Power management for Compute Module based design - Page 1
Forum section: electronics / projects

author: bpye
author_rank: Contributor
timestamp: September 06, 2016, 12:46:55 pm
content: Hey all,Currently working on a design based around the Raspberry Pi compute module. In part because it's the cheapest SOM I can find, but also because it supports DPI(18 bit + HSYNC/VSYNC/DE/DOTCLK video). I currently have most of the design worked out in my head and here in various development boards all hooked up, however I'm not too sure what to do for power management.I need for the Compute Module 2.3-5V @ 2W, 3.3V @ 250mA, 1.8V @ 250mA, and 2xIO @ 50mA (this will probably also be 3.3V though half could be 1.8V, the SD card on one needs to be 3.3V I believe). This is in addition to an LCD which requires a constant current 80mA backlight driver at ~3.3V and a 3.3V supply (unless SD cards work at 1.8V). This should be around 50mA. In total I need (including some margins for other peripherals)2.3 - 5V @ 2W (straight from battery/USB after regulation?)3.3V @ 500mA1.8V @ 250mA80mA CC LED driver (~3.3V)The compute module specifies that the supplies should come up from highest to lowest voltage or simultaneously. It seems like the former would be easier so 2.3-5V, 3.3V then 1.8V.I have considered two power sources for this. I initially planned to have a rechargeable LiPo, charged over USB. However as a simplification I think I using 4xAA batteries (which can be rechargeable externally), but also support running over USB would be good enough. Also considered USB OTG support as the CM supports it, but I think it's probably unnecessary, it would mean that it would need a 5V boost converter to supply the OTG port when there is a USB device plugged in.Most integrated PMICs seem specific to a SoC, so trying to find something flexible enough for this (which I believe is ultimately pretty standard). I have an I2C and SPI bus already so it wouldn't be hard to add a PMIC that has either interface, I'm welcome to suggestions. A PMIC is also nice that it provides a method for the device to turn itself off (in most cases), so if not using a PMIC this would need to be added.Thanks!

author: ajb
author_rank: Super Contributor
timestamp: September 06, 2016, 01:31:38 pm
content: As long as your power supply will always be >3.3V, then a single 3.3V buck converter seems like it gets you most of the way there. If you really need a full 250mA @ 1.8V, then a second buck converter is probably the way to go, but if your current requirements are much lower then an LDO may be acceptable depending on your battery life requirements. Sequencing can be as simple as using the output of the 3.3V regulator to control the enable input of the 1.8V regulator, as long as there's not a maximum interval that might be violated. For the LED driver, assuming it's a typical 3V-ish white LED setup, you may be able to get away with a linear current source running from 3.3V. A linear 3.3V->3V conversion is 91% efficient, so a switching converter isn't likely to gain you much unless you need to boost the voltage.Is there a dedicated power input for the USB OTG power rail, or would this be derived from the main system voltage?

author: bpye
author_rank: Contributor
timestamp: September 06, 2016, 01:58:21 pm
content: If I go with the 4xAA then system power will certainly always be above 3.3V, I think even with a LiPo 3.3V might be an acceptable cut off, need to look into the discharge of them more to be completely sure however. The 1.8V current spec is based upon the compute module documentation, it's used for various PHYs on the SoC and the SDRAM so I suppose it's not impossible that it might need up to 250mA so another buck regulator is probably the right approach.The backlight is just 4 white LEDs, the datasheet specs them together as 80mA I believe, they have a shared cathode and four anodes, fairly certain their development board gets away with powering all 4 in parallel though I know that that isn't always the best idea.If I do go for two buck regulators + 1 LDO then I am still going to need to work out how I am going to get the ability to control the power rail from the device. Battery charge level would also be useful, though, if using a LiPo there are battery charging ICs with I2C or SPI interfaces, and I do already have an MCU so for the 4xAA approach just adding a voltage divider then reading through the ADC is probably sufficient.If USB OTG were to be supported it'd have to be derived from the main system power rail, there is no external voltage input. As I said though, I am leaning to making this device only anyway as I'm struggling to think of applications where OTG would be useful. Perhaps for network connectivity but it can always act as an RDNIS gadget then you should be able to bridge the device to a network.EDIT: Looks like the TPS65011 (http://www.ti.com/product/TPS65011) could work. The two DC/DC converters can do the two main rails, and I'm pretty sure LDO1 could be used as a constant current driver for the backlight supply.