application note AN238/0488 ease stepper-motor-drive design high-power, dual-bridge ics in addition to simplifying design problems, a family of dedicated chips improves stepper-motor drive-circuit reliability by significantly reducing the component count. figure 1 : the simplest stepper-motor drive technique is the basic l/r configuration. adding series re- sistors and raising the supply to make an l/4r drive improves torque at high steps rates but reduces efficiency. the l293, l293e and l298n dual-bridge ics (see box , oinside the dual-bridge icso) significantly redu- ce the problems encountered in the design of step- per-motor drive circuitry. they can, for example, simplify the design and increase the efficiency of constant-current choppers. and with a single chip replacing the transistors and predriver stages, cir- cuit performance improves. best of all, the devices have applications in complex as well as basic driver networks. 1/8
simplest design is an l/r drive the simplest motor-drive configuration (fig. 1) con- sists of a m c that performs the translator function in software (see box , ogenerating switching sequen- ceso) and drives the motor through a l293 dual brid- ge. only eight external components are required ; these are diodes that protect the device's output transistors against inductive spikes generatedwhen a winding de-energizes. the l293 handles 1a continuous (for higher current use and l298n). however, if you plan to run the mo- tor continuously with two phases on, dissipation will be the limiting factor. you can improve the performance of this basic l/r drive by increasing the series resistance and raising the supply voltage to restore the original phase cur- rent. at high speeds,torque improves, but efficiency decreases. normally, you increase each winding's resistance by a factor of four through the addition of a 3r series resistance, resulting in the l/4r drive. dual-bridge driver ics reduce the parts count ofbipolar steppermotors andsimplify design. the sche- matic of the l298n is functionally similar to those of the l293 and l293e. inside the dual-bridge ics the l293, l293e and l298n (figure) contain two po- wer-transistor bridges, predriver stages, control logic and protection circuitry. there's a control input for each bridge and an enable input for each half bridge ; inputs connect directly to m cs, cmos or ttl. the ics inte- grate level shifters with a separate logic-supply pin. for current sensing, the l293e and l298n have external emitter connections. a single package drives a 2-phase bipolar stepper mo- tor, challenging the assumption held by many that uni- polar motors are easier to drive. you can use a bipolar motor - simpler and less expen- sive than a unipolar motor - without building complex power stages. furthermore, you don't have to worry about simultaneous conduction of a half bridge's sour- ce and sink transistors - a basic problem with discrete- component bridge circuits. chip design makes it impossible for both transistors to be on at the same time. designers should also discard the mistaken idea that constant-current chopper drivers are complicated and expensive. you can build one with two bridge ics and a few passive components. type i o i o(peak) v s package sensing connections l293 1a 1.5a 36v dip16 l293e 1a 1.5a 36v dip20 one per half bridge l298n 2a 2.5a 46v multiwatt15 one per bridge application note 2/8
multiple supplies boost perfor- mance a dual-level supply also improves the performance of a basic l/r circuit. a high supply voltage yields good torque characteristics when the motor is run- ning. a lower-than-rated voltage provides some hol- ding torque when the motor is at rest, therebysaving power when the motor is idle. fig. 2 shows a suitable voltage-switch circuit. r x sets the holding current, which can be low because a permanent magnet or hybrid stepper motor provi- des some holding torque at zero current. however, make certain the l293's motor-supply input never goes below the logic-supply voltage. while there's no danger of damaging the device, it's imposssible to drive the output transistors correctly under such conditions. the dual bridge's enable inputs offer a means of ex- tending the chip's flexibility. for example, you can connect them directly to the logic supply - no resi- stors are needed - to enable the chip permanently. as an alternative, use the enable inputs to disable the motor during the power-on reset sequence. in wave-drive and half-step modes, use the enable inputs to increase torque at high speeds. when a winding de-energizes, flux collapse is a function of the current-decay rate. during this decay, the de- energized winding opposes the efforts of the next winding in sequence, partially cancelling the torque. you can minimize this effect by disabling a bridge only when the winding it drives is turned off ; becau- se the d i/ d t of an inductor equals e/l, disabling the bridge accelerates the current decay.this action di- scharges the winding's stored energy through its supply and maintains the terminal voltage e at v s plus two diode drops. if you were to leave the bridge enabled, the current would flow to ground through one diode and one transistor, and it would lower the terminal voltage.this scheme doesn't applywith dri- ves with two phases on becauseno winding ever de- energizes. figure 2 : switching the supply to a lower volt- age when the motor is idle saves cur- rent without compromising driving power. figure 3 : maintaining a constant-average phase current this fixed-ripple chopper provides improved performance and efficiency v ref controls the phase current. application note 3/8
generating switching sequences in addition to selecting a motor and determining power- stage design, you must also decide how to generate the switching sequences that step the motor. programming a m c or using a special piece of hardware called a tran- sistor accomplishes this task. software translation is more economical, and it is the first choice for large-volume products. fig. a shows a basic step procedure (a) that you can integrate into a routine (b) ; the routine executes a clockwise rotation of n steps at a fixed rate. the step rate is defined by a software loop, but you can also use programmed timer interrupts. fig. a :a m c can generate the phase sequence (a) for a stepper motor. a routine (b) expands a single-step routine into are that executes a move of n steps. a simpler approach uses the software equivalent of a shift register. for example, you can load a 99 (hex) into a register and take the phases from bits 0 to 3. a rotate left instruction yields a clockwise step ; a rotate right instruction causes a counterclockwise move. when software translation ties up your m c, lighten the load by adding a hardware translator. in applications in- volving unidirectional motors, this logic circuit (fig. b) re- quires only one pulse for each step ; you'll also need a direction signal (b) if your motor rotates in both direc- tions. by adding a 7408 to a 2-phase translator, you can satisfy a wave-drive application (c), while the addition of two or gates provides fast turn-off in wave-drive mode. fig. b : built a simple 2-phase hardware translator using a dual flip-flop, for single (a) or bidirec- tional (b) rotation. add some extra ics for wave-drive signals (c) and to provide fast turn-off (d). a) b) b) c) d) a) application note 4/8
