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application note AN243/1288 switchmode drivers for solenoid driving this design guide describes the operation and applications of the l294 and l295 switch-mode solenoid drivers. integrating control circuitry and power stage on the same chip, these devices replace complex di- screte circuits, bringing space and cost savings. many applications, particularly in computer peri- pherals, require a high power, fast solenoid driver circuit. in the past these circuits have been reali- sed with discrete components because the high powers required precluded the use of monolithic technology. sgs-thomson microelectronics has overcome this problem with a new high power bipolar techno- logy that uses an innovative implanted isolation te- chnique. this technology is used to fabricate two switchmode solenoid driver chips, the l294 and l295,which both incorporatehigh power outputsta- ges and control circuitry. both circuits are designed for efficient switmode operation and are mounted in multiwatt ? plastic package. the l294 solenoid driver the l294 is designed for solenoid driving applica- tions where both very high speed and high current are essential ; needle and hammer driving in printer mechanisms, for example. itdelivers 4 a with supply voltages up to 46 v, handling effective powers up to 180 w. shown in figure 1, the l294 is controlled by a ttl - level logic input and the peak load current is pro- grammed by a reference voltage applied to the pin labelled v i . internal switchmode control circuitry regulates the solenoid current by turning the output stage on and off repeatedly to keep the load current between the figure 1 : internal block diagram of the l294 switchmode solenoid driver. 1/12
programmed peak value, i p , and a lower limit of 0.9 i p . other features of the l294 include thermal shut- down, output short circuit protection, overdriving protection and a latched diagnosticoutput. this out- put indicates fault conditions such as a short circuit solenoid. circuit operation in most applications the l294 is used with a fixed ref- erence voltage (v i ) and the solenoid is controlled by negative-going pulses on the enable input. when the enable input is active (low level), the output stage is enabled and the load current rises as shown in figure 2. the load current is sensed by an external resistor (r s ) in the emitter of the sink stage. through the op amp and transconductance amplifier (ota), the sensed voltage charges an external rc network (r1c1) which determines the switching characteris- tics of the device. figure 2 : output current waveforms of the l294. the output current in regulated by swit- ching between a peak value, ip, and a lo- wer limit of 0.9 ip. figure 3 : two level current control can be implemented by switching vi between two values. application note 2/12
the voltage across this rc network is compared with the voltage v i , which fixes the output peak cur- rent. when the current has reached the program- med peak value this comparator switches, turning off the output source stage and closing a switch which reduces the voltageon the non-inverting input to 0.9 v i . the load current now recirculates in d1. the voltage on pin 8 falls with a time constant de- termined by r1c1 or the load characteristics, whi- chever is the longest. in other word. r1c1 sets the minimum recirculation time constant. when the voltage across r1c1 has fallen to the 0.9 v i thresholdthe comparator switches on, turning the output stage back on and restoring the v i compari- son threshold. the output source stage is switched in this way, re- gulating the load current, until the enable input goes high again. at this point the output stage is di- sabled - both source and sink - and the load current recirculates through d1 and d2 to ensure a fast de- cay. by varying the voltage v i the peak load current can be programmed to any value in the range 0.6 a to 4 a. this feature can be exploited to implement two-level current control if the fixed reference is re- placed by a switched referenceas shown in figure 3. protection to protect the load and the l294 from overdriving an on-time limiter inhibits the output stage inde- pendentlyof the enable input if the duration of the input pulse exceeds a period set by the external ca- pacitor c2 (figure 4). this circuit is reset by taking the enable input high. the on-time limiter can be disabled by grounding pin 3. protection against overheatingis incorporated in the form of a thermal shutdown circuit which disables the output stage when the junction temperature ex- ceeds 150 c. the circuit restarts when the tempe- rature has fallen about 20 c. the l294 is also protected against short circuits to ground, to supply and across the load. triggered when the source stage current exceed 5 a or the sink stage current exceed 1 v/r s , the short circuit protection block inhibits the output stage and sets a flip flop which is supplied by a separate supply volt- age v ss . this flip flop is connected to the diagnostic output and signals that all is not well - a shorted so- lenoid, for example. the diagnostic flip flop is reset by removing the supply v s . a led can be connected to the diagnostic output as shown in figure 5. if the diagnosticfunction is not re- quired the v ss supply can be omitted. the short cir- cuit protection, however, still functions, evenwithout v ss . using the l294 the basic