Patent Publication Number: US-7902646-B2

Title: Multiphase synchronous buck converter

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to and is a divisional of co-pending U.S. patent application Ser. No. 12/020,892, filed Jan. 28, 2008, and is a continuation of co-pending U.S. patent application Ser. No. 12/405,525, filed Mar. 17, 2009, which applications are incorporated herein by reference in their entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates, in one embodiment, to a multiphase buck converter package. A single molded package is provided which incases at least four MOSFET dice and a plurality of leads. The dice are electrically connected in a predetermined using lead frame. Wirebonding techniques are not required to connect the dice. In one embodiment, the leads all extend in the same direction. 
     BACKGROUND OF THE INVENTION 
     Buck converters are switching circuits that downgrades a received voltage to a lower, output voltage. Such circuits are commonly used to power microprocessors, wherein the voltage desired by the microprocessor is substantially lower than the voltage supplied by the power supply. In a multiphase buck converter, a plurality of dice are operated in sequence to downgrade the voltage. Thus, each die contributes a portion of the desired current. Reference may be had to U.S. Pat. No. 6,803,750 to Zhang (Multiphase Synchronous Buck Converter with Improved Current Sharing). 
     Unfortunately, prior art multiphase buck converters are not modular units. Therefore, a multiphase buck converter package is desired which would easily permit a user to add such a converter to an existing circuit. 
     SUMMARY OF THE INVENTION 
     The invention comprises, in one form thereof, a multiphase buck converter package. The package includes at least four dice and at least nine parallel leads. The dices are electrically connected through a plurality of die attach pads, thus eliminating the need for wirebonding. 
     An advantage of the present invention is the modular nature of the package. A user may easily add a multiphase buck converter package into an existing circuit to optimize a system to increase its ability to respond to dynamic changes in the load current, resulting in a significant improvement in performance. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention is disclosed with reference to the accompanying drawings, wherein: 
         FIG. 1  is a perspective view of a first embodiment of the invention; 
         FIG. 2  is a flow diagram of a process for forming the first embodiment; 
         FIG. 3  is a schematic diagram of the circuit of the first embodiment; 
         FIG. 4  is a top view of the first embodiment illustrating the various leads; 
         FIG. 5  is an exploded view of the first embodiment; 
         FIG. 6  is a perspective view of a second embodiment of the invention; 
         FIG. 7  is a flow diagram of a process for forming the second embodiment; 
         FIG. 8  is a schematic diagram of the circuit of the second embodiment; 
         FIG. 9  is an end view of the second embodiment. 
         FIG. 10  is a top view of the second embodiment illustrating the various leads; and 
         FIG. 11  is an exploded view of the second embodiment. 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The examples set out herein illustrate several embodiments of the invention but should not be construed as limiting the scope of the invention in any manner. 
     DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
     Referring to  FIG. 1 , a multiphase buck converter package is shown. The embodiment of  FIG. 1  has a certain width  100  and a height  102  suitable to accommodate a plurality of die attach pads  104  in a vertical configuration. In other embodiments (see  FIG. 6 ) the width and height may be altered to accommodate non-vertical configurations.  FIG. 2  is a flow diagram depicting a process for forming the buck converter package of  FIG. 1 . 
     Process  200  of  FIG. 2  begins with first lead frame  202 . First lead frame  202  includes a plurality of leads  210 , a first die attach pad  204 , a second die attach pad  206 , and a bridge  208  connecting pad  204  and pad  206 , as well as first die  212 . In the embodiment shown in  FIG. 2 , first lead  205  is the gate lead which is separated from first pad  204 , while second lead  207 , bridge  208 , first pad  204  and second pad  206  are unitary. In one embodiment, connecting bridge  208  is thinner than pads  204  and  206 . Such a configuration facilitates the bending operation performed in step  226 . The other leads shown in  FIG. 2  are not connected to die attach pad  204 . 
     As shown in  FIG. 2 , first die  212  has been mounted to first pad  204  such that the source of first die  212  is electrically connected to second lead  207  and the gate of first die  212  is electrically connected to first lead  205 . The drain of first die  212  is exposed. 
     In step  214  of process  200 , a third die attach pad  216  is attached to the underside of first pad  204 . After attachment, the third die attach pad  216  is disposed in a plane that is parallel to, but separated from, the planes of both first pad  204  and second pad  206 . Third die attach pad  216  includes second die  218  whose source is connected to third lead  222  and whose gate is connected to fourth lead  220 . In  FIG. 2 , third pad  216  is shown in an inverted state to better depict second die  218 . The bottom surface (not shown) of third pad  216  is protected with a layer of insulating material. In step  214 , third pad  216  is attached to die attach  204  such that the insulating material is adjacent to the bottom surface of pad  204 . Third lead  222  and fourth lead  220  are extended in length by contacting corresponding leads from the plurality of leads  210 . In this manner, the source of second die  218  has been connected to third lead  222  and the gate of second die  218  has been connected to fourth lead  220 . The drain of first die  212  and second die  218  are exposed in step  214 . 
