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Simple backup power supply for 3.3 V rail

Posted by: Cnc Machining China 2022-08-05 Comments Off on Simple backup power supply for 3.3 V rail

Data loss is a problem in telecom, industrial, and automotive applications where embedded systems require reliable power supply. Sudden interruptions in power can corrupt data as hard drives and flash memory perform read and write operations. Designers often use batteries, capacitors, and supercapacitors to store enough energy to provide short-term power support for critical loads during power outages.

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

The LTC3643 can easily be used as a backup solution 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 an isolation diode is used to separate the backup voltage supply from non-critical circuits, as shown in Figure 1a. If D1 were a Schottky diode, the forward voltage drop (as a function of load current and temperature) would be 0.4 V to 0.5 V, enough to place the voltage on the LTC3643 VIN pin below the 3 V minimum. Therefore, the backup power circuit may not start up.

Simple backup power supply for 3.3 V rail

Figure 1(a) and (b). The location of the blocking diode in the backup system schematic.

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

  3.3 V backup power operation

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

The key to operating backup power in a 3.3 V environment is the addition of 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 creates a potential across RA 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. As soon as 3.3 V is applied to the converter, the converter starts, pulls down the gate of Q1 and the PFO pin, and begins charging the storage capacitor.

Simple backup power supply for 3.3 V rail

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

When the 3.3 V rail reaches steady state, the ICA current decreases to a point at which the voltage across RA drops below the Q2 gate threshold level and Q2 turns off, thus no longer affecting the function of the backup converter . In addition, the PFO pin ties R3A to ground, which resets the PFI pin supply-fail voltage level to a minimum of 3 V to ensure that the converter maintains normal operation when the input voltage supply is disconnected.

  circuit function

The waveforms in Figure 3 show the results at startup with a 3.3 V rail. When the input voltage rises, the gate voltage of Q2 also rises, thus pulling the gate of Q1 low. Q1 is in a hardened state, allowing the full 3.3 V to reach the LTC3643, bypassing the Q1 body diode. Finally, 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.

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

Simple backup power supply for 3.3 V rail

Figure 3. Waveforms of the 3.3 V Rail at Power-Up

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) and Q2 is turned off. In contrast, the gate voltage of Q1 rises sharply to 3.3 V. This turns off Q1, and the body diode of Q1 acts as a blocking diode, thereby separating the load from the input. At this point the backup power supply takes over and the LTC3643 delivers 3.3 V to the critical load by discharging the energy from the storage capacitor.

Figure 4. Waveforms of the 3.3 V Rail at Power Down

  in conclusion

The circuit described in this article allows the LTC3643 to be used as a backup power solution for the 3.3 V rail. The LTC3643 simplifies backup power by using low-cost electrolytic capacitors as energy storage elements.

  About the Author

Victor Khasiev is a Senior Applications Engineer at Analog Devices. Victor has extensive experience in power electronics including AC/DC and DC/DC conversion. He holds two patents and has authored several articles. These articles relate to the automotive and industrial applications of ADI’s semiconductor devices. Articles cover boost, buck, SEPIC, positive-to-negative, negative-to-negative, flyback, forward converters, and bidirectional backup power supplies. His patents are high efficiency power factor correction solutions and advanced gate drivers. Victor is happy to support ADI customers: answering questions about ADI products, designing and validating power supply schematics, laying out printed circuit boards, troubleshooting, and participating in testing the final system.

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