Dual devices drive down power-device sizes

-- EDN, 1/20/2000

Your power-related design can see significant space savings from dual MOSFETs, because you so often use these functions in pairs. The P-channel FDZ2552P and similar N-channel FDZ2551N MOSFETs from Fairchild Semiconductor (Picture) are for battery-protection applications with low threshold (V_GS=2.5V) and low on-resistance. These dual devices are available in BGA packages, which take 0.10 cm² of pc-board area, approximately one-third the area of comparable SO-8 devices. (It wasn't that long ago that we considered SOT-23 packages small.)

The N-channel dual device features R_DS(ON) of 0.018W at V_GS=4.5V and 0.030W at V_GS=2.5V; comparable figures for the P-channel device are 0.045 and 0.075W. The BGA package, only 0.70 mm high when mounted on a pc board, has excellent thermal characteristics as well as size. The N-channel device, for example, supports continuous drain current as high as 9A. These dual MOSFETs are also available in SuperSOT-8 and TSSOP-8 packages; prices begin at 85 cents (10,000).

If you need lower voltage dual devices, you might consider using Siliconix/Vishay Intertechnology's P- and N-channel Si3552DV, dual-P-channel Si3909DV and Si3905DV, and dual-N-channel Si3948DV. These components operate at voltages as low as 1.8V and come in TSSOP-6s, but, because of on-resistance as great as 61% lower than their predecessors, they can replace larger SOT-23 MOSFETs. Depending on which device you select, you get combinations of critical parameters, including R_DS(ON) =0.20W at V_GS=-4.5V (-20V breakdown), R_DS(ON)=0.265W at V_GS=-1.8V(-8V breakdown); and R_DS(ON)=0.105W and R_DS(ON)=0.200W, both at V_GS=10V. You can access the TSOP-6 dual MOSFET for 26 cents (100,000).

Finally, the IRF7901D1 from International Rectifier puts a high-side control FET and low-side synchronous FET with a parallel Schottky diode in an SO-8 package, for synchronous buck dc/dc-converter architectures. This IC minimizes external pc-board traces and reduces stray inductances, which adversely affect efficiency and performance. With this 65-cent (100,000) IC, you can provide peak outputs as high as 5A at 5, 3.3, and 1.8V output with efficiencies as high as 96%. The vendor also offers a design kit that lets you evaluate discrete-versus-dual-FET approaches in your application.

Fairchild Semiconductor, www.powertrench.com, www.fairchildsemi.com.

Siliconix/Vishay Intertechnology Inc, www.siliconix.com.

International Rectifier, www.irf.com.

-by Bill Schweber