Nanopower Buck Converter Runs on 720nA, Easily Fits into Energy Harvesting and Other Low Power Applications

The LTC3388-1/LTC3388-3 integrated synchronous step-down regulator provides a regulated output while consuming a mere 720nA of quiescent current. It accepts inputs up to 20V and can deliver up to 50mA of load current. Eight pin-selectable output voltages are offered: 1.2V, 1.5V, 1.8V, and 2.5V on the LTC3388-1 and 2.8V, 3.0V, 3.3V, and 5.0V on the LTC3388-3. The extremely low quiescent current prolongs battery life for keep-alive circuits in portable electronics and makes it a good fit for energy harvesting applications.

The LTC3388-1/LTC3388-3 (Figure 1) achieves its low quiescent current by entering a sleep state once the output is in regulation. In the sleep state, load current is provided by the output capacitor while the output voltage is monitored. When the output falls below a fixed hysteresis window, the converter wakes up and refreshes the capacitor.

Figure 1: Low quiescent current buck converter.

This hysteretic method of providing a regulated output minimizes losses associated with FET switching and makes it possible to efficiently regulate at very light loads. The total quiescent current at VIN in the sleep state is 720nA when VIN is 4V and increases to only 820nA when VIN is 20V. At light loads, the time the buck is active is miniscule relative to the time it sleeps, so the average quiescent current required to maintain regulation approaches the DC sleep quiescent current.

Enable and Standby

Two pins, EN and STBY, control enable and standby functions on the LTC3388-1/LTC3388-3. When EN is low, the buck is turned off and only 520nA of quiescent current appears at VIN. When EN is high, the STBY pin places the LTC3388-1/LTC3388-3 in standby. In this mode the buck is prevented from switching, resulting in a quiet supply. The PGOOD pin, which is high when the output is in regulation, remains active while in standby. PGOOD transitioning low can serve as an indicator that the output has fallen and that the LTC3388-1/LTC3388-3 should leave standby mode to refresh the output.

High Efficiency at Light Loads

The extremely low quiescent current of the LTC3388-1/LTC3388-3 allows for high efficiency at loads as low as 10µA. This is especially useful for low power systems that spend a long time idling and only periodically wake up to perform a task. Figure 2 shows typical efficiency of the LTC3388-1 for the 1.8V output, which is suitable for powering low power microprocessors.

Figure 2: LTC3388-1 Efficiency vs load current for the 1.8V output.

Supports Energy Harvesting Applications

The LTC3388-1/LTC3388-3 is especially well suited for energy harvesting applications where only low amounts of energy are available. Figure 3 shows the LTC3588-1 piezoelectric energy harvesting power supply harvesting ambient vibration energy with a piezoelectric transducer and producing a 3.3V output. The LTC3388-3 is powered from this output and is configured to provide a −3.3V rail, producing a low power dual output supply. The LTC3388-3’s low quiescent current, combined with the LTC3588-1’s own low quiescent current results in a complete solution that draws only 1600nA with no load at both outputs.

Figure 3: Piezoelectric energy harvester with dual ±3.3V outputs.

Conclusion

The LTC3388-1/LTC3388-3 monolithic buck converter’s extremely low quiescent current makes it ideal for low power applications. A quiescent current of less than a microamp prolongs battery life for keep-alive circuits in portable electronics and enables a new generation of energy harvesting applications.

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Michael Whitaker