I'm planning to use the LM2623 to boost a single AA for a 5V load that draws 400mA. This graph seems useful
The graph seems to suggest that at Vin=1.5V, there's no way 400mA could be drawn without a large voltage drop. At 200mA, Vout can be kept at 5V. I'm guessing that the load (if USB) will refuse to draw if the voltage is too low (<4.4V).
Is there a way to let it draw as much as it can from the input source? Could two DC/DC converters be used so that the first boosts the input from 1.5V to, e.g., 3V, and the second boosts the 3V to 5V. This way, at Vin=3V, the load could draw higher current (not for very long though).