A simple backup power supply, do you know?

The LTC3643 backup power supply enables designers to use a relatively inexpensive energy storage element: low-cost electrolytic capacitors. In the backup or holdup power supply mentioned here, the LTC3643 charges a storage capacitor to 40 V when power is present, and when power is lost, the LTC3643 discharges the energy of the storage capacitor to the critical load. The load (output) voltage can be any voltage between 3 V and 17 V.

The LTC3643 can be easily used in backup solutions for 5 V and 12 V rails, but 3.3 V rail solutions require extra caution. The minimum operating voltage of the LTC3643 is 3 V, which is relatively close to the nominal input voltage level of 3.3 V. This margin is too tight when a blocking diode is used to separate the backup voltage source from non-critical circuits, as shown in Figure 1a. If D1 is a Schottky diode, its forward voltage drop (as a function of load current and temperature) can reach 0.4 V to 0.5 V, enough to put the voltage on the LTC3643 VIN pin below the 3 V minimum. As a result, the backup power circuit may not start.

Figure 1. (a) and (b). Location of isolation diodes in a backup system schematic.

One possible solution is to move the diode to the input of the supply DC/DC converter, D2, as shown in Figure 1b. Unfortunately, in this case, non-critical loads connected to the upstream DC/DC supply will draw power from the backup supply, leaving less energy for the critical loads.

Figure 2 shows a solution for generating a 3.3 V backup supply using an isolation MOSFET to reserve energy for critical loads. The isolation diode shown in Figure 1 is replaced by a low gate threshold voltage power P-channel MOSFET Q1.

Figure 2. Enhanced schematic of the LTC3643 solution for a 3.3 V rail.

The key to operating the backup supply in a 3.3 V environment is to add the RA-CA series circuit. At startup, as the input voltage rises, the current flowing through capacitor CA is determined by the formula ICA = C × (dV/dt). This current generates a potential across RA that is sufficient to enhance a low gate threshold voltage small signal N-channel MOSFET Q2. When Q2 turns on, it pulls the gate of Q1 to ground, providing a very low resistance path between the input voltage and the LTC3643 supply pin VIN. Once 3.3 V is applied to the converter, the converter immediately starts, pulls down the gate of Q1 and the PFO pin level, and begins charging the storage capacitor.

When the 3.3 V rail reaches steady state, the ICA current decreases to a point where the voltage across RA drops below the Q2 gate threshold level and Q2 turns off, no longer affecting the backup converter functionality. In addition, the PFO pin grounds R3A, resetting the PFI pin power-fail voltage level to a minimum of 3 V to ensure normal operation of the converter when the input voltage supply is disconnected.

The waveforms in Figure 3 show the results when the 3.3 V rail is started. As the input voltage rises, the gate voltage of Q2 also rises, pulling the gate of Q1 low. Q1 is in an enhanced state, allowing the full 3.3 V voltage to reach the LTC3643, bypassing the Q1 body diode. Eventually, the gate voltage of Q2 drops below the threshold level and Q2 turns off, at which point the LTC3643 is fully operational and controlling the gate of Q1.

Figure 3. Waveform of the 3.3 V rail at power-up

The versatility of the LTC3643 is on display here: in particular, it can limit the charge current of the boost converter used to charge the storage capacitor. In situations where the total current must be minimized, such as when there are long wires or high impedance voltage sources, the boost current can be set at a lower level to minimize the effect of the charging current on the input voltage drop. This is especially important for 3.3 V rails. In Figure 2, the 0.05 Ω resistor RS sets a limit of 0.5 A (10.5 A load) for the boost converter charging current (the maximum possible setting limit is 2 A); the rest of the current is delivered to the load.

Figure 4 shows the waveforms when the 3.3 V rail is lost. When the input voltage drops, the gate voltage of Q2 remains constant (close to ground potential) and Q2 is off. In contrast, the gate voltage of Q1 rises sharply to 3.3 V. This turns Q1 off, and the body diode of Q1 acts as an isolation diode, isolating the load from the input. The backup supply now takes over, and the LTC3643 delivers 3.3 V to the critical load by discharging the energy of the storage capacitor.

Figure 4. Waveform of 3.3 V rail when powered off

The circuit presented in this article enables the LTC3643 to be used as a backup power supply solution for a 3.3 V rail. The LTC3643 uses low cost electrolytic capacitors as energy storage elements, simplifying the backup power supply.