Abstract:
A driving circuit for generating a required firing current for a safety device comprising an arrangement of a first transistor (M 2 ) connected in series with a second transistor (M 3 ); and a power control transistor (M 1 ) connected in series with the first transistor; characterised in that the first and second transistors operate in fully switched on mode (Rds(on)) and the required firing current (I(squib)) is generated by means of varying the voltage (Vc) across the gate source of power control transistor and the first and second transistors in a predetermined manner.

Description:
FIELD OF THE INVENTION 
     This invention relates to a digital squib driver architecture and design, in particular but not exclusively for the automotive industry. 
     BACKGROUND OF THE INVENTION 
     The automotive industry has for many years provide vehicles with electronically controlled elements. For example, the elements may be an airbag for deployment in the event of an accident or crash. The crash is detected typically in about 10-15 milliseconds. Then an electric signal is sent to fire the airbag. This is typically accomplished by means of squib which ignites a propellant (e.g. sodium azide) which generates a gas (e.g. nitrogen) which in turn inflates the airbag. This stage typically takes about 45-55 milliseconds and then within a further 75-80 milliseconds the airbag deflates. A squib is a small explosive device or other firing device for a safety device or other type of device: for example, an airbag activating component. 
     The energy required to fire the squib is generally provided by a reservoir capacitor. This capacitor must store as much energy as possible and as such the voltages is typically high (˜35V). An example of a current squib driver circuit has an output driver comprising two switches (a high-side and a low-side). The high-side is driven in a current limitation mode which generates a very high energy requirement for the power MOSFET. For example 35V and 2.1 A for a 1 ms period would equate to 73.5 mJ. As the MOSFET size is related to the amount of energy to be absorbed, the higher the energy the larger the MOSFET. This can have a large impact on the size and cost of the ASIIC. 
     Airbag applications also call for a safing switch in order to increase the overall safety by adding redundancy. This is achieved by providing a power switch that is off until a redundant microchip allows the firing of the squib. The redundant chip and squib are in series. These guarantees that the squib does not fire the airbag until all crash algorithms and validation are completed to confirm the need for airbag deployment. The safing switch is generally an external MOSFET device  110 . 
     One object of the present invention is to overcome at least some of the disadvantages of the prior art. A further object of the present invention is to reduce the size of components in a driving circuit. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and apparatus as described in the accompanying claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Reference will now be made, by way of example, to the accompanying drawings, in which: 
         FIG. 1  is a basic circuit diagram of a digital squib driver in accordance with one embodiment of the present invention. 
         FIG. 2  is a basic circuit diagram of a two stage firming digital squib driver in accordance with one embodiment of the present invention. 
         FIG. 3  is a basic circuit diagram similar to  FIG. 3  showing a first balancing technique in accordance with one embodiment of the present invention. 
         FIG. 4  is a basic circuit diagram similar to  FIG. 3  showing a second balancing technique in accordance with one embodiment of the present invention. 
         FIG. 5  is a basic circuit diagram similar to  FIG. 3  showing a third balancing technique in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1  a digital squib driver  100  is shown. The squib driver may be used to drive a squib firing in any application including automotive applications. The squib driver includes a safing MOSFET M 1  a hi-side switch M 2  and a lo-side switch M 3 . Both M 2  and M 3  are driven in full Rds (on) mode. This means that the transistor is fully turned on. The squib current I (squib) is regulated by means of the safing MOSFET M 1 . The Vc voltage across the gate-source of M 1 , M 2  and M 3  regulates the voltage on Vz based on previous diagnostics, including for example resistance measurements of the squib (R squib ) high-switch (R HS ) or low switch (R LS ). The value of I(squib) is thus equal to Vz/R total. R total is equal to the sum of R squib , R HS  and R LS . R HS  and R LS  are known and vary with temperature in a predictable manner which can be determined by measurements. R squib  typically varies from 1.7Ω to 4.7Ω but may be higher in certain circumstances. R squib  may also be determined by measurements. 
     With these resistance measurements a control circuit (not shown) can control V C  such that the firing current is of the required value whilst M 2  and M 3  are kept fully switched on in Rds(on) mode. This circuit arrangement in accordance with an embodiment of the present invention is much less complex and overcomes many of the disadvantages associated with the prior art. Notably, some of the problems of size of MOSFETs M 2  and M 3  and overall circuit complex predrivers; thermal problems associated with the switches; and energy issues are all ameliorated or overcome. 
     In certain circumstances two stage firing is required. The circuit in  FIG. 2  is an example of an embodiment which operates two or more firings at a time. The circuit shown generally at  200  includes a safing MOSFET M 1  and two firing circuits  202  and  204 . Each firing circuit is equivalent to that in  FIG. 1  and includes respectively high-side and low-side switches which include MOSFETS (M 2  A &amp; B and M 3  A &amp; B). All the MOSFETS are driven in full Rds(on) mode. If the squib resistance for each are different there will be different currents required for firing the squib. In order to ensure that both squibs fire the value of V C  need to be adjusted to provide the higher of the two calculated currents. This is clearly less than optimal for the squib which requires the lower current and for overall current consumption. 
     This can be rectified, at least in part by using a balancing technique within each circuit  202  and  204 . There are many different ways in which this can be achieved including:
         rebalancing with a series impedance;   preventing simultaneous firing   including variable resistors in one or other circuit.       

