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An Excel workbook to calculate recommended external component values
and
A continuous time behavioral model of the modulator that runs in PSPICE A/D v 9.1 or above. The model is
small enough to run under Cadence's Orcad Lite Edition (includes Orcad Capture and PSPICE A/D), which can
be ordered on CD or may be downloaded at:
http://www.orcad.com/downloads/demo/default.asp
Information on the FAN5068, including the datasheet can be found on:
http://www.fairchildsemi.com/pf/FA/FAN5068.html
This package of design aids (including this document) is can be downloaded from:
http://www.fairchildsemi.com/collateral/AN-6006.zip.
To install, copy AN-6006.ZIP to an empty folder (e.g. 鈥淔AN5068Design鈥?. Then unzip AN-6006.ZIP into that
folder.
Recommended design procedure:
1.
2.
3.
4.
Use the spreadsheet (FAN5068 Design calculation aid.xls ) to calculate the output filter using the 鈥淥utput
Filter鈥?tab.
Use the 鈥淢ain Sheet鈥?tab of the spreadsheet to calculate the other component values once you have selected the
output L and C.
Use the 鈥淐ompensation鈥?tab to design the compensation network.
Input the values you have selected into the PSPICE model. Generate a Bode plot by simulating with the
鈥渁pplication circuit-ac sweep鈥?simulation profile. Make sure to simulate over the corners of VIN and IOUT for
your application.
Once you are satisfied with the small signal stability, you can view the transient response by simulating using
the 鈥渁pplication circuit-transient response鈥?simulation profile. Be sure the follow the instructions at the bottom
of the Application Circuit, Page 1 sheet for resetting RLOAD before simulating transient response.
5.
Design Calculation Spreadsheet: FAN5068 Design calculation aid.xls
For instructions on the use of this spreadsheet, see the 鈥淚nstructions鈥?tab of the spreadsheet.
PSPICE Simulation Model
The simulation model is a sampled data continuous time model, which is adapted from Ray Ridley and Dennis
Feucht's modeling work for current mode controllers
, ,
. It is set up to provide a bode plot where the red trace is
Phase Margin (in degrees) and the green trace is gain (in dB). For stable response, we recommend at least 45擄 of
phase margin when the gain crosses 0dB. The model also provides transient response using a pulsed current source
(I1) as the load. The IC鈥檚 error-amp behavioral model is based on Ray Kendall鈥檚 Macromodelling article in EDN.
To run the model start Capture (9.1 or higher), open
FAN5068.opj
(this is the "project" file for Capture). Double
click on Page 1 Under .\fan5068.dsn\Application Circuit.
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