Output Voltage Set
The voltage adjustment is set by the voltage divider R1-R3 in my schematic circuit. Microchip labels these resistors as RTOP (for R1) and RBOT (for R3). Here is the formula that Microchip provides:
VFB is always 1.21V. Keep RBOT (R3) close to 300K or so. You can use the above formula to calculate RTOP (R1). Here are two examples provided by Microchip for 3.3 and 5V outpus:
VOUT = 3.3V
RBOT = 309K
RTOP = 533.7K
VOUT = 5.0V
RBOT = 309K
RTOP = 967.9K
Maximum Output Voltage
There is a device limit output voltage which depends upon the input voltage and the output current. The following characteristics shows this relation for 3 output voltages, 2V, 3.3V and 5V:
So, so ensure reliable operation at 3.3V output 100mA, the input voltage should not fall bellow 0.9 Volts.
Efficiency and Power Dissipation
The efficiency of the system mostly depends on the input and output voltage and the current. Due to the mosfet switching use, the main cause of losses are resistive loses, so a bigger gap between input and output voltages will result in less efficiency. Another efficiency factor is the inductor resistance. Larger inductors have less resistance and thus better efficiency, but they come in bigger and more expensive packages. Also, poor capacitors with high leaking current have negative effect on efficiency.
Here is the efficiency to output current characteristics that Microchip provides:
The output voltage is 3.3V for all situations. It is obvious that the efficiency is less when the input voltage is lower.
The following graph shows how a sophisticated inductor selection can lead to better efficiency.
The first inductor (1.8A ISAT) has results in betetr efficiency than the second inductor (0.7A ISAT) due to the fact that it has mush less resistance
And here is how a good capacitor selection results in the overall efficiency:
To calculate the power dissipation of the system, first you need to estimate the efficiency. You can use the first characteristics (efficiency VS Output Current) to have an approximation on the system's efficiency. Then use the following formula to get the power dissipation:
Read the next page with the PCB design considerations and drawings