27 lines
1.5 KiB
Markdown
27 lines
1.5 KiB
Markdown
#pcb/musicplayer #pcb
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## TPS63020
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### Choosing an Inductor
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Neither of the options given in the datasheet are still available so a different inductor will need to be chosen
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The equations needed to find the peak current in steady state operation are
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Duty cycle boost: $$D=\frac{{V_{OUT} - V_{IN}}}{V_{OUT}}$$
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and peak current: $$I_{PEAK} = \frac{I_{out}}{\eta * (1 - D)} + \frac{{V_{IN} * D}}{2 * f * L}$$
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where
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- $D$ = duty cycle in boost mode
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- $f$ = converter switching frequency
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- $L$ = inductor value
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- $\eta$ = estimated converter efficiency
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(This is taken from section 8.2.2.2 of the datasheet)
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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
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$$D = \frac{{3.3 - 2.304}}{3.3} = 0.302$$
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Now calculating for $I_{PEAK}$ using the values
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- $D = 0.302$ (calculated previously)
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- $f = 2.5$ (MHz, typical stated in datasheet)
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- $L=1.5$ (uH, value for inductor used)
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- $\eta = 0.9$ (efficiency assumption stated in datasheet)
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$$I_{PEAK} = \frac{3.3}{0.9 * (1 - 0.302)} + \frac{{2.304 * 0.302}}{2 * 2.5 * 1.5} = 5.346$$
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The datasheet states to pick an inductor with a saturation current 20% higher than the calculated value which would mean $$I_{sat} \geq 6.415$$
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The [EXLA1V0402-1R5-R](https://www.digikey.com/en/products/detail/eaton-electronics-division/EXLA1V0402-1R5-R/16893520) 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. |