Ferrite having a Curie temperature Tc of 140.degree. C. and initial permeability of 25 at a frequency of 100 kHz, which comprised main components comprising 47.0 mol % of Fe.sub.2O.sub.3, 36.7 mol % of NiO, 11.0 mol % of CuO, 5.0 mol % of ZnOand 0.3 mol % of CO.sub.3O.sub.4, and 1.0% by mass of Bi.sub.2O.sub.3 per the total amount of the main components, was formed into sheets by an LTCC method, and each sheet was provided with a predetermined coil pattern by an Ag paste. Insulating layerswere laminated such that a magnetic gap GP1 made of Cu--Zn ferrite (Curie temperature: -60.degree. C.) comprising main components comprising 45.7 mol % of Fe.sub.2O.sub.3 and 4.0 mol % of ZnO, the balance being CuO, and 0.3% by mass of Bi.sub.2O.sub.3per the total amount of the main components was formed on an insulating layer S2, pressure-bonded, and sintered to produce a mother board in which pluralities of multi-layer substrates each having a laminated coil having inductance of 3.3 .mu.H wereconnected. After degreasing, the sintering was conducted in an electric furnace with an air atmosphere, with temperature elevation at 150.degree. C./hr, keeping at 900.degree. C. for 1 hour, and then temperature decrease at about 300.degree. C./hr.
Conductor patterns formed by electric plating on an outer surface of the mother board were subjected to Ni--P plating and Au plating to form first external terminals. Connected to the first external terminals were a semiconductor-integratedcircuit device IC, and capacitors Cin (10 .mu.F), Cout (4.7 .mu.F) by soldering. With a device-mounting surface sealed by an epoxy resin, the mother board was divided along dividing grooves given in advance, to obtain DC-DC converters of 4.5mm.times.3.2 mm.times.1.4 mm.
When the multi-layer substrate was viewed from above, a region inside the laminated coil had an area of 4.2 mm.sup.2, and a region outside the region had an area of 4.3 mm.sup.2 A portion with the laminated coil was as thick as 0.3 mm, with anupper insulating layer region and a lower insulating layer region each having a thickness of 0.2 mm. The magnetic gap was 20 .mu.m with an area of 2.1 mm.sup.2. When the resultant DC-DC converter was operated to provide an output voltage Vo of 3.2 Vfrom an input voltage V1 of 3.6 V, a voltage conversion efficiency of more than 95% was obtained when an output current Io was 150 mA.
A coil inductor (inductance: 3.3 .mu.H) having a drum ferrite core was mounted on a printed circuit board together with the same semiconductor integrated circuit device IC and capacitors Cin, Cout as in Example 1, to produce a DC-DC converter ofComparative Example 1. Measurement under the same conditions as in Example 1 revealed that the DC-DC converter of Comparative Example 1 had substantially comparable voltage conversion efficiency of more than 95%.
However, the DC-DC converter of Example 1 comprising capacitance elements suffered little influence of parasitic inductance generated by the first and second connecting wires, etc., while exhibiting comparable voltage conversion efficiency tothat of Comparative Example 1. The parasitic inductance in the DC-DC converter of Example 1 was less than about one-fifth in the DC-DC converter of Comparative Example 1. Such a terminal structure as LGA, etc. suppressing a magnetic flux from leakingfrom the inductor enabled the high-density mounting of other circuit elements near the DC-DC converter on the printed circuit board.
Although the DC-DC converter of the present invention has been explained above referring to the attached drawings, the present invention is not restricted to the depicted embodiments, but may be modified variously within the scope of the presentinvention. As shown in FIG. 25, for instance, the first main surface of the multi-layer substrate may be covered with an overcoat, and the first connecting wires may be formed on a layer near the first main surface in the multi-layer substrate. Withsuch a structure, the elongation of the plating and short-circuiting by an excess solder can be prevented, even if the first connecting wires on the first main surface have narrow gaps. Also even large first external terminals would not interfere withthe first connecting wires.
The use of the same insulating material as the multi-layer substrate for the overcoat is not preferable because it increases parasitic inductance. Thus, the overcoat is made of non-magnetic materials such as glass and dielectric materials, orlow-permeability magnetic materials. The formation of the first external terminals and the first connecting wires on the overcoat is preferable because parasitic inductance generated by the first connecting wires can be further reduced.
Effect of the Invention
While being small, the DC-DC converter of the present invention integrally comprising a semiconductor integrated circuit IC and an inductor is excellent in the reduction of a magnetic flux leaking from the multi-layer substrate, the reduction ofparasitic inductance, the heat dissipation of the semiconductor integrated circuit IC, etc.
