LM2576S Worst Case Analysis

Hello Danny,

Worst case analysis can usually be subdivided into different tasks based on what parameter is most important to the end application.

From your spec I am assuming that V0UT = 7.0  +/- 0.5V over the operating range.

First select the feedback resistors:  For Vout = 7.0V and VFB = 1.23V then the best choice is RFBT = 4.99K and RFBB = 1.07K.  (There are web applications that can select these for you.)

For worst case over temp use the data sheet values of 1.180V and 1.280V for the min and max reference voltage over the -40C to +125C temperature range. I realize you only need -20C to 75C but these are the only numbers shown on the data sheet.  Further the data sheet numbers have a notable guard band to give high production yield. So keep in mind that these two voltages will be pessimistic.

Next you need to recalculate output voltage based on the actual resistor worst case values. Start with the room temperature tolerance (i.e. +/- 1%) and multiply by the temperature coefficient  (+/- 100PPM/degree C typical Philips brand 0805)  for -20C and +70C.  So determine the worst case high and low resistor values at the temperature extremes based on both positive and negative temperature coefficients and high and low room temperature tolerance.

Then for the very worst case conditions use the following values in the data sheet VOUT equation.

 VOUT(max) based on VFB(max) RFBT(max) and RFBB(min).

 And

 VOUT(min) based on VFB(min) RFBT(min) and RFBB(max).

 A spreadsheet helps with all of this.

 So this should get you started for worst case VOUT level.

 Are there other factors of concern?

 Alan Martin TI/SVA