application circuit for the l294 is shown in figure 5 ; a suggested layout is given in figure 6. the circuit is complete except for the source of v i . in most cases this will be provided by a simple resi- stive divider dimensioned to set the desired peak current. with a 0.2 w sense resistor as shown, the l294 has a transconductanceof 1a/v for v i above 600 mv. the device will not work with v i less than 450 mv and operation is not guaranteed for v i bet- ween 450 mv and 600 mv. the on-time limiter delay - set by c2- is approxima- tely 120 000 x c2. pin 3 must be grounded if the on- time limiter isn't used. switching frequency depends partly on the timing network r1c1 and partly on the load characteristics. r1c1 determines the minimum value of t 1 (see fig- ure 2), which is given by t1 0.1 x r1c1. c1 must be in the range 2.7 - 10 nf to ensure stability of the amplifier ota. r1 must be at least 10 k w to give suf- ficient gain for ota. the standard application circuit of figure 5 has a switching frequency of about 10 khz. the recirculation diodes should be fast typesand ra- ted at 3a (d1) and 1a (d2). if the full 4a capability of the l294 is not used these can be reduced. figure 4 : ontime limiter waveforms. after a pe- riod defined by c2 the output is disabled regardless of the state of enable, pro- tecting against overdriving. application note 3/12
the peak current, i oex , (see figure 7a) is found from : v z r2 1 i oex = .. 5r s r1 + r2 v z is the zenervoltage.the zenerand r5 can be omit- ted if a regulated 5 v supply is available for point a. the holding current, i hold , is found from : v z (r2 // r4) 1 i hold = .. 5r s r1 + (r2 // r4) theduration of the peak is determined by r3c1 and is increased by raising r3 or c1. typical component values are listed in the table be- low : figure 5: standard solenoid driving application of the l294. pin 7 must be connected to suitable reference voltage to set the peak current. d1: 3a fast diode d2: 1a fast diode trr 200ns a high initial peak and low holding current can be ob- tained with the circuit shown in figure 7a. this exam- ple supplies a current peak for about 10 ms. +i oex =4a i hold =1a +i oex = 2.5 a i hold = 0.5 a r1 r2 r3 r4 r5 d1 d2 c1 10 k w 47 k w 150 k w 2.7 k w 0.2 w (1 w) 3a 0.5 a 0.2 m f 10 k w 27 k w 150 k w 1.5 k w 0.27 w (0.5 w) 1.5 a 0.5 a 0.2 m f figure 6: suggested printed circuit board laypout for the application circuit of figure 5. application note 4/12
figure 7a : application circuit for two level current control. this circuit generates a high peak current for a period determined by r3c1 then a lower holding current. figure 7b : output current waveform obtained with the circuit of fig. 7a. application note 5/12
figure 8 : pin functions of the l294. n function 1 solenoid supply voltage v s (12-46 v) 2 output, source stage 3 on-time limiter time constant. a capacitor to ground sets delay period (120 000 x c2 seconds). on-time limiter is disabled by grounding this pin. 4 supply input (5 v) for diagnostic flip flop. 5 diagnostic output, open collector. signals intervention of latched short circuit protection. reset by removing pin 1 supply. 6 ground. 7 v i reference input. peak output current is proportional to v i . transconductance is 1 a/v for r s = 0.2 w and v i 600 mv. 8 timing. a parallel rc network from this pin to ground sets the minimum recirculation time constant. the capacitor must be 2.7-10 nf to ensure stability. the resistor must be greater than 10 k w . 9 enable. ttl-compatible logic input that controls the solenoid current. the solenoid is driven when this input is at a low level. the on-time limiter overrides enable. 10 connection for load current sense resistor. 11 output, sink stage the l295 dual switchmode driver the l295 is a dual switchmode solenoid driver which handles up to 2.5 a per channel at voltages up to 46 v - a total effective power handling of 220 w. compared to the l294 it offers a more economi- cal solution when 2.5 a is sufficient because there are two drivers per chip. like the l294 it features switchmode regulation of the output current and thermal shutdown. additionally it has a separate lo- gic supply input so thatthe logic can be run at a lower voltage, reducing dissipation. intended for inductive load driving, the l295 is par- ticularly suitable for solenoids and stepper motors. one l295 drives two solenoids and two l295s can drive the four phases at a unipolar stepper motor or the two phases of a bipolar stepper motor in bridge configuration. each channel of the l295 is controlled by a ttl-le- vel digital input and the peak load current is pro- grammed, independently for each channel, by a voltage reference input. a chip enable input is also provided to disable both channels together. inside the l295 internally the l295 (figure 9) bears little resemblan- ce to the l294. looking at channel one, when the v in1 input goes high the output transistors q1 and q2 are switched on (the enableinput en isassumed to be active, i.e. low). the current in the load then rises exponentially, as shown in figure 10, until the voltage across the external sense resistor r s1 rea- ches the current program reference voltage v ref1 . the comparator comp1 switches and sets the flip flop ff1 which turns off the source transistor q1. the load current now recirculates through d2-q2- r s1 and decays. what happens next is determined by the oscillator componentsr and c on pin 9. if