ADI Linear Regulator LT3073 - How to Implement a Fast Start-Up Circuit?

Semiconductor Integrated Circuits (ICs)
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To reduce noise, traditional LDOs require connecting a large-capacitance capacitor in parallel with the input pin. However, charging a large-capacitance capacitor takes more time, which fails to meet the requirements of scenarios where start-up speed is critical. How to resolve this contradiction and achieve a fast start-up circuit?

We take the ADI LT3073 as an example.

The Implementation Logic of the LT3073 Fast Start-Up Circuit

During the fast start-up phase, the voltage at the LT3073’s REF pin charges gradually from 0V to the target value. Since VOUT follows the REF voltage, the charging process of the REF voltage directly corresponds to the start-up process of VOUT — the faster the REF voltage charges, the quicker VOUT reaches the target value. In other words, to achieve fast start-up, the REF pin must be charged rapidly.

The process specifically consists of 2 key steps:

1. Start-Up Phase: Actively Increase the Charging Current of the REF Pin

When the LT3073 is enabled via the EN pin, the fast start-up circuit is activated immediately: this increases the charging current of the REF pin from 100μA (during normal operation) to 2mA (an increase of 20 times), significantly accelerating the charging speed of the large capacitor at the REF pin.

Example: For a 22μF REF capacitor, the 2mA large current reduces the charging time from the traditional 100ms to 5ms, increasing the start-up speed by 20 times — meeting the fast power-on requirements of most scenarios.

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LT3073 Functional Block Diagram (Image source: ADI)

2. Judgment of Start-Up Completion: Accurate Trigger for Current Shutdown

The fast start-up circuit does not operate continuously; instead, it uses REF voltage threshold detection to determine whether start-up is complete, avoiding impacts on normal noise performance. When the voltage at the REF pin rises to 98.8% of its final stable value (this is the “Fast Start-Up Turn Off Threshold” parameter in the datasheet, with a typical value of 98.8%), the fast start-up circuit automatically turns off the 2mA boosted current, and the REF pin reverts to the normal 100μA operating current.

This threshold design not only ensures that the REF voltage has stabilized (preventing incomplete start-up due to premature shutdown) but also avoids additional noise introduced by excess current, balancing fast start-up and low noise.

Finally, Application Precautions

1. Fast Start-Up Current Specification

During the start-up phase, the maximum current at the REF pin is 2mA (this is the “Fast Start-Up REF Pin Current” parameter in the datasheet). It is necessary to ensure that the external circuit (e.g., a series resistor at the REF pin, if any) can withstand this current to prevent current limiting from reducing the start-up speed.

2. REF Capacitor Compatibility

The fast start-up function supports REF capacitors ranging from 4.7μF (conventional low-noise configuration) to 22μF (ultra-low 1/f noise configuration). The start-up time for different capacitances increases linearly with the capacitance, but all are far lower than those of traditional LDOs (e.g., the start-up time for a 10μF REF capacitor can be reduced to 2.3ms).

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