Stacked synchronous buck converter having chip embedded in outside recess of leadframe

A power supply system comprising:

• a QFN leadframe having a plurality of leads and a planar pad with a first and a second pad surface, at least some of the plurality of leads having protrusions beyond the plane of the first surface, the protrusions having a height suitable for attaching a semiconductor chip to the first pad surface while keeping the un-attached chip surface coplanar with the lead protrusions;
• the pad tied to a switch node terminal of the system;
• a first FET chip vertically stacked to the pad by having a drain terminal of the first FET chip attached to the second pad surface;
• a clip vertically stacked on the first FET chip by having a first flat side attached to a source terminal of the first FET chip, the clip tied to a lead from the plurality of leads as a grounded output terminal of the system;
• and
• a second FET chip having a source terminal of the second FET chip attached to the first pad surface, and further having a drain terminal of the second FET chip and a plurality of gate terminals co-planar with the lead protrusions, the drain of the second FET chip tied to a board terminal as an input terminal to the system.

A method for fabricating a power supply system comprising the steps of:

• providing a leadframe having leads and a pad with a first and a second surface, the first surface having a portion recessed with a depth and an outline suitable for attaching a semiconductor chip;
• attaching a first FET chip with its drain terminal vertically on the second surface of the pad;
• attaching a clip with its first flat side vertically on the source terminal of the first FET chip;
• encapsulating the clip, the first FET chip and the second pad surface in a packaging compound, while leaving the first pad surface un-encapsulated;
• and
• attaching a second FET chip with its source terminal to the recessed portion of the first pad surface so that the drain and gate terminals of the second FET chip are co-planar with the un-recessed portion of the first pad surface and with the leads.