This circuit is used in a battery-powered portable handy light source, consisting of an array of LEDs, powered by a Panasonic lithium battery.
This circuit can be divided into these modules:
This module consists of the following:
- Microcontroller and it's driving components
- Shift register chip for driving the 7-Segment
- A 7-Segment and it's accompanying components
This module steps down the battery's voltage down to 3 Volts which is suitable for the LEDs.
If battery voltage drops below 3 Volts however, this module is turned off and the battery's voltage is directly passed through to the LEDs.
The signals required for control of the buck converter are provided by the microcontroller.
The Control and Buck Converter modules require voltages upwards of 5 Volts for their function, mainly needed by MOSFET's gates or the microcontroller (When operating near 20MHz).
For that reason, this module consists of a 'Dickson charge pump' that converts the 2.5v ~ 4.2v of the battery to 8v ~ 15v.
The signals required for driving the charge pump are provided by the microcontroller.
Consisting of a LTC1733 Single-Cell Lithium Charge Controller, this module takes care of topping off the battery when a charger is connected.
A single CD4094 Shift-and-Store Register chip (similar to the infamous 595 Shift Register), Takes CLOCK, DATA, STROBE and OUTPUT-ENABLE signals from the microcontroller and outputs the signals needed for controlling the 7-Segment.
These signals are connected to bases of 7 PNP Transistors which drive the LEDs of the 7-Segment directly.
Taking advantage of two schottky diodes, power to the control module will get sourced from either the battery or the charge pump (Which is regulated down to 5 Volts.).
Because of the redundant-psu style dual-diode circuit, current will get sourced from whichever source providing a higher voltage.
The diode accross Vout and Vin of the regulator is there to protect the regulator if the Vout of the regulator goes higher than the Vin. This shouldn't be much of a problem because of relatively small leakage current of the dual-diode circuit, but is put there 'Just-in-Case™'.