1.5 KiB
#pcb/musicplayer #pcb
TPS63020
Choosing an Inductor
Neither of the options given in the datasheet are still available so a different inductor will need to be chosen
The equations needed to find the peak current in steady state operation are
Duty cycle boost: D=\frac{{V_{OUT} - V_{IN}}}{V_{OUT}}
and peak current: I_{PEAK} = \frac{I_{out}}{\eta * (1 - D)} + \frac{{V_{IN} * D}}{2 * f * L}
where
D= duty cycle in boost modef= converter switching frequencyL= inductor value\eta= estimated converter efficiency (This is taken from section 8.2.2.2 of the datasheet)
First, to calculate the duty cycle boost we can plug in the output voltage (3.3V) and the "minimum input voltage in boost mode" which for the BQ25985 is listed as 2.304V
D = \frac{{3.3 - 2.304}}{3.3} = 0.302
Now calculating for I_{PEAK} using the values
D = 0.302(calculated previously)f = 2.5(MHz, typical stated in datasheet)L=1.5(uH, value for inductor used)\eta = 0.9(efficiency assumption stated in datasheet)
I_{PEAK} = \frac{3.3}{0.9 * (1 - 0.302)} + \frac{{2.304 * 0.302}}{2 * 2.5 * 1.5} = 5.346
The datasheet states to pick an inductor with a saturation current 20% higher than the calculated value which would mean I_{sat} \geq 6.415
The EXLA1V0402-1R5-R has a saturation current of 6.7A which should be a good value for this application. The DC resistance is also low and comparable to the options listed in the datasheet.