# Boost Converter LiPo Charger

## Summary Figure 1: Schematic for the Boost Converter/LiPo Charger Test Board. Figure 2: PCB Layout for the Boost Converter/LiPo Charger Test Board.

The TPS61085, made by TI, is a 650kHz/1.2MHz boost converter capable of boosting an input voltage of 2.3-6V up to an output of 18.5V, with a 2A switch current.  This device will be used to produce the 9V required by the lithium polymer (LiPo) charging IC from the 5V provided by a standard USB port or 5V regulated wall adapter. The MCP73844 is a dual-cell LiPo charge management controller, providing an adjustable charge current via an external sense resistor and P-Channel MOSFET. The IC can precondition cells that have dropped below 2.85V per cell, after which a constant current/constant voltage charge cycle begins. The test board provides all the necessary support components for the TPS61085 and MCP73844, including the datasheet suggested components for a 5V to 9V step up, and an external MOSFET and sense resistor, specifying a charge current of 220mA, selected for the maximum charge current given the boost converter’s efficiency and maximum USB port supply current of 500mA.  The charge current was selected using the following formulas:

$\large P_{in}\cdot \eta = P_{out}$

Where $\eta$ is the switching regulator efficiency, approximately 85% at 220mA load.

$\large P_{in} = I_{in}\cdot V_{in}$

$\large P_{out} = I_{out}\cdot V_{out}$

$\large I_{out}=\frac{I_{in}\cdot V_{in}\cdot \eta}{V_{out}}$

Thus, given a 500mA 5V supply, the maximum output current at 9V is approximately 240mA.

The schematic for this test board is shown in Figure 1, and the board is shown in Figure 2.

## Testing

• Output Voltage = 9.009V
• Charge Current = 212.3mA
• Charge Termination Voltage = 8.382V
• Output Power = 1.913W
• Input Voltage = 4.974V
• Input Current = 437.5mA
• Input Power = 2.176W
• Efficiency = 87.9%
• Heat Power = 263mW
• Reverse Leakage Current (Charger disabled) = 0.2uA
• Reverse Leakage Current (Charger unpowered) = 895uA

The results show that the charger and boost converter produce the necessary voltages to function (within 0.2%), and maintain a current draw of less than 500mA at the full charge current, as required by USB. Additionally, the reverse leakage current is negligible; a concern for battery drain when in use and when the device is off. The device has a very high efficiency of 87.9%, so little power is wasted as heat.

## Bill of Materials

 Part Quantity Unit Cost Total Cost Source TPS61085DGKT 1 $3.30$3.30 Digi-Key MCP73844-840I/MS 1 $1.56$1.56 Digi-Key 0603 Capacitor 3 $0.0096$0.03 eBay - mib_instruments 0603 Resistor 5 $0.00294$0.02 eBay - mib_instruments 1206 10uF Capacitor 4 $0.297$1.19 Digi-Key PMEG3020ER 1 $0.48$0.48 Digi-Key STS5PF20V 1 $0.24$0.24 Digi-Key 0603 Green LED 1 $0.11$0.11 Digi-Key SDR0302-3R3ML 1 $0.42$0.42 Digi-Key Male Header 3 $0.01$0.03 eBay PCB 1.225"x0.5125"/3 $5.00$1.05 Laen's PCB Total Unit Cost \$8.43