AN-6002| Application Note

www.fairchildsemi.com

AN-6002
Component calculations and simulation tools for FAN5234 or FAN5236
Background / Overview
To simplify designs using the FAN5234 and FAN5236 PWM modulators, Fairchild has developed: A spreadsheet to calculate recommended component values Capability added 6/19/04: Phase Gain Bode Plot inside the spreadsheet. and A continuous time behavioral model of the modulator that runs in PSPICE A/D. Provides transient analysis and Bode plot outputs. 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 at
http://www.ema-eda.com/products/Default.aspx

or downloaded from:
http://www.orcad.com/download.orcaddemo.aspx.

These links have been verified as of this publication date but may change over time. This package of design aids (including this document) is contained in AN-6002.ZIP which can be found on: http://www.fairchildsemi.com/collateral/AN-6002.zip. To install, copy AN-6002.ZIP to an empty folder (e.g. FAN5236Design). Then unzip AN-6002.ZIP into that folder. Bode PlotsA new feature has been added to the Spreadsheet tool

Recommended design procedure:
1. 2. 3. 4. 5. 6. Use the spreadsheet (FAN5236 Design calculation aid.xls ) to calculate the output filter using the "Output Filter" tab. Use the "Main Sheet" tab of the spreadsheet to calculate the other component values once you have selected the output L and C. Most applications will not require C2. Try your first simulation with this value set to 1pF. View results on the Bode plot inside the spreadsheet. Use the "input Filter" tab to determine the ripple current rating required for the input filter capacitor. Input the values you have selected into the PSPICE model (see Figure 2). Generate a Bode plot by simulating with the "application circuit-ac sweep" 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 "application circuit-transient response" simulation profile.

Design Calculation Spreadsheet: FAN5236 Design calculation aid.xls
This spreadsheet calculates external components and provides a bode plot for stability analysis. Instructions can be found in the "Instructions" tab of the spreadsheet.. Macro Security Note: FAN5236 Design calculation aid.xls uses macros extensively. For the spreadsheet to operate properly, check the "Always trust macros from this source" box if a security warning appears, then click the "Enable Macros" button.. This is only required the first time you run a Fairchild spreadsheet tool with macros.

REV. 1.0.5 01/04/06