application note AN271/0189 multipower-bcd technology by c.cini, c. diazzi, d. rossi, s. storti high side monolithic switch in recent advances in integrated circuit technology have allowed the realization of a new mixed process integrating isolated dmos power transistors in combination with bipolar and cmos signal struc- tures on the same chip. called multipower-bcd, this technology has been used to realize a monolithic self-protected high-side switch mainly intended for automotive applications. driven by ttl, cmos in- put logic it can supply resistive or inductive loads up to 6a dc allowing a current peak of 25a with an r ds(on) = 0.1 w . fault conditions are signaled on a diagnostic output pin. electronic power switches in the car the increase of the number of the electrical compo- nents in the car (today more than 50) and the in- crease in assembly costs shall soon make economical multiplexed power supply and control systems. these systems consist of a single line for power supply and a multiplexed signal network for control ; in this way it is not necessaryto have a wire for every load, but only a common power line and a common signal line for all the loads (fig. 1). figure 1 : example of a multiplexed system. 1/9
figure 2 : block diagram of a peripheral unit. the control system is made, for example, with acen- tral unit near the dashboard, for the user interface, a serial data transmission line and some peripheral units near the loads (fig. 2). the multiplexed system not only makes it possible to reduce weight and overall dimensions of the cable harness, now critical in some places (e.g. the junc- tion between the vehicle body and the doors), but, also makes it possible to have a bidirectional signal betweenperipheral units and the central unit without any extra line, this is useful for fault detection and, in a future, for data transmission to make a more complex informatic system. today the key problem, from the system engineer- ing point of view, is data transmission whereas for semiconductor technology the key problem is the electronic power switches. the electronicswitch, in additionto its main function, must be able : 1) to withstand a very high peak current (20a) with total battery voltage ( 14v) applied. 2) to protect itself, the power network and the load against overvoltages (load dump 60v) and overload (protection with fuses is impractical), 3) to make some fault detections e.g. detect short circuit or open load condition. for this reasons a simple electromechanic or elec- tronic switch standing alone is not sufficient, a more application note 2/9
high side driver the problem of electrochemical corrosion is of pri- mary importance in automotive systems because the electrical components are in an adverse envi- ronment (temperature, humidity, salt), for this rea- son the series switch is connected between the load and the positive power source. therefore when the electrical component is not powered (that is for the greatest part of the lifetime of the car) it is at the low- est potentialand electrochemical corrosion does not take place. for this connection, components such as power pnp bipolar transistor or power p-channel mos would be integrated with low level signal circuitry (fig. 3a), but this kind of element is less efficient and more difficult to realize than their complementary one. npn bipolar transistors or n-channel mos, if directly driven by the supply voltage, are not a good solution because the minimum voltage drop on the switch is v be or v t (threshold voltage) ; the best so- lution is to have a driving voltage for the power tran- sistor, higher than the positive supply. nevertheless a power junction npn transistor (fig. 3b) needs a certain amount of base current ( b = 10-60 to have deep saturation) that could be obtained with a dc- dc converter; if centralized it complicates the power supply distribution network, if decentralized it com- plicates the peripheral unit always critical for size, re- liability and cost. figure 3 : possible high side driv- a) with pnp pass transistor b) with npn pass transistor (needs c) with n application note 3/9
table 1 : devices in multipower-bcd technology. vertical d-mos bv dss > 60v v th =3v f t = 1ghz p-channel drain extension bv dss > 75v v th = 1.9v f t = 200mhz c-mos n-channel bv dss > 15v v th = 0.9v c-mos p-channel bv dss > 15v v th = 1.9v bipolar pnp v ceo > 20v b =30 f t = 10mhz bipolar npn1 v ceo > 20v b =30 f t = 300mhz bipolar npn2 v ceo > 20v b = 250 f t = 1ghz bipolar npn3 v ceo > 20v b = 250 f t = 140mhz on the other side a power mos n-channel (fig. 3c), being a voltage driven device, requires for the driving only a capacitive charge pump which can be fully integrated on the switch chip. bipolar transistors moreover need driving power and principally, are limited in maximum peak power by second breakdown. the process technology for the realization of the device a mixed bipolar- cmos-dmos process has been utilized. this proc- ess integrates the following components (tab.1) (fig. 4) : figure 4 : a schematic cross section of bipolar, cmos and dmos structures. application note 4/9
