application note 6a door lock motor driver for automotive by stefano vergani the l9937 device is a full bridge for bidirectional motor driver applications realized in bipolar tech- nology; it can deliver up to 6a output current with low saturation voltage. two diagnostic informations are provided to moni- tor overload conditions and the internal tempera- ture, and the device is assembled in the multi- watt-11 package with the case connected to the ground terminal. the l9937 is particularly suitable to drive bidirec- tional dc motors in m c based systems. fig. 1 shows a possible application circuit, with an analog interface between the power devices and the m c. in the following, the functions of each block of the analog interface are described. 1 - overvoltage and reverse battery protection l9937 is particularly suitable to drive the door lock motors in automotive applications. fig. 2 shows the circuit schematics; due to the hostile automotive environment, it is necessary a transil (suggested type ldp24a) between v d and gnd, to protect the l9937 against overvoltages higher than 50v. the diode d1 suggested type by239- 200a) supplies the voltage vd necessary for the correct device's operation at the same time it protects the device against the reverse battery. AN456/0191 t is advanced information on a new product now in development or undergoing evaluation. details are subject to change without an application of the l9937 device (full bridge motor drive) is described. the interface between the l9937 and a m c is discussed. a complete protections circuitry description is also given. figure 1. 1/6
2 - switch-off sequence referring to fig. 2 and supposing i.e. t1 and t4 on, t2 and t3 off (this means en=h in1=h in2=l), the following steps have to be observed to allow a correct recirculation of the current in the motor at the switch off (ref. fig. 3): a)switch off t1 and wait for 100 m s about in this condition (en = l in1 = h in2 = l) b)after the a.m. delay switch on t2 (en = l in1 = h in2 = h) c)switch off both t2 and t4 after the motor stop (en = l in1= l in2 = l) step a) allows the recirculation of the motor cur- rent due to the inductive component of the motor itself between ds1 and t4; the 100 m sec delay time is needed to avoid the cross-conduction in the left half bridge. in step b) the motor is short circuited to gnd (t2 and t4 on) and this allows the dynamic braking. in step c) t1, t2, t3 and t4 are off to allow a very low current consumption of the bridge. if the dynamic braking is not requested, step b) can be omitted. in any case the lower power tran- sistor of an half bridge must be kept on, after the switch off of the upper transistor of the other half bridge, for a time longer than t = 5 ? r l /l l , where rl and ll are the resistance and the inductance of the load. figure 2 figure 3: switch-off sequence application note 2/6
3 - input driving voltage to allow a correct operation of l9937 over the full temperature range, the driving voltage at the input pins must be higher than 5.5v, with 4ma current capability. 4 - short circuit protection it is possible to protect l9937 against short circuit to ground and across the motor in the full bridge application. the circuit schematics shown in fig. 4 uses two voltage comparators (u1a, u1b) to detect the vce of the upper power transistors. u2a and u2b are open drain nand gates (i.e. part no. hcc40107) and u3a/b/c/d are non inverting buffer to drive the l9937 (i.e. part no. 74hc4050). u1a and u1b sense the differential voltage vd- out2 and vd-out1 respectively. referring to fig. 4, chosen r1=100k and r2=20k, the values of r3 and r4 may be calculated according to the following formula: r3 = ( v d - v ceth )- 0.166 v d ) 0.166 v d * r4 where : v d = bridge power supply v ceth = collector to emitter detection threshold. choosing: v ceth =2v@v d = 12v and r4 = 1k, the above formula gives r3 = 4k. when all signals from m c are at low level (motor off), the inputs to the bridge are low too; in these conditions the output voltage of the two compara- tors is high and therefore the outputs of u2a/u2b are free. when the m c sends, for example, in2 and en high, out2 of the bridge goes high and out1 goes low. at this point the output of u1a pulls down the input of u2a before that the delay capacitor c1 is charged (through r5) up to the u2a threshold; in this way the u2a output remains free and the bridge drives the motor. if a short circuit occurs, the vce of the upper power transistor increases above the threshold and then the u2a output pulls down the enable input of l9937. at the sdme time the sc signal to m c, high in normal conditions, goes low; at this point the m c executes the switch-off sequence. we have just explained what happens when a short circuit occurs during the motor running phase. another faulting condition occurs switch- ing on the bridge when a short circuit is present; in this case the bridge is driven for a time depend- ing on the time constant r5 ? c1 = r6 ? c2. choosing r5 = r6 = 3.3k and c1 = c2 = 1nf, then the time constant will be t = 3.3 m sec, that is 5 m sec about delay time. longer delay time might allow the short circuit current to reach val- ues beyond the absolute maximum ratings. figure 4 application note 3/6
5 - thermal protection the l9937 has 5 built-in diodes series-connected that can be used to implement a thermal protec- tion for the device. fig. 5 shows the relationship between the voltage across the diodes and the temperature at 100 m a diode current. fig. 6 shows the simplest solution to do a thermal protection; an a/d converter of the m c is used to detect the voltage drop across the 5 diodes. the 15k resistor sets the current in the diodes and the 100nf capacitor acts as a filter against the noise. when the m c detects a voltage lower than the low threshold chosen according to the diagram in fig. 5, it executes the switch-off sequence and re- jects any command to the bridge until the diodes voltage increases beyond the high threshold. the recommended hysteresis value is 30 c. 6 - overvoltage protection at point 1 we suggest a way to protect the l9937 against the voltage transients. this protection al- lows the device to withstand overvoltages only if the bridge is not operating. to protect the device against the overvoltages in all the operating con- ditions it is possible to implement the circuit shown in fig. 7. (note: a-b are connected to the nodes between r3 and r4 (left side and right side) in fig. 4; c is connected to the node between r1 and r2 in fig. 4). when v d reaches 18v the comparator output pulls down a and b, causing the intervention of the hardware protection showed in fig. 4; at the same time the ovv signal is sent to m c, which executes the switch off sequence. the m c must reject any command to the bridge during the over- voltage conditions. with the values shown in fig. 7, a 1v hysteresis is provided. it is possible to enhance the performances of the system just described avoiding the braking of the motor also for short duration voltage transients; to do this the m c, once received the overvoltage di- agnostic signal (ovv), put at low level the enable of the l9937, confirming the hardware switch-off of the motor; in this condition an output of the half bridge is in high impedance state and the other one is low, allowing the recirculation of the current and the free running of the mo- tor. the system holds this condition until ovv is ac- tive; when the ovv signal is released the m c re- sets the hardware protection, sending en = in1 = in2 = l and then restore the previous command to the bridge. it is mandatory, however, to wait for the complete current recirculation of the motor before to reset the hardware protection; in facts, when en = in1 = in2 = l both the l9937 outputs are in high im- pedance conditions. 7 - diagnostic feedback output df pin is an open drain output to monitor overcur- rent and overtemperature conditions. the overcurrent detection threshold is inversely dependent from the temperature of the chip. typical application of this function is to send the df signal, with an external pull-up to vcc, to a digital input of the m c; when the df signal goes at low level, the m c executes the switch-off se- quence. figure 5 figure 6 figure 7 application note 4/6
l9937 in a body multiplex environment all the functions described above can be imple- mented in a custom integrated circuit together with a bus transceiver and a protocol handler. it is then possible to obtain a very small size mod- ule that can be integrated directly in the actuator. fig. 8 shows a typical application of these mod- ules as peripheral units in a oclass ao wired multi- plex system. figure 8: oclass ao wired peripheral application application note 5/6
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. specifications men- tioned 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 ex- press 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 6/6
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