Hi, all.
When R2 is used smallar than recommended value, the OUT terminal voltage rises.
Why is it?
Hello Kobayashi,
The FB resistor divider doesn’t only determine VOUT_MAX level but also determines the range of the adaptive voltage control loop. So if R2 is too small, this will limit the VOUT_MIN level as well, preventing Vout level to be adjusted to the correct level, in this case increasing the boost output higher than expected.
Some customers require lower value resistors, for this we recommend using three feedback resistor configuration instead. This way we end up with more dynamic range of the adaptive control loop and smaller value resistors since the adaptive control loop is now a function of both R2 & R3. Recommended values for three resistor configuration are R2 ~6kΩ, R3 ~27kΩ and varying R1 to obtain desired VOUT_MAX. The following equation can be use to calculate the VOUT_MAX for specific R1, R2 & R3 values:
VOUT_MAX = [[[((ISEL_MAX x R3) + VBG) x (1 / R2)] + ISEL_MAX] x R1] + (ISEL_MAX x R3) + VBG
Where,
VBG = 1.2V
ISEL_MAX = 38.7uA
R2 = Recommended 6kOhm value
R3 = Recommended 27kOhm value
Best Regards,
Salome
