This board contains main voltage regulator that converts unregulated LiPo battery voltage into a stable 3.3V voltage suitable to be used by other boards like MCU or anything else. Primary regulator used is a fixed-voltage Texas Instruments TPS63001 that operates in buck and boost modes and also provides a power-save functionality. This regulator is enabled when EN pin is high so I'm using a LTC2950-1 push button controller to provide that signal to regulator. When system is powered on, LTC2950 keeps EN pin low and only after a push button is pressed for about ~0.6sec (can be changed by modifying Con and Coff capacitors, I'm using a 0.1uF ones) EN pin is pulled high and regulator turns on.

Power save mode of TPS63001 allows to turn off switching and directly connect input to output when voltages between them are close enough to 3.3V. This pin is pulled up by resistor on board and can be pulled low by MCU or by connecting it to ground. Power save mode LED would come on when regulator is in power save mode.

TPS63001 allows currents of up to 1.2A in buck mode and up to 800mA in boost mode, so there's plenty of current for most MCU-based projects.

LTC2950 is configured in a way to monitor input voltage that's coming to TPS63001 regulator using a resistor divider network. If voltage on KILL pin falls below ~0.6V - LTC2950 immediately shuts down the regulator. I've configured resistor divider to shut down system when input voltage drops to about 3.1V so LiPo battery can be safe. You can change this voltage down to ~2.8 volts as LiPo battery undervoltage protection circuit kicks in at about 2.7V, but it's really not recommended to go below 3.0V when dealing with LiPo batteries.

LTC2950 also provides KILL and INT signals so MCU can shut down regulator by bringing KILL down or execute an interrupt when button pressed while system is powered on using INT line.

PS, PB, KILL and INT signals are routed to GPIO pins of this board and can be used to control this circuit.

Characteristics:

• Input voltage: 2.7V - 5.5V (yes, you can use anything else to power up this circuit, not only a LiPo battery, any other battery type or USB power would work just fine) - Voltage is limited down to 2.7V only because LTC2950 can operate not below that voltage. TPS63001 can be used in boost mode from 1.8V
• Current limit: from 3.3V to 5.5V - 1.2A, from 2.7V to 3.3V - 0.8A (without voltage drop on the output of regulator)
• Switching frequency: 1.31MHz (measured), so be sure to use shielded inductors or modify the PCB design to have a cutout in ground plane around inductor and below it and tracks to it
• Turn on time: ~0.6s (with 100nF capacitor)
• Turn off time: ~0.6s plus shutdown time of LTC2950 as per datasheet (with 100nF capacitor)