATA3) One of four (MSB) control bits for the internal digital-to-analog converter. (VCC) Supply voltage for the logic circuitry. Typically = 5 V. The PHASE input determines the direction of current in the load. One of two output load connections. No internal connection. Connection to the sink-transistor emitters. Sense resistor RS is connected between this point and ground. No internal connection. Return for the logic supply (VCC) and load supply (VBB); the reference for all voltage measurements. One of two output load connections. (VBB) Supply voltage for the load. No internal connection. * For the ‘A’ package, pins 1 and 8 must be externally connected together. 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 3957 FULL-BRIDGE PWM MICROSTEPPING MOTOR DRIVER Functional Description Two A3957S— full-bridge PWM microstepping motor drivers are needed to drive the windings of a bipolar stepper motor. Internal pulse-width modulated (PWM) control circuitry regulates each motor winding’s current. The peak motor current is set by the value of an external current-sense resistor (RS), a reference voltage (VREF), and the digital-to-analog converter (DAC) data inputs (D0, D1, D2, and D3). To improve motor performance, especially when using sinusoidal current profiles necessary for microstepping, the A3957S— has three distinct current-decay modes: slow decay, fast decay, and mixed decay. PHASE Input. The PHASE input controls the direction of current flow in the load (table 1). An internally generated dead time of approximately 1.5 µs prevents crossover currents that could occur when switching the PHASE input. DAC Data Inputs (D0, D1, D2 , D3). A non-linear DAC is used to digitally control the output current. The output of the DAC is used to set the trip point of the current-sense comparator. Table 3 shows DAC output voltages for each input condition. When D1 , D2, and D3 are all logic low, all of the power output transistors are turned off. Internal PWM Current Control. Each motor driver IC contains an internal fixed off-time PWM current-control circuit that limits the load current to a desired value (ITRIP). Initially, a diagonal pair of source and sink transistors are enabled and current flows through the motor winding and RS (figure 1). V BB DRIVE CURRENT RECIRCULATION (SLOW-DECAY MODE) RECIRCULATION (FAST-DECAY MODE) RS Dwg. EP-006-15 Figure 1 — Load-Current Paths When the voltage across the sense resistor equals the DAC output voltage, the current-sense comparator resets the PWM latch, which turns off the source drivers (slow-decay mode) or the sink and source drivers (fast- or mixed-decay mode). With the DATA input lines tied to VCC, the maximum value of current limiting is set by the selection of RS and VREF with a transconductance function approximated by: ITRIP ≈ VREF/3RS = IOUT + I SO. where ISO is the sense-current offset due to the base-drive current of the sink transistor (typically 30 mA). The actual peak load current (IPEAK) will be slightly higher than ITRIP due to internal logic and switching delays. The driver(s) remain off for a time period determined by a user-selected external resistor-capacitor combination (RTCT). At the end of the fixed off time, the driver(s) are re-enabled, allowing the load current to increase to ITRIP again, maintaining an average load current. The current-sense comparator has a fixed offset of approximately 16 mV. With RS = 0.5 Ω, the sense-current offset (ISO) is effectively cancelled (VIO(S) ≈ ISO • RS). The DAC data input lines are used to provide up to eight levels of output current. The internal 4-bit digital-to-analog converter reduces the reference input to the current-sense comparator in precise steps (the step reference current ratio or SRCR) to provide half-step, quarter-step, eighth-step, or “microstepping” load-current levels. ITRIP ≈ SRCR x VREF/3RS Slow Current-Decay Mode. When VPFD ≥ 3.5 V, the device is in slow current-decay mode (the source drivers are disabled when the load current reaches ITRIP). During the fixed off time, the load inductance causes the current to recirculate through the motor winding, sink driver, ground clamp diode, and sense resistor (see figure 1). Slow-decay mode produces low ripple current for a given fixed off time (see figure 2). Low ripple current is desirable because the average current in the motor winding is more nearly equal to the desired reference value, resulting in increased motor performance in microstepping applications. For a given level of ripple current, slow decay affords the lowest PWM frequency, which reduces heating in the motor and driver IC due to a corresponding decrease in hysteretic core losses and switching losses respectively. Slow decay also has the advantage that the PWM load current regulation can follow a more rapidly increasing reference before the PWM frequency drops into the audible range. For these reasons slow-decay mode is typically used as long as good current regulation can be maintained. 3957 FULL-BRIDGE PWM MICROSTEPPING MOTOR DRIVER Under some circumstances slow-decay mode PWM can fail to maintain good current regulation: 1) The load current will fail to regulate in slow-decay mode due to a sufficiently negative back-EMF voltage in conjunction with the low voltage drop across the load during slow decay recirculation. The negative back-EMF voltage can cause the load current to actually increase during the slow decay off time. A negative back-EMF voltage condition commonly occurs when driving stepping motors because the phase lead of the rotor typically causes the back-EMF voltage to be negative towards the end of each step (see figure 3A). 2) When the desired load current is decreased rapidly, the slow rate of load current decay can prevent the current from following the desired reference value. A — Slow-Decay Mode 3) When the desired load current is set to a very low value, the current-control loop can fail to regulate due to its minimum duty cycle, which is a function of the userselected value of tOFF and the minimum on-time pulse width ton(min) that occurs each time the PWM latch is reset. Fast Current-Decay Mode. When VPFD ≤ 0.8 V, the device is in fast current-decay mode (both the sink and source drivers are disabled when the load current reaches ITRIP). During the fixed off time, the load inductance causes the current to flow from ground to the load supply via the motor winding, ground-clamp and flyback diodes (see figure 1). Because the full motor supply voltage is across the load during fast-decay recirculation, the rate of load current decay is rapid, producing a high ripple current for a given fixed off time (see figure 2). This rapid rate of decay allows good current regulation to be maintained at the I PEAK I TRIP B — Fast-Decay Mode SLOW (VPFD ≥ 3.5 V) MIXED (1.2 V ≤ V PFD ≤ 2.9 V) FAST (V PFD ≤ 0.8 V) PFD t OFF C — Mixed-Decay Mode Dwg. WP-031-2 Figure 3 — Sinusoidal Drive Currents Figure 2 — Current-Decay Waveforms 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 3957 FULL-BRIDGE PWM MICROSTEPPING MOTOR DRIVER cost of decreased average current accuracy or increased driver and motor losses. Mixed Current-Decay Mode. If VPFD is between 1.2 V and 2.9 V, the device will be in a mixed current-decay mode. Mixed-decay mode allows the user to achieve good current regulation with a minimum amount of ripple current and motor/ driver losses by selecting the minimum percentage of fast decay required for their application (see also Stepper Motor Applications). As in fast current-decay mode, mixed-decay starts with the sink and source drivers disabled after the load current reaches ITRIP. When the voltage at the RC terminal decays to a value below VPFD, the sink drivers are re-enabled, placing the device in slow current-decay mode for the remainder of the fixed off time (figure 2). The percentage of fast decay (PFD) is user determined by VPFD or two external resistors. PFD = 100 ln (0.6[R1+R2]/R2) where V CC R1 PFD R2 Dwg. EP-062-1 Fixed Off-Time. The internal PWM current-control circuitry uses a one shot to control the time the driver(s) remain(s) off. The one-shot off-time, tOFF, is determined by the selection of an external resistor (RT) and capacitor (CT) connected from the RC timing terminal to ground. The off-time, over a range of values of CT = 470 pF to 1500 pF and RT = 12 kΩ to 100 kΩ, is approximated by: tOFF ≈ RTCT. When the load current is increasing, but has not yet reached the sense-current comparator threshold (ITRIP), the voltage on the RC