Abstract:
An integrated circuit driver is disclosed. The driver comprises a high side transistor and a low side transistor connected in series. The output of the driver is taken from the source of the high side transistor and the drain of the low side transistor. A bootstrap contact pad is connected to the output node. Connected to the bootstrap contact pad is a bootstrap capacitor that is also connected to a high side gate drive that selectively controls the high side transistor.

Description:
TECHNICAL FIELD 
   The present invention relates to integrated circuit drivers that use a bootstrap supply to drive the gate of the high side switch, and more particularly, to a method and apparatus for providing a stable bootstrap voltage to the gate of the high side switch. 
   BACKGROUND 
   One common type of integrated circuit driver utilizes two power MOSFET switches in a totem pole (half-bridge) topology. The MOSFET switches are typically NMOS switches that are connected in series. The power MOSFET switches are driven to conduct alternately. One of the MOSFET switches is designated as a high side switch, and the other MOSFET switch is designated as the low side switch. In one application, by selectively switching the power MOSFET switches in an alternating fashion, a load can be driven with an alternating current. In such a manner, a DC to AC inverter is formed. Likewise by controlling the switches according to an input signal (such as an acoustic signal), a class D audio amplifier is formed. Further, the same half bridge topology using a stable DC reference as the input can be used to create a DC power supply. 
   The gate of the high side switch is typically driven by a bootstrapped power supply. This is done to allow use of an NMOS switch, which has roughly half the on resistance of a PMOS switch of the same area. A bootstrap capacitor is used to increase the voltage available to the gate of the high side switch.  FIG. 1  shows a prior art simplified schematic of an integrated circuit driver (IC) used in conjunction with a bootstrap capacitor to drive a load. The IC driver provides current to drive a load. A bootstrap capacitor C b  has one terminal connected to the output of the IC driver. The other terminal of the bootstrap capacitor C b  is provided back to the IC driver to drive the gate of the high side switch. 
   A more detailed schematic of the IC driver of  FIG. 1  is shown in FIG.  2 . As seen in  FIG. 2 , the IC driver  101  includes the high side switch  107  and the low side switch  109 . The high side switch  107  is driven by gate drive and fault circuit  111 . Similarly, the low side switch  109  is driven by gate drive and fault circuit  113 . The gate drive and fault circuits  111  and  113  are operative to control the switching of the high side and low side switches  107  and  109 . In addition, the gate drive and fault circuits  111  and  113  typically include fault detection circuitry and a bootstrap supply monitor. These additional functions are generally needed to measure whether there is a fault condition on the switch or whether the bootstrap supply is sufficient for the IC to operate properly. 
   The precise configuration of the gate drive and fault circuits  111  and  113  may be varied, but generally the configuration and operation is well known in the prior art. Note that the gate drive and fault circuit  113  used to control the low side switch  109  operates using a first supply voltage V sp1 . The low side switch  109  does not require a bootstrapped power supply. In contrast, the gate drive and fault circuit  111  that controls the high side switch  107  is connected to the bootstrap capacitor  103 . 
   The output of the IC driver  101  is taken from the node connecting the high side switch and the low side switch. In physical terms, the output node is a conductive pad on the integrated circuit, designated in  FIG. 2  as SW pad    115 . The integrated circuit die is then set into a package wherein the pad SW pad    115  is connected to a package pin SW pin    117 . The connection between the pad  115  and the package pin  117  is typically made through a bond wire formed of gold, copper, or other highly conductive material. 
   Nevertheless, the bond wire between the pad  115  and the package pin  117  includes some finite amount of parasitic inductance L p1  and parasitic resistance R p1 . When current is supplied through the pin  117  to the load  105 , invariably there will be a loss of voltage across the parasitic inductance L p1  and parasitic resistance R p1 . 
   The amount of the voltage drop is important because any voltage that develops across the bond wire between SW pad and SW pin, subtracts directly and instantaneously from the bootstrap supply. Because of the large value of current and high rate of change of that current in the bondwire, the voltage drop can be significant, on the order of two or more volts. This sudden drop in the internal bootstrap supply voltage will adversely affect any signal processing operating under the internal bootstrap supply, such as the bootstrap supply monitor and fault check circuits. 
