vault backup: 2026-07-01 14:44:21

PCB Design/Music Player/Process.md

@@ -16,25 +16,26 @@ where
 (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 (3.6V) and the "minimum input voltage in boost mode" which for the BQ25985 is listed as 2.304V
-$$D = \frac{{3.6 - 2.304}}{3.6} = 0.302$$
+$$D = \frac{{3.6 - 2.304}}{3.6} = 0.36$$
 Now calculating for $I_{PEAK}$ using the values 
-- $D = 0.302$ (calculated previously)
+- $D = 0.36$ (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)
-$$I_{PEAK} = \frac{3.3}{0.9 * (1 - 0.302)} + \frac{{2.304 * 0.302}}{2 * 2.5 * 1.5} = 5.346$$
+$$I_{PEAK} = \frac{3.6}{0.9 * (1 - 0.36)} + \frac{{2.304 * 0.36}}{2 * 2.5 * 1.5} = 6.36$$
-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 datasheet states to pick an inductor with a saturation current 20% higher than the calculated value which would mean $$I_{sat} \geq 7.63$$
+(FIND NEW INDUCTOR)
 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.
 ### Output Capacitors
 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.
 ### Input Capacitors
 Other than being X5R or X7R no other major considerations are needed for the input capacitors.
 ### Output Voltage Selection
-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.
+Since 3.6V are needed the values of 180K and 1.1M will be used for the feedback voltage divider as stated in the table of common output voltage values in the datasheet.
 
-| $V_{OUT}$ | $R1$        | $R2$          |
+| $V_{OUT}$ | $R1$          | $R2$          |
-| --------- | ----------- | ------------- |
+| --------- | ------------- | ------------- |
-| 3.3 V     | 1 M$\Omega$ | 180 k$\Omega$ |
+| 3.3 V     | 1.1 M$\Omega$ | 180 k$\Omega$ |
 ### Layout Considerations
 Figure 31 in section 10.2 of the datasheet should be used as an example layout for the regulator
 ## PCM5102A
@@ -57,3 +58,5 @@ There are a few considerations needed for effectively routing USB. The first is
 There are two different modes that can be used to read and write data to the microSD card, SPI and SD-MMC. SPI is simpler but SD-MMC can get faster speeds and is supported by the ESP32-S3 so that is what I will be using. the ESP32-S3 can multiplex this functionality to any of the GPIO pins so the pinout of the microcontroller is not important. 10K pullup resistors are also needed on the CMD and data lines for the micro SD card to ensure that they do not enter an undefined state. I have also included a 10K pullup resistor on the card detect pin so that it is easier for the microcontroller to read and the in is in a defined state when there is no card in the slot (the pin is pulled to ground when there is a card inserted).
 ## Control Scheme
 This device will not have a touchscreen (the display doesn't have one and I don't want to use one) so I will be controlling everything with physical buttons. To do this my goal is to have physical buttons on the top for play/pause and skip forwards and backwards. I was also able to find a "thumb-wheel switch" on DigiKey which looks like a great option for volume control and navigation. It should have not only up and down selection but also a push in button for confirm. It also has two different angles for each direction that it measures although I am not sure yet if I will make use of both. I would also like a physical power switch that cuts off power to the device (except for the battery charger) to conserve battery. 
+## Analog Power
+One of the challenges of designing this PCB was designing around analog power needed for the DAC. For the audio quality to be good and have minimal noise the analog power inputs need to be clean and separated from the digital power on the board. Since a switching buck-boost converter is used the power rails will have a lot of noise that is not good for the DAC. A buck-boost is still needed to step up the voltage if needed and use the full capacity of the battery and not have to cut out early. To fix this two LDOs were added to clean the power rails and provide a stable source of power. One is for the analog power and one is for digital to provide separation between the two. For this to work the buck-boost output needs to be pushed to 3.6V so that there is enough headroom for the LDOs to properly operate.