terminal is approximately 0.6VCC. When I TRIP is reached, the PWM latch is reset by the current-sense comparator and the voltage on the RC terminal will decay until it reaches approximately 0.22VCC. The PWM latch is then set, thereby re-enabling the driver(s) and allowing load current to increase again. The PWM cycle repeats, maintaining the peak load current at the desired value. With increasing values of tOFF, switching losses will decrease, low-level load-current regulation will improve, EMI will be reduced, the PWM frequency will decrease, and ripple current will increase. A value of tOFF can be chosen for optimization of these parameters. For applications where audible noise is a concern, typical values of tOFF are chosen to be in the range of 15 µs to 35 µs. RC Blanking. In addition to determining the fixed off-time of the PWM control circuit, the CT component sets the comparator blanking time. This function blanks the output of the currentsense comparator when the outputs are switched by the internal current-control circuitry (or by the PHASE input, or when the device is enabled with the DAC data 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 tOFF time, the comparator’s output is blanked and CT begins to be charged from approximately 0.22VCC by an internal current source of approximately 1 mA. The comparator output remains blanked until the voltage on CT reaches approximately 0.6VCC. The blanking time, tBLANK, can be calculated as: tBLANK = R TC T ln (RT/[RT - 3 kΩ]). When a transition of the PHASE input occurs, CT 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, CT is charged by an internal current source of approximately 1 mA. The comparator output remains blanked until the voltage on CT reaches approximately 0.6VCC. Similarly, when the device is disabled, via the DAC data inputs, CT is discharged to near ground. When the device is reenabled, CT is charged by an internal current source of approximately 1 mA. The comparator output remains blanked until the voltage on CT reaches approximately 0.6VCC. The blanking time, tBLANK′, can be calculated as: tBLANK′ ≈ 1900 CT. The minimum recommended value for CT is 470 pF ± 5 %. This value ensures that the blanking time is sufficient to avoid false trips of the comparator under normal operating conditions. For optimal regulation of the load current, this value for CT is recommended and the value of RT can be sized to determine tOFF. 3957 FULL-BRIDGE PWM MICROSTEPPING MOTOR DRIVER Thermal Considerations. Thermal-protection circuitry turns off all output transistors when the junction temperature reaches approximately +165°C. This is intended only to protect the device from failures due to excessive junction temperatures and should not imply that output short circuits are permitted. The output transistors are re-enabled when the junction temperature cools to approximately +150°C. Stepper Motor Applications. The A3957SB or A3957SLB are used to optimize performance in microstepping/sinusoidal stepper-motor drive applications (see figures 4 and 5). When the load current is increasing, the slow current-decay mode is used to limit the switching losses in the driver and iron losses in the motor. This also improves the maximum rate at which the load current can increase (as compared to fast decay) due to the slow rate of decay during tOFF. When the load current is decreasing, the mixed current-decay mode is used to regulate the load current to the desired level. This prevents tailing of the current profile caused by the back-EMF voltage of the stepper motor (see figure 3A). BRIDGE A BRIDGE B VBB 1 V PFD 2 V REF 3 NC NC VBB D 1B 24 23 14 22 NC NC 15 + 4 NC 12 13 11 D2B 10 PHASE B 47 µF NC 21 16 VCC NC 9 +5 V 8 D3B D 0A 18 11 D3A 8 17 +5 V 9 10 15 D2A 11 NC 14 NC 13 6 20 47 µF 21 5 NC NC + PHASE A 12 LOGIC 19 D0B NC 16 7 18 22 VBB D 1A 23 24 30 kΩ 7 VCC 0.5 Ω 19 LOGIC 0.5 Ω 30 kΩ 470 pF 6 17 470 pF 20 5 VBB NC NC 4 3 V REF 2 V PFD 1 Dwg. EP-047-5 Figure 4 — Typical Application MIXED DECAY SLOW DECAY MIXED DECAY SLOW DECAY Dwg. WK-004-5 Figure 5 — Microstepping/Sinusoidal Drive Current 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 3957 FULL-BRIDGE PWM MICROSTEPPING MOTOR DRIVER Table 4 — Step Sequencing Full 1/2 1/4 1/8 1/16 Step Step