You said you looked at the LM2576. This part uses a fixed switching frequency that is at 52KHz. This will have an effect on the transient performance.

You may want to look at the data folder for this part on the National web site (http://www.national.com/mpf/LM/LM...#Overview). There they recommend a whole bunch of alternative parts that offer some upside benefits.

Also Recommended
LM25005 - Enhanced Package, More Efficient And Better Transient Response
LM25576 - Enhanced Package, More Efficient And Better Transient Response
LM2673 - Much Improved Switching Frequency
LM2676 - Much Improved Efficiency And Switching Frequency
LM2696 - Higher Efficiency, Enhanced Package, Better Transient Response
LM5005 - Higher Input Voltage, Higher Switching Frequency With Sync
LM2596 - More Efficient
LM2599 - Improved Switching Frequency, And Efficiency

You may also want to consider the wide range of parts offered at www.linear.com. They have a tool you can download to your computer called Switcher CAD (a.k.a LT Spice) that you can use to model ready made tests made for many of their parts. With this you can see first hand the transient response and ripple on the outputs at loads that you would select.

Michael Karas