A 5V, 2.5A Uninterruptible Power Supply (UPS) based on the Linear Technology (now Analog Devices) LTC4040 2.5A Battery Backup Power Manager.

The LTC4040 includes a step-up DC-DC converter to power your 5V equipment from a single cell Li-Ion or LiFePO4 Battery in the event of loss of external power. When external power is available, the step-up regulator operates in reverse as a step-down battery charger.

Battery Chemistry and Charge Voltage

Battery chemistry and charge voltages can be set up via inputs F0, F1 & F2. For Li-Ion cells, regulated charge voltages of 3.95V/4.0V/4.05V/4.1V are available. For LiFePO4 cells the thresholds change to 3.45V/3.5V/3.55V/3.6V. As the batteries are used for back-up purposes and as capacity is degraded when cells remain fully charged, it is recommended to trade off capacity (up-time) for battery lifetime.

Charge Current

A fixed resistor on the PROG pin, sets the maximum charge current. The current design uses a 2K resistor to set the charge current to 1A. The LTC4040 includes a safety timeout or approximately 4.25h for Li-Ion batteries and 2.13h for LiFePO4.

The current shunt (R1) can be used to prioritise system load over charge rate, ensuring the input power supply is not overloaded. At present the 0.01 ohm shunt sets the maximum input load to 2.5A (0.025V/0.01).  This means if the load is drawing a full 2.5A, the charger will be throttled back to almost nothing. You can change R1 to 0.007 ohms to set the current at 3.5A. This allows the battery to charge at 1A, while the load can consume 2.5A.

Backup / Power Failure Threshold

R16/R15 sets the power failure threshold at 4.6V. When the input exceeds 4.6V, the device will operate in pass-through mode (via external Q1 MOSFET) and the output voltage will track that of the input. When the input voltage drops below this threshold, the DC-DC converter will switch on and power the load from battery. In this mode, the output voltage is set via R3/R4 to 5.0V.

Status

20/06/2019 - Prelimary testing with DC Load and LiFePO4 returns good results. Waiting on extra parts - USB-C and Output Capacitors in order to complete design.