TPS40170: Temperature Variation and Converter Efficiency

(1)  In reviewing the datasheet, it seems the MOSFET R_on is used to sense current (according to Figure 4 of the datasheet).  If so, how do you deal with the R_on temperature variation?

TPS40170 controller includes a temperature compenated current source out of the ILIM pin.  This temperature compensated current source adjusts the voltage drop across the external ILIM resistor.  This increases the voltage threshold required to trigger over-current threshold proportionally to the increased Rdson of the MOSFET at elevated temperature.

This compensates for Rdson increase due to an increase in ambient temperature, providing a more consistant current limit over ambient temperature, however the selection of the ILIM resistor should account for self-heating of the MOSFETs while carring the current limit current.  Failure to account for self-heating will result in low current limit since the IC die temperature and MOSFET temperature are not consistantly equal.

(2)  What kind of conversion efficiency is possible with this device?

Conversion efficiency is heavily dependant on the Input Voltage, Output Voltage and Current Levels.  5V ouptut efficiencies are provided in the datasheet design example and are 90-95% depending on input voltage (higher at lower input voltages)

Efficiency will tend to be lower for lower output voltages, due to less output power and similar power loss.

Efficiency will tend to be higher for higher output voltages, again due to similar power losses, but with greater ouptut power.