       FIG. 3  shows a first balancing technique. Assuming that squib resistance does not substantially change during the squib life, it is possible to add on additional resistance R bal1  or R bal2  as shown in  FIG. 3 . This means that the total resistance from V Z  to ground is roughly equal for each squib output and thus the generated firing current is more optimal for both squibs. 
       FIG. 4  offers an alternative rebalancing technique. If the squib resistance varies with time, temperature or whatever then real time rebalancing is required. This squib resistance is known in real time as diagnostics are carried out regularly, for example every 2 ms. Based on this measurement a logic module (not shown) can select the low side resistance by using one or more parallel low side transistors  402 . In this example the MOSFET is split into 3 pieces, but may be more or less divided as the case requires. In the example show, P 1  has a resistance of 3Ω, P 2  of 6Ω and P 3  of 2Ω. This results in an overall resistance of: 3Ω for just P 1 ; 2Ω for the parallel connection of P 1  and P 2 ; and 1Ω for the parallel connection of P 1 , P 2  and P 3 . This ability to vary the resistance ensures that V Z  for each squib circuit can be adjusted so that firing current can be optimised, by connecting the sections or pieces of the MOSFET in parallels. 
       FIG. 5  shows a further improvement with respect to rebalancing particularly where temperature impacts the circuits and resistances. The high side MOSFET M 2 , can be split  504  in the same way as the low side  502  into X parallel structures (where X is a number greater than 1). In addition the total resistance can be further split by inclusion of a poly-silicon resistance  506  in series with each MOSFET. In the example shown at  502  and  504  the poly-silicon resistance may be 0.5Ω. For  502 : P 1  is 1.0Ω and P 3  is 1.3Ω. For  504 , the parameters are such that the MOSFET has a resistance Rds(on) of about 1.0Ω maximum at room temperature. This is derived from 1 d 45V, wg=325 ng=50. 
     As has previously been noted this digital squib driver has many advantages over the previous analogue squib drives. Not least in respect of size, reduced requirement for sustaining energy, no regulator current needed for high and low side switches, simplified pre-driver circuits (not shown) can be used; improved current management especially when squib resistance changes with time and/or temperature. 
     The embodiments of the present invention shown in the drawings provide a number of advantages. These include:
         Smaller scale MOSFETS are required as energy is totally dissipated which brings about increased reliability at lower cost.   The high-side switch does not need to sustain the same levels of energy and are thus dramatically reduced in size. For example sizes of &lt;0.08 mm 2  may be achieved.   The high-side predriver is converted to a pure inverter driver that does not need to regulate current. This provides faster design time and faster test time.       

     The low side switch is operated in Rds(on) mode which again gives size reduction as for the high-side. There is further no thermal coupling from the high-side switch. The predriver of the low side switch can also be simplified leading to still further size reductions. 
     The examples of circuit arrangements, values, devices and applications are shown by way of example only and may be varied whilst still providing the advantages of the invention.