Nada Labs

Having determined the battery and power requirements, a full schematic of the GPS logger was made. Buck regulators and Li-Ion battery chargers were investigated from a variety of manufacturers. The TC105 3.3V Buck regulator and the MCP73863 4.2V Li-Ion battery charger, both from Microchip, were chosen to provide the power needs for the project. The circuit is capable of detecting if a USB port is a Charging Port and adjusting the battery charge current to suit.

The circuit consists of several sections, covering battery charging, voltage regulation, usb interface and the PIC and support components.

Battery Charger

Based around U1, a MCP73863 single cell Li+ battery charger from Microchip, the charging circuit allows for charging at up to 1.2A from a 5V power supply. While U1 is a single cell charger, it can be used in this application because BT1 is two 2800mAh cells connected in parallel, appearing to the charger as one large capacity single cell. R8 sets the charge current when powered from a standard USB port to 500mA. This current is also used to power the system so the actual charge current will be closer to 400mA. When a USB Charging Port is detected, R8 is bypassed by driving the gate of Q1, yielding a charge current of 1.2A.

R1, R9 and RT1 are used to monitor the temperature of the battery and will disable charging if the temperature is lower than -4°C or above 55°C. R2 and R3 are used as a voltage divider to allow the PIC to monitor the battery voltage for indicating a low battery condition.

The MCP73863 was chosen mainly for its availability and its SOIC packaging, as the GPS Logger will be hand soldered. There are other devices available which are a better suit to this application but QFN packages are difficult to populate on a board.

Voltage Regulation

A TC105 buck converter forms the heart of the voltage regulator, providing the rest of the circuit with a stable 3.3V independent of the battery voltage. A buck converter needed to be used here as standard linear regulators have a drop out voltage of ~1V, requiring the input voltage in this case to always be above 4.3V. Buck converters can operate down to the required voltage and are more efficient than linear regulators. Information on how buck converters work can be found on the wikipedia Buck Converter page.

USB Charge Port Detection

A recent addition to the USB class specifications allows for the implementation and detection of USB Charging Ports and Dedicated Chargers. The Battery Charging Specification(zip file) was released on 15th April 2009 and allows devices to draw up to 1.5A from a USB port under certain conditions. The detection of a charging port is covered in section 3 of the specification and is implemented in this circuit by Q2, Q3, R4, R10 and D2. Q3, R4 and D2 provide VDP_SRC to D+ which needs to be in the range of 0.5V – 0.7V. D2 provides a reference voltage 0f ~0.6V due to the conduction drop across it when Q3 is on. R4 limits the current flowing through the diode. Q2 and R10 are used to detect VDP_SRC on D-, which if present indicates that a charging port is available. R10 provides the current sink of IDM_SINK to pull D- down on a standard USB port. The PIC samples the voltage present on AN1 when both Q2 and Q3 are on. If the voltage is 0.4V – 0.8V a charge port is present and 1.2A can be used to charge the battery.

PIC Microcontroller

The PIC is very similar to the previous circuit, with the major addition of Q4 to power off the GPS module. R5 was added to pull up the Dout line from the SD card. R13 and C10 provide a low pass filter to debounce the pushbutton. This should be connected to a schmitt input on the PIC however none of the interrupt on change ports provide this capability and it was considered to costly in component count and PCB space to add another IC. The only time the button needs to be debounced is when it is released after being held in to put the logger to sleep. A contact bounce at this point will wake the system back up, at all other times the switch is debounced in software.

Bill of Materials

The following components will be used in the assembled version of the GPS Logger. 

The total component cost is $87.00 (not including shipping) which will be larger once the cost of PCB manufacture is added.

Next Step

A case needs to be decided upon, components orderd and a PCB designed to fit. Once the PCB has been ordered the user software can be designed and written.