ADI Linear Regulator LT3073 - What is "Output Type: Programmable"?

Semiconductor Integrated Circuits (ICs)
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You may notice that on the DigiKey website, the LDO’s output type is listed as “programmable”. How do you program the LDO’s output?

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We take the ADI LDO LT3073 as an example:The programming of the LT3073’s output voltage mainly has two methods: digital pin programming (basic preset levels) and external drive of the REF pin programming (flexible custom voltage).

1. Basic Method: Digital Pin (VO₀, VO₁, VO₂) Programming

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Digital Pin (VO₀, VO₁, VO₂) Programming (Image source: ADI)

Core Principle: Interaction Between Internal Current Source and Resistor Network

The LT3073 integrates internally:

  • A current source with a typical value of 100μA;
  • An internal nominal resistor (RINT) associated with the VOₓ preset levels.

The output voltage is strongly correlated with the voltage of the REF pin. Therefore, the output voltage can be customized by controlling the REF voltage through an external circuit.

The LT3073 selects preset output voltage levels (such as 1.0V, 1.1V, 1.15V, 1.2V, etc.) through the combination of level states (low level, high level, high-impedance state) of the VO₀, VO₁, and VO₂ pins, as shown in the table below.

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LT3073 Preset Output Voltage Levels Table (Image source: ADI)

Advantages & Limitations

  • Advantages: No external components required; the circuit is extremely simple, suitable for general scenarios.
  • Limitations: The output voltage can only select preset levels, and arbitrary custom values are not supported.

2. Voltage Divider Network Driving the REF Pin

Voltage Divider Network Driving the REF Pin (Image source: ADI)

When a non-preset voltage (such as 1.025V, 2.5V, etc.) is required, the REF pin can be directly or indirectly driven by an external voltage source to flexibly customize the output voltage (range: 0.5V~4.2V).

The external voltage source VEXT is connected to the REF pin through a resistor voltage divider network (REXT1 and REXT2), enabling more flexible adjustment of the REF voltage.

At this time, the superposition principle is used to calculate the REF voltage:

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Where:

  • IINT: Internal 100μA current source;
  • RINT: Internal nominal resistor corresponding to the VOₓ preset level;
  • REXT1, REXT2: External voltage divider resistors;
  • VEXT: External voltage source.

Example

Use a 1.25V external reference source to set the REF voltage to 1.025V, and configure VOₓ to the “1V preset” (at this time, RINT = 10kΩ). Steps:

  1. Select REXT2 = RINT / 10 = 1kΩ (to ensure accuracy and offset process deviations of the internal resistor);
  2. Calculate the parallel resistance: RINT || REXT2 = 10kΩ || 1kΩ ≈ 909.09Ω;
  3. Substitute into the superposition formula and solve to get REXT1 ≈ 221Ω (select the closest 1% precision resistor).

Summary

The LT3073 supports two output voltage programming methods:

  1. Digital pin programming: Simple and fast, compatible with preset voltages, suitable for general scenarios;
  2. External drive of the REF pin: Flexibly customizes any voltage (0.5V~4.2V), suitable for high-precision/non-standard voltage scenarios, but requires additional circuits and calculations.

Precautions

  1. When dividing the voltage, REXT2 should be ≤ 10% of the internal nominal resistor RINT to ensure accuracy under process variations;
  2. When driving the REF pin externally, the output voltage threshold of “Power Good” in the data sheet may not be guaranteed (since the output voltage is customized).

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