The key to improving the transient response is to start with the best topology. For example, the TPS61094 supports low I_Q and fast transient response. The TPS61094 is a bi-directional buck/boost converter that has a low I_Q of 60nA in supercapacitor-charging (buck) and supercapacitor-discharging (boost) modes. The TPS61094 monitors dv/dt slopes at the output and adjusts its regulation behavior to optimize the transient performance at any given moment. This allows to quickly detect an output voltage drop while maintaining low I_Q at the same time. As a result, the output voltage will remain nearly constant when the TPS61094 starts supporting backup power or peak load support from a supercapacitor.

You must reduce the number of current-consuming blocks as much as possible, so the simpler the topology, the better. For example, the TPS63900 four-switch buck-boost converter, which has an I_Q of 75nA, uses one single mode to regulate the output voltage above, below or equal to the input level. Besides the core architecture, using sample-and-hold techniques when entering light load minimizes the I_SHDN of all internal support functions.

You can conserve even more current with a zero-current feedback divider, digital assisted control and dynamic biasing. Dynamic biasing is a well-known technique, but becomes challenging when operating with just a few nanoamperes. To avoid the gain falloff at low bias currents, optimally shaping both the transconductance and output resistance as a function of the bias current achieves an I_Q-efficient constant gain amplifier.

Another technique uses fast startup circuits. By reducing the startup time of the sample-and-hold reference systems, the on time of the band-gap core and scaling amplifier circuits are reduced significantly. This improves the on-to-off time ratio, thus bringing the average current down in the nanoampere range while maintaining noise and accuracy levels.

To improve the line transient response, feed-forward techniques are applied to the voltage regulation loop in an energy-efficient way. Using transient detection circuits to adjust bias currents or enable circuitry further reduces both voltage dips at the output and settling times.

Figure 11 illustrates the application of these techniques in the TPS63900. The line transient is barely visible on the output voltage and far below the switching ripple, whereas other devices show 100-mV changes.