figure 5 : block diagram of high side switch l9801. - n-channel power dmos able to withstand v ds = 60v for the series element. - bipolar npn and pnp transistors mainly employed in analog circuitry where low offsetand highgain are needed e.g. voltage com- parators and references, operational amplifiers. - cmos transistors to realize a dense logic network with stand by currents practically negli- gable. - passive components as resistors with a great variety of sheet resistivity (30+8500 w /) to optimize both very high and very low resistive circuitry and gate oxide capacitors (e.g. to realize charge pump capacitors). the circuit the circuit (fig. 5) is made by a power dmos series element, a driving circuit with a charge pump, an in- put logic interface and some protection and fault de- tection circuits. application note 5/9
the power dmos the power dmos transistor is an array to 10,000 elementary dmos cells that occupies an area of about 19,000 mils 2 and has a r ds(on) = 80m w with v gs = 10v. the low value of r ds(on) is required both to increase the power transferred to the load and to minimize the power dissipated in the device. in fact the switch must be operative also at very high am- bient temperature(125 c) as required in automotive applications. for example to drive a 5a (60w) load, the drop on the switch is 400 mv and the dissipated power is 2w (r th j-case 1.25 c/w). the charge pump the charge pump is a capacitive voltage doubler (fig. 6) starting from power supply (car. battery), driven by a 500 khz oscillator. the pump capacitor is an integrated 80 pf capaci- tor, the storage capacitor is the gate capacitance of the power itself ( ~ 500 pf). figure 6 : charge pump. input interface considering the very wide operating temperature range (tj = 40 to + 150 c) it is not possible to ob- tain the logic threshold from the conduction thre- shold of any elementary device, because of its tem- perature coefficient, respecting ttl input levels. nevertheless a solution with a voltage reference and comparator is not suitable because it needs a bias current flowing also when the device is in the off state. this point is of great importance because the switches are directly connected to the car battery without the interposition of the ignition switch, thus also a little current (> 50 m a) multiplied for the num- ber of the switches (e.g. 50), causes an appreciable discharge current always flowing. for this reason a threshold circuit has been designed derived from a well known voltage reference (fig. 7). figure 7 : input interface. fixed a threshold value v in =v in * for this value must be, by design i 1 =i 2 =i 0 *. a q1 a q2 if a = area ratio a = d v be r1 must be i 1 = (i 1 +i 2 )r2+v be (q2) = v in r2 kt r1 q that is v in *=2 1n a +v be q2 reasoning around the threshold point it can be noted that the transco nductance of q2 is greater than the transconductance in q1 branch (q1series r1). for this if v in >v in *i 2 >i 1 q3 on if v in the choice of the values is made imposing : 1) v in * v bg 1.250v band-gap voltage of sili- con. in this case v in * is practically stable in tempera- ture and centered respect ttl input levels (v lmax = 0.8v. v hmin =2v). the idle current i abs in the worst case, that is when v in =v lmax = 0.8v. tj = 150 c, it must be i abs =i 1 +i 2 <50 m a. the proposed circuit has also a third working region : when v in conclusion - future developments a process allowing the integration of power dmos, cmos and bipolar transistor makes possible the construction of a monolithic switch comprehending also protection and fault detection functions. the power dmos approach allows also the possi- bility to make a large range of power switches with different on resistance and current capability only scaling proportionally the power area. moreover the cmos structures can be utilized to make also the coder/decoder circuit to interface di- rectly the transmission line. those features and the possibility to integrate more than one power element on the same chip makes possible, in a near future, the integration of the whole peripheral unit. application note 8/9
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 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-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 9/9
|