often a m c controls the translator, setting the direction line and providing a pulse for each step. software is thus simplified, and if you use a programmed interrupt scheme, the m c is free to handle other tasks. fig. c de- scribes an absolute-positioning routine for a step with a direction-control translator ; fig. d outlines how pro- grammed timer interrupts are used to relieve the bur- den on the c. two special cases call for hardware translation. the first is in a system for which you have already designed in control circuitry to provide step and direction signals. the second case involves single-quantity and small- run applications, in which the cost of a few ics is a small price to pay for simplified software. figure c : for use with a hardware trans- lator this a absolute-positioning routine sets the direction line and sends the appropriate number of step pulses. figure d : to set a motor step rate, used programmed timer interrupts in place of software timing loops. notes : enter with de- sired position current po- sition in register or mem- application note 5/8
chopper circuit offers more en- hancements adding a chopper circuit to maintain a constant-ave- rage phase current improves performance and effi- ciency. fig. 3 shown a simple constant-currentdrive that employs a dual bridge, dual comparator and a few passive components. this circuit requires an l293e, because this dual-bridge ic offers access to the lower emitter connections, thus letting you insert current-sensing resistors. operation of this fixed-ripple chopper drive is strai- ghtforward. when the m c or translator activates a bridge,the increasing load current raises the voltage across the sensing resistor until it equals the com- parator's reference voltage. the comparator then switches, clamping the translator signals through the diodes to deactivate the bridge. as the current decays, the voltage across the sensing resistor de- creases until it equals the comparator's lower thre- shold. the comparator switches again, allowing the m c or translator to activate a bridge and restart the cycle. as long as the translatordrives the bridge,this sequence repeats to provide a constant-average phase current with fixed ripple. v ref adjusts the lower current limit, while the compa- rator's hysteresis sets the ripple, and hence the peak current. although a divider establishes the va- lue of v ref in this case, you can employ the dual-sup- ply design approachand switch v ref toa lower value when the motor is idle. in addition, use of a d/a con- verter establishes v ref for micro-stepping applica- tions. this drive circuit, with its fixed-ripple current characteristics, is well suited for such service. fixed-frequency chopper is motor independent fig. 3's drive has some disadvantages. first, the chopper frequency depends on motor characteris- tics, and these parameters vary from unit to unit. in addition, it's impossible to synchronize the chop- pers, and this shortcoming can cause trouble on the ground plane. using a flip flop/comparator arrangement (fig. 4) to develop a fixed-frequency chopper overcomes the- se problems. in this circuit, the ne555 timer gene- rates negative pulses that reset the flip flops to enable the phase-controlsignals from the translator. if these signals are set to energize a winding, the figure 4 : this fixed-frequency constant-current chopper driver enables the synchronization of several drives, thus minimizing potential ground-plane problems. application note 6/8
figure 5 : a special translator-chopper control circuit cuts the drivers components count to the mini- mum. current in that winding rises until the voltage across the sensing resistor switches the comparator, thus setting the flip flop. this disables the phase signals and deactivates the bridge. current in the winding falls until the next clock pulse resets the flip flop, and the sequence repeats to maintain a constant cur- rent. controller ic reduces component count if you're using a hardware translation and constant- current choppers, you can further reduce the com- ponent count by using a controller chip such as the l297 a 20-pin dip that houses a translator and a dual fixed-frequencychopper circuit. under the con- trol of step and direction inputs, the l297 generates normal, wave-drive and half-step sequences. as shown in fig. 5, the controller connects directly to a dualbridge. externalcomponentrequirementsare minimal : and rc network to set the chopper fre- quency and a resistive divider to establish the com- parator reference voltage (v ref ). to accommodatemotors with a phase current as great as 3.5 a, replace the single dual-bridge ic with two de- vices configured in parallel (input to input, enable to enable, etc) to form a single bridge. it's extremely im- portant that you pair the half bridges 1 with 4 and 2 with 3 to ensure optimum current sharing. reprinted from edn. 11/24/83 ? 1986cahnerspublishing company division of reed publishing usa. application note 7/8
information furnished is believed to be accurate and reliable. however, sgs-t homson microelectronics assumes no responsibility for the consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. no license is granted by implication or otherwise under any patent or patent rights of sgs-thomson microelectronics. specifications mentioned in this publication are subject to change without notice. this publication supersedes and replaces all information previously supplied. sgs-thomson microelectronics products are not authorized for use as critical components in life support devices or systems without express written approval of sgs-thom son microelectronics. ? 1995 sgs-thomson microelectronics - all rights reserved sgs-thomson microelectronics group of companies australia - brazil - france - germany - hong kong - italy - japan - korea - malaysia - malta - morocco - the netherlands - singapore - spain - sweden - switzerland - taiwan - thaliand - united kingdom - u.s.a. application note 8/8
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