52 lines
4.1 KiB
Markdown
52 lines
4.1 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.
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### Output Capacitors
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The datasheet states no major considerations except for capacity and being small and ceramic. Either ones I already have or any 22uF ceramic capacitor will be used.
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### Input Capacitors
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Other than being X5R or X7R no other major considerations are needed for the input capacitors.
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### Output Voltage Selection
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Since 3.3V are needed the values of 180K and 1M will be used for the feedback voltage divider as stated in the table of common output voltage values in the datasheet.
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| $V_{OUT}$ | $R1$ | $R2$ |
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| --------- | ----------- | ------------- |
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| 3.3 V | 1 M$\Omega$ | 180 k$\Omega$ |
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### Layout Considerations
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Figure 31 in section 10.2 of the datasheet should be used as an example layout for the regulator
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## PCM5102A
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## LCD Selection
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Initially I was planning to use the [NHD-2.4-240320DA-CTXN](https://www.digikey.com/en/products/detail/newhaven-display-intl/NHD-2-4-240320DA-CTXN/26743729) but I found that it requires 9.0V to drive the back light. Since I am driving this off of a single cell lipo I didn't want to add the additional complexity of another boost converter to get to 9.0V. I was able to find [another very similar display](https://www.digikey.com/en/products/detail/newhaven-display-intl/NHD-2-4-240320AF-CSXP/22204988) that instead uses 3.0V (up to 3.3V) to power the back light.
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The display controller gives the option of using SPI or parallel RGB to control the display. I decided to go with SPI since it is much simpler to route and manage and I don't need the performance gained by the parallel RGB interface.
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### Backlight Resistors
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To limit the current of the backlight for the display series resistors are needed for the anode (power supply) pins. To calculate the value for the resistor the equation
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$$R = \frac{{V_{in} - V_{f}}}{I_{f}}$$
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is used which gives the current limiting resistor needed for a LED. The display used has a LED voltage of 3.0V and will be supplied with 3.3V. It also typically draws 160 mA of current. Plugging in these values gives
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$$R = \frac{{3.3 - 3.0}}{0.16} = 1.875 \Omega$$
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For a standard value a $2.0 \Omega$ resistor will be used. This may make the display a little bit less bright but won't change it very much.Vs - Vf) / If using the maximum forward voltage:
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### Backlight Control
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Since this device will be battery-powered and the display is not needed most of the time it needs to be able to be controlled by software by the ESP32-S3. To do this a transistor will be added to the cathode pins to control the backlight with an io pin from the microcontroller. |