Battery - A large portion of the board is dedicated to power-supply, the design will ideally run of a single lithium-polymer cell. This is run through a standalone TI BQ25606, which is a 3A capable SMPS charger which provides Narrow-Voltage DC (NVDC), ensuring that the main system rail always runs between 3.5 and 4.5V, even while charging. This simplifies the later power stages somewhat, as we can use a simple Buck converter or LDO for the primary 3.3V rail instead of requiring a more expensive Buck-Boost. For battery State-of-Charge monitoring, I also included a MAX17048 fuel-gauge on an I2C interface.
Primary - The primary rail is where things get interesting, I made the design as low-power as physically possible in the off-state without software. To achieve this, I took the enable pin of the 3.3V regulator (TLV62569) and connect this to an output on the core. This might seem unwise, however the core has a backup power supply, provided by a low Iq 3.0V LDO (MCP1810). The reference manual states that the pull-ups will remain operational during the lowest power modes on certain ‘Backup IO’. The power button for the device is connected to the backup voltage, so that when pressed the voltage goes through a ‘bootstrap’ Schottky diode and onto the enable pin of the regulator. Once the microcontroller has started, it can take over control of this pin and hold enable high itself. The Schottky diode allows the button to be utilise in the software.
Core - While there is a version of the STM32H72x produced with a built in SMPS to improve efficiency, The 100-pin LQFP variant cannot be configured to use OctoSPI. In order to use both I would need to step up to the 100-Ball TFBGA with a 0.8mm pitch, stock of which is currently non-existent. For sake of comparison, I included an external SMPS (ST1PS02) with an adjustable output of 1-1.35V. The voltage required is dependant on clock frequency. This external core voltage can be fed in via the VCAP pins when the internal LDO is bypassed.
LCD - There are two other primary regulators on the board, both of which are boost converters. One 5V boost converter (MCP1640) produces the analog voltage required for the LCD, the other is a constant current LED driver (LV52204), and is almost the same design that I implemented years ago for Atom/Aim. A PWM signal can be used to control the backlight brightness.
From Suffolk, currently living in Hampshire.
Electrical and Electronic Engineer.
A keen cyclist, runner, hiker and photographer.