     In step  226 , second die attach pad  206  is bent about bridge  208  to form assembly  224  such that second die attach pad  206  contacts the exposed drain of second die  218 . In  FIG. 2 , bridge  208  is bent into a simple U shape, although other shapes are permissible. Step  214  connects the drain of second die  218  to second lead  207  by forming an electrical pathway from the drain of second die  218 , through the second pad  206 , the bridge  208 , first pad  204 , and finally to second lead  207 . The second die attach pad  206  is now disposed in a plane parallel to, but separated from, the planes of the first pad  204  and third pad  216 . The drain of first die  212  remains exposed. 
     In step  230  a fourth die attach pad  232  is mounted atop the drain of first die  212 . The fourth die attach pad  232  is disposed in a plane that is parallel to, but separated from, the planes of the first pad  204 , second pad  206 , and third pad  216 . Fourth die attach pad  232  includes third die  234  whose gate is connected to fifth lead  238  and whose source is connected to sixth lead  236 . Fifth lead  238  and sixth lead  236  are extended in length by contacting corresponding leads from the plurality of leads  210 . The bottom surface of fourth pad  232  contacts the exposed drain of first die  212 . In this manner, the source of third die  234  has been connected to sixth lead  236 , the gate of third die  234  has been connected to fifth lead  238 , and the drain of first die  212  has been connected to both the source of third die  234  and to sixth lead  236  (both through third pad  232 ). In assembly  240 , the drain of third die  234  is exposed. 
     In step  242  of process  200 , fifth die attach pad  244  is mounted atop the exposed drain of third die  234 . Fifth die attach pad  244  includes fourth die  246  whose source is connected to seventh lead  248 , and whose gate is connected to eighth lead  250 , and whose drain is connected to ninth lead  252 . Seventh lead  248 , eighth lead  250  and ninth lead  252  are extended in length by contacting corresponding leads from the plurality of leads  210 . After mounting, seventh lead  248  is now connected to the source of fourth die  246  and the drain of third die  234  (through fifth die attach pad  244 ). 
     In step  256 , assembly  254  is encased in molding compound  258  to form package  260 . The resulting assembly is comprised of four dies which are connected in series. Such a configuration is represented schematically in  FIG. 3 . 
       FIG. 3  is a schematic diagram of one embodiment of the invention for a two phase buck converter. In  FIG. 3 , seventh lead  248  connects to an inductor L 1  and second lead  207  connects to an inductor L 2 . High side gate lead  250  and low side gate lead  238  of the first buck converter are connected to a driver chip IC 1  which controls the high side and low side switch of the first buck converter. High side gate lead  205  and low side gate lead  220  of the second buck converter are connected to a driver chip IC 2  which controls the high side and low side switch of the second buck converter. 
       FIG. 4  is a depiction of the function of the various leads of package  260 . Lead  238  is the gate lead for third die  234 . Lead  248  connects to the source of the fourth die  246  and drain of the third die  234 . Lead  205  is the gate lead for first die  212 . Lead  236  connects to the source of third die  234  and to the drain of first die  212 . Lead  207  connects to the source of first die  212  and the drain of second die  218 . Lead  222  is the source lead for second die  218 . Lead  220  is the gate lead for second die  218 . Lead  250  controls the gate of fourth die  246 . Lead  252  is the drain lead for fourth die  246 . 
       FIG. 5  is an exploded view of the components of the package of  FIG. 1 . The insulating material  500  that is disposed between first pad  204  and second pad  216  is visible in this view. 
       FIG. 6  is a perspective view of another embodiment of the invention. In  FIG. 6 , the package has a width  500  and height  502 . Width  500  of  FIG. 6  is substantially longer than width  100  of  FIG. 1 , while height  502  is substantially smaller than height  102 . Each of the embodiments shown in  FIGS. 1 and 2  are useful in different situations. 
       FIG. 7  is a flow diagram of a process  700  for forming the package of  FIG. 6 . Process  700  begins with lead frame  702 , which includes first die attach pad  708   a , second die attach pad  708   b , and third die attach pad  710 . Second pad  708   b  and third pad  710  are joined by conductive bridge  706 . Lead frame  702  also includes a plurality of leads  704 . In the embodiment depicted in  FIG. 7 , the two flanking leads, the first pad  708   a , second pad  708   b , third pad  710  and bridge  706  are unitary. The non-flanking leads will be connected to the internal components of the package during subsequent steps. Third pad  710  includes first lead  712 . 