Step Step Step 1 1 1 1 2 2 3 4 2 3 5 6 4 7 8 2 3 5 9 10 6 11 12 4 7 13 14 8 15 16 3 5 9 17 18 10 19 20 6 11 21 22 12 23 24 4 7 13 25 26 14 27 28 8 15 29 30 16 31 32 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 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 Bridge A PHASEA D3A H H H H H H H H H H H H H H H H X L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L X H H H H H H H H H H H H H H H H H H H H H H H H H L L L L L L L L L L L L L H H H H H H H H H H H H H H H H H H H L L L L L L L L L L L L L H H H H H H H H H Bridge B D2A D1A D0A ILOAD A H H H H H H L L L L H H H H L L L L L H H H H L L L L H H H H H H H H H H H L L L L H H H H L L L L L H H H H L L L L H H H H H H H H H L L H H L L H H L L H H L H H L L H H L L H H L L H H H H H H H L L H H L L H H L L H H L H H L L H H L L H H L L H H H H H H L H L H L H L H L H L H L X L H L H L H L H L H L H L H H H H H L H L H L H L H L H L H L X L H L H L H L H L H L H L H H 100% 100% 100% 95.7% 91.3% 87.0% 82.6% 78.3% 73.9% 69.6% 60.9% 52.2% 43.5% 34.8% 26.1% 17.4% 0% -17.4% -26.1% -34.8% -43.5% -52.2% -60.9% -69.6% -73.9% -78.3% -82.6% -87.0% -91.3% -95.7% -100% -100% -100% -100% -100% -95.7% -91.3% -87.0% -82.6% -78.3% -73.9% -69.6% -60.9% -52.2% -43.5% -34.8% -26.1% -17.4% 0% 17.4% 26.1% 34.8% 43.5% 52.2% 60.9% 69.6% 73.9% 78.3% 82.6% 87.0% 91.3% 95.7% 100% 100% PHASEB D3B X H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H H X L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L L H H H H H H H H H H H H H H H H H H H L L L L L L L L L L L L L H H H H H H H H H H H H H H H H H H H L L L L L L D2B D1B D0B ILOAD B L L L H H H H L L L L H H H H H H H H H H H L L L L H H H H L L L L L H H H H L L L L H H H H H H H H H H H L L L L H H H H L L L H H L L H H L L H H L L H H H H H H H L L H H L L H H L L H H L H H L L H H L L H H L L H H H H H H H L L H H L L H H L L H H X L H L H L H L H L H L H L H H H H H L H L H L H L H L H L H L X L H L H L H L H L H L H L H H H H H L H L H L H L H L H L H L 0% 17.4% 26.1% 34.8% 43.5% 52.2% 60.9% 69.6% 73.9% 78.3% 82.6% 87.0% 91.3% 95.7% 100% 100% 100% 100% 100% 95.7% 91.3% 87.0% 82.6% 78.3% 73.9% 69.6% 60.9% 52.2% 43.5% 34.8% 26.1% 17.4% 0% -17.4% -26.1% -34.8% -43.5% -52.2% -60.9% -69.6% -73.9% -78.3% -82.6% -87.0% -91.3% -95.7% -100% -100% -100% -100% -100% -95.7% -91.3% -87.0% -82.6% -78.3% -73.9% -69.6% -60.9% -52.2% -43.5% -34.8% -26.1% -17.4% Step angle 0° 45° 90° 135° 180° 225° 270° 315° 3957 FULL-BRIDGE PWM MICROSTEPPING MOTOR DRIVER A3957SA Dimensions in Inches (controlling dimensions) 0.014 0.008 9 16 0.430 MAX 0.280 0.240 0.300 BSC 1 0.070 0.045 0.100 0.775 0.735 8 0.005 BSC MIN 0.210 MAX 0.015 0.150 0.115 MIN 0.022 0.014 Dwg. MA-001-16A in Dimensions in Millimeters (for reference only) 0.355 0.204 9 16 10.92 MAX 7.11 6.10 7.62 BSC 1 1.77 1.15 2.54 19.68 18.67 BSC 8 0.13 MIN 5.33 MAX 0.39 3.81 2.93 MIN 0.558 0.356 Dwg. MA-001-16A mm NOTES:1. Exact body and lead configuration at vendor’s option within limits shown. 2. Lead spacing tolerance is non-cumulative 3. Lead thickness is measured at seating plane or below. 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000 3957 FULL-BRIDGE PWM MICROSTEPPING MOTOR DRIVER A3957SLB Dimensions in Inches (for reference only) 24 13 0.0125 0.0091 0.491 0.394 0.2992 0.2914 0.050 0.016 0.020 0.013 1 2 3 0.050 0.6141 0.5985 0° TO 8° BSC NOTE 1 NOTE 3 0.0926 0.1043 0.0040 MIN. Dwg. MA-008-25 in Dimensions in Millimeters (controlling dimensions) 24 13 0.32 0.23 10.65 10.00 7.60 7.40 1.27 0.40 0.51 0.33 1 2 3 15.60 15.20 1.27 BSC 0° TO 8° NOTE 1 NOTE 3 2.65 2.35 0.10 MIN. NOTES:1. Exact body and lead configuration at vendor’s option within limits shown. 2. Lead spacing tolerance is non-cumulative 3. Webbed lead frame. Leads 4, 5, 12, and 13 are internally one piece. Dwg. MA-008-25A mm 3957 FULL-BRIDGE PWM MICROSTEPPING MOTOR DRIVER 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 design of its products. The information included herein is believed to be accurate and reliable. However, Allegro MicroSystems, Inc. assumes no responsibility for its use; nor for any infringements of patents or other rights of third parties which may result from its use. 115 Northeast Cutoff, Box 15036 Worcester, Massachusetts 01615-0036 (508) 853-5000