   Therefore, the arrangement shown in  FIG. 2  having an imprecise and noisy bootstrap supply is undesirable. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram showing a bootstrap capacitor and an integrated circuit driver for driving a load. 
       FIG. 2  is a detailed schematic of the integrated circuit driver of FIG.  1 . 
       FIG. 3  is a schematic circuit diagram illustrating one embodiment of the present invention. 
       FIG. 4  is an illustration of an integrated circuit die mounted on an integrated circuit package. 
   

   DETAILED DESCRIPTION 
   The present invention is an integrated circuit driver having a “quieter” bootstrap power supply. The integrated circuit driver has an output pin and output pad that is dedicated to the bootstrap capacitor thereby maintaining a stable bootstrap supply voltage. In the following description, some specific details, such as example values for the circuit components, are provided to provide a thorough understanding of embodiments of the invention. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention. 
   Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment of the present invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. One skilled in the relevant art will recognize, however, that the invention can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the invention. 
     FIG. 3  shows one embodiment of the present invention. As seen,  FIG. 3  is substantially similar to the prior art IC driver  101 , except that an additional pad SW 2   pad    303  is also attached to the output node between the high side switch  107  and the low side switch  109 . Additionally, a second output pin  305  is provided from the IC driver  301 . Having the second pad  303  and the second package pin  305  connected to the bootstrap capacitor  103 , the bootstrap capacitor  103  is not affected by any voltage drop caused by current flowing to the load  105  through a first package pin  117 . 
   Note that substantially all the current provided by the high side switch  107  and the low side switch  109  flows to the load  105  through the package pin  117 . Little if any current flows through the second package pin  305 , thereby eliminating any voltage drop through the parasitic resistance and inductance of the bond wire connecting the second package pin  305  to the second pad  303 . Thus, the bootstrap supply voltage provided by the bootstrap capacitor  103  maintains its value and is less noisy. 
   As seen, the IC driver  301  of the present invention includes an additional package pin  305  that is connected directly to the bootstrap capacitor  103 . In an alternative embodiment, the second package pin  305  has a bond wire directly attached to the same pad  115  as the first package pin  117 . This saves the requirement for forming the second pad  303 . In one embodiment, the IC driver  301  may be used to drive, for example, a cold-cathode fluorescent lamp. However, typically, the lamp is connected through a secondary winding of a transformer whose primary winding is connected to the output of the IC driver  301 . 
     FIG. 4  further illustrates the arrangement of the present invention. In  FIG. 4 , an integrated circuit package  401  is adapted to mount an integrated circuit die  403 . The integrated circuit die  403  includes various circuitry, such as the low side switch, the high side switch, and the gate drive and fault circuitry. In addition, the integrated circuit die  403  includes an output contact pad  409 , a bootstrap contact pad  407  (referred to as a second pad SW 2   pad    303  in FIG.  3 ), a high side gate drive input pad  421 , and various other contact pads  405  and  411 . 
   The output contact pad  409  is connected to an output pin  413  of the integrated circuit package  401  by an output bond wire  417 . The output bond wire  417  is secured to the output pin  413  and the output contact pad  409 . The bootstrap contact pad  407  is connected to bootstrap pin  415  of the integrated circuit package  401  by a bootstrap bond wire  419 . The bootstrap bond wire  419  is secured to the bootstrap pin  415  and the bootstrap contact pad  407 . 
   The bootstrap capacitor C b  is connected between the bootstrap pin  415  and the gate drive circuitry on the integrated circuit  403  through another package pin and high side gate drive input pad  421 . Finally, the load is connected to the output pin  413 . The other various pins of the integrated circuit package  401  are used in known configurations, such as for power supply, ground, control lines, and such. 
   As noted above, in an alternate embodiment, the output contact pad  409  and the bootstrap contact pad  407  is one and the same. Whichever pad conducts current to the load is made large, such as contact pad  411 , but not changed in size if also attached to Cb. 
   Thus, the above described IC driver provides a stable bootstrap power supply, even when large amounts of power are being delivered. This is accomplished by connecting the bootstrap capacitor to a dedicated bootstrap pin and contact pad. 
   From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.