     In step  714  of process  700 , first die  716  and second die  718  are mounted to lead frame  702 . First die  716  is flip-chip mounted, such that the drain of first die  716  contacts first pad  708   a . Second die  716  is mounted in a conventional fashion such that the source of second die  718  contacts third pad  710 . This places lead  712  in electrical contact with both the drain of first die  716  and the source of second die  718 . 
     In step  720 , fourth die attach pad  722  is mounted to second die  718  such that is contacts the drain of die  718 . Fourth pad  722  includes second lead  724 , which is now in electrical contact with the drain of second die  718 . 
     In step  726  of process  700 , bridge  706  is bent such that third pad  710  is disposed atop second pad  708   b . When first lead  712  and second lead  724  contact corresponding leads of the plurality of leads  704 , the length of first lead  712  and second lead  724  are extended. Contact between the exposed surface of fourth pad  722  and second pad  708   b  is prevented. Such contact may be prevented by coating the exposed surface of fourth pad  722  with an insulating material or by simply leaving a space between fourth pad  722  and second pad  708   b.    
     In step  728  fifth die attach pad  732 , which includes third lead  734 , is mounted atop first die  716  such that the source of first die  716  contacts fifth pad  732 . The length of third lead  734  is extended by contact with one of the leads of the plurality of leads  704 . Also in step  728 , fourth lead  736  is attached to the gate of first die  716  and fifth lead  730  is attached to the gate of second die  718 . 
     In step  738 , the assembly is flipped to perform operations on the bottom side of lead frame  702 . In another embodiment, not shown, the operations are performed on the bottom of lead frame  702  without flipping the frame  702 . 
     In step  740  of process  700 , third die  742  is mounted to the bottom of second pad  708   b  such that the drain of third die  742  is connected to first lead  712 . 
     In step  744 , sixth die attach pad  746 , which includes fourth die  748  and sixth lead  750 . Fourth die  748  is mounted to sixth die attach pad  746  such that the drain of fourth die  748  contacts sixth pad  746 , and thus sixth lead  750 . The length of sixth lead  750  is extended by contact with one of the leads of the plurality of leads  704 . 
     In step  752 , seventh die attach pad  754  is mounted to fourth die  748  such that seventh pad  754  is connected to the source of fourth die  748 . Seventh pad  754  includes seventh lead  756 , which is extended in length by contact with one of the leads from the plurality of leads  704 . In step  752 , eighth lead  760  is connected to the gate of third die  742  and ninth lead  758  to the gate of fourth die  758 . 
     In step  762 , the assembly is encased within molding compound  764  to form assembly  766 . In the embodiment depicted in  FIG. 7 , assembly  766  is show front a viewpoint such that third pad  710  is on top and sixth pad  746  is on the bottom. 
       FIG. 8  is a schematic diagram of the circuit of the embodiment shown in  FIG. 6 . This is a more flexible circuit connection which allows the users to have an option to add the embodiment to their own defined circuitry. The users may build or add more parallel buck converters under a switch through leads  734  and  756  to form any system the users wish to obtain. 
       FIG. 9  is an end view of package  766  looking from the perspective of arrow  900 . First die  716 , second die  718 , third die  742  and fourth die  748  are shown. First die attach pad  708   a  and second pad  708   b  are disposed in first plane  902 . Third pad  710  is disposed in second plane  904 . Fourth pad  722  and fifth pad  732  are both disposed in third plane  906 . Sixth pad  746  is in fourth plane  908 . Seventh pad  754  is in fifth plane  910 . 
       FIG. 10  illustrates the function of the various leads of package  766 .  FIG. 10  includes first die  716 , second die  718 , third die  742  and fourth die  748 , which has a gate, source and drain. First lead  712  is connected to the third drain, first drain and second source. Second lead  724  is connected to the second drain. Third lead  734  is connected to the first source. Fourth lead  736  is connected to the first gate. Fifth lead  730  is connected to the second gate. Sixth lead  750  is connected to the third source and fourth drain. Seventh lead  756  is connected to fourth source. Eighth lead  760  is connected to the third gate. Ninth lead  758  is connected to the fourth gate. 
       FIG. 11  is an exploded view of the components of the embodiment shown in  FIG. 6 . 
     While the invention has been described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof to adapt to particular situations without departing from the scope of the invention. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope and spirit of the appended claims.