these components are present the flip flop is reset by the next clock pul- se before the current decays very far. the output stage is therefore turned on again and the load cur- rent rises. when it reaches the peak value comp1 switches again, setting the flip flop and disabling the output stage. this process is repeated, regulating the load current until v in1 goes low. the output stage is then disabled and the current falls off rapidly, recircula- ting through d1 and d2 (figure 10). if the oscillator components are omitted and pin 9 grounded the current simply decays slowly until v in1 goes low. the output stage is then disabled and the load current recirculates through d1 and d2. this case is illustrated by the waveforms of figure 11. note that in this case the peak current level is con- trolled. unlike the l294, the switching frequency of the cur- rent regulation loop is determined by the oscillator components r and c (the l294 is also affected by the load). typically, the switching frequency will be 10-30 khz. another difference between the two de- application note 6/12
vices is that the l294 gives a constant ripple, the l295 does not. two level control since the peak load current is programmed by the reference voltage (for each channel), two level cur- rent control can be obtained by switching between two referencevoltages. a high v ref is selected initial- ly to give a high initial current peak. then, after a sui- table interval, v ref is reduced to give the lower holding current (figure 12). two level current control is very useful for solenoids which require a high ini- tial current peak for fast actuation. figure 9 : internal block diagram of the l295 dual switchmode driver. application note 7/12
figure 10 : waveforms illustrating normal opera- tion of the l295. figure 11 : when the oscillator components are omitted and pin 9 grounded the l295 delivers a simple current peak to the load. figure 12 : two level current control is obtained by switching v ref between two values. application note 8/12
l295 application hints the basic application circuit of the l295 is shown in figure 14. a suitable layout is given in figure 15. suitable values for the oscillator components, r and c, can be found from the nomogram, figure 16. the value for the reference voltages dependson the de- sired peak current and is equal to i p r s ; it must be in the range 0.2 v to 2 v. if the v ref inputs are left open circuit the l295 assu- mes an internal default value of 2.5 v giving a peak current of 2.5/r s amperes. the l295 can also be used to drive unipolar stepper motors. for a four phase motor two devices are used, connected as shown in figure 17. this circuit provides switchmode regulation of the load current with a chopper rate of about 25 khz. the enable in- puts (en, connected together) enable/disable the whole circuit and the channel inputs v in1 ...v in4 are driven by a suitable translator circuit. phases 1 and 2 must not be energised togetherbecause theysha- re the same sense resistor. the same applies to channels 3 and 4. however, ' two phase on' drive is still possiblefor bifilar motorswhere phases one and two represent one winding and 3 & 4 the other, and also for variable reluctance motors with phase 1 ad- jacent to phase 3 etc. two l295s could also be used to drive a bipolar stepper motor in systems where a translator already exists. figure 13 : pin functions of the l295. n function 1 solenoid supply voltage, v s (12-46 v). 2 channel one output, source stage. 3 channel one output, sink stage. 4r s1 . sense resistor connection, channel one. 5v ref1 . a voltage on this pin sets peak current of channel one. if this pin is left open or connected to v ss a default v ref of 2.5 v is assumed. an externally applied v ref must be in the range 0.2 to 2 v. 6 vin1. logic input for channel one. driver is active when vin1 is high and en low. 7 en. chip enable (active low). when high both channels are disabled. 8 ground. 9 oscillator timing network. this pin is grounded to produce a single peak. 10 v ss . logic supply voltage, internally regulated. (4.75-10 v). 11 v in2 . logic input for channel two. driver is active when v in2 is high and en low. 12 v ref2 . voltage input, controls peak current of channel two. if left open or connected v s an internal 2.5 v reference is assumed. an externally applied v ref must be in the range 0.2 to 2 v. 13 r s2 . sense resistor connection, channel two. 14 channel two output, sink stage. 15 channel two output, source stage. application note 9/12
figure 14 : typical pplication circuit of the l295. r1 l1 and r2 l2 are solenoids. figure 15 : suggested printed circuit board layout for the circuit of figure 14. d1 : 3a fast diode trr 200ns d2 : 1a fast diode application note 10/12
figure 16 : nomogram for the selection of values for the oscillator components. rc. figure 17 : two l295s, connected as shown, can be used to drive a four phase unipolar stepper motor. application note 11/12
information furnished is believed to be accurate and reliable. however, sgs-thomson 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. specifica- tions mentioned in this publication are subject to change without notice. this publication supersedes and replaces all information pre- viously 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-thomson 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 12/12

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