Information About the
LM2650 Evaluation Board
Rev. 1
INTRODUCTION
Aprinted circuit board (PCB) has been developed. This ap-
plication note contains information about the board.
GENERAL DESCRIPTION
TheLM2650evaluationboardisprovidedasatoolfordevel-
oping DC/DC converters based on the LM2650 IC. It is con-
figured for single-output, step-down DC/DC converters. Fig-
ure 1 is a complete schematic of the board which can
accommodate up to 28 components including the LM2650.
Table 1 is a complete list of pads for placing components.
Note:Notallapplicationswillrequiretheplacementofall28
components. The number of components placed depends
on the requirements of the application and the use of fea-
tures like programmable soft-start. The LM2650 evaluation
boardisintendedtobeareusabletoolonwhichmanydiffer-
ent converters meeting the requirements of many different
applications can be built. It is not intended to demonstrate
only one application of the LM2650.
For convenience, a sample of the LM2650 and eight other
components have been assembled: a 0.1 µF capacitor at
each of C1, C2, C6, and CB, a 0.2 µF capacitor at CVDD, a
0.01µFcapacitoratCSS,a24.9kΩ resistoratR2,anda0Ω
resistor at R4. Of the eight, the first seven are common to
manyapplications,andthelastissimplyajumpergrounding
theSYNCinput.Whenthesynchronizationfeatureisnotbe-
ing used, the SYNC input should be grounded.
EXAMPLE CIRCUITS
ThecomponentscontainedinTables2,3,and4canbeused
to build typical application circuits.As with the design of any
DC/DC converter, the design of these involved tradeoffs be-
tween efficiency, size, and cost. The converters detailed in
Table2weredesignedwithefficiencyasthenumberonecri-
teria.ThosedetailedinTable4tradeslightlyhigherswitching
losses for a much smaller inductor.
THERMAL PERFORMANCE
The24-leadSOpackageisamoldedplasticpackagewitha
solid copper lead frame. Most of the heat generated at the
die flows through the lead frame into the 3-ounce copper
planesontheboard.Theboardthenactsasaheatsink.The
junction-to-ambient thermal resistance of the packaged IC
mountedontheboardhasbeenmeasuredtobe38˚C/W,37
˚C/W, and 35 ˚C/W for the dissipation of 1.0W, 1.5W, and
2.0W respectively. These measurements were made in still
air. The junction-to-ambient thermal resistance of the pack-
agedICaloneinstillairis78˚C/W.Theboardis0.063″ thick
FR-4 material.
ART
Figure 2 through Figure 4 show the PCB art work.
AN100011-5
FIGURE 1. The LM2650 Evaluation Board Rev. 1 Schematic
National Semiconductor
AN-1071
Steven Hunt
September 1997
Information
About
the
LM2650
Evaluation
Board
Rev
.
1
AN-1071
©1997 National Semiconductor Corporation AN100011 www.national.com