Overview

The LT1083 (and TO-220 / TO-3P packaged LT1083CT / LT1083CP) is a 3-terminal low-dropout (LDO) positive adjustable linear voltage regulator engineered by Linear Technology (now Analog Devices). Designed to deliver up to $7.5\text{ A}$ continuous output current, it offers a massive current upgrade over standard LM317 regulators while slashing required input-to-output headroom down to $1.0\text{ V}$ typical ($1.5\text{ V}$ maximum at full 7.5A load).

The output voltage is continuously adjustable from $1.25\text{ V}$ to $30.0\text{ V}$ using two external resistors. Because of its outstanding ripple rejection, minimal noise, and high current capability, the LT1083 is a staple in linear laboratory bench power supplies, high-end audio power amplifier heater rails, microprocessor supplies, and post-regulators for switching supplies.

Quick reference

Pinout (TO-220 / TO-3P Package)

Looking at the front labeled face with leads pointing down and metal heatsink tab at the top:

        ┌──────────────┐
        │ [LT1083 Tab] │  (Metal Tab connected internally to Pin 2 OUT)
        ├──────────────┤
        │    LT1083    │  (Front Face)
        └─┬────┬────┬──┘
          1    2    3
         ADJ  OUT   IN

Standard Adjustable Voltage Circuit

       +V_IN Unregulated Input
          │
       [Pin 3: IN]
        LT1083
       [Pin 2: OUT] ──────────────┬─────────────── +V_OUT Regulated DC Output
          │                       │
       [Pin 1: ADJ]          [ R1 = 121Ω 1% ]
          │                       │
          ├───────────────────────┘
          │
     [ R2 Pot / Resistor ]
          │
         GND

Output Voltage Formula

The output voltage is set by the ratio of $R_1$ and $R_2$, referenced to the internal $1.25\text{V}$ bandgap voltage:

$V_{OUT} = 1.25\text{V} \times \left(1 + \frac{R_2}{R_1}\right) + (I_{ADJ} \times R_2)$

Because the adjust pin current ($I_{ADJ} \approx 50\ \mu\text{A}$) is negligible, choosing $R_1 = 121\ \Omega$ simplifies the calculation:

$V_{OUT} \approx 1.25\text{V} \times \left(1 + \frac{R_2}{R_1}\right)$

Specifications

Common mistakes

• Omitting output capacitor or using high-ESR capacitor: The LT1083 requires a minimum output capacitor of $10\ \mu\text{F}$ (tantalum or low-ESR electrolytic) for control loop stability. Without an output capacitor, the regulator will oscillate.
• Shorting heatsink tab to chassis ground: The tab is OUT, not GND. Bolting directly to a grounded chassis shorts the output directly to ground, triggering current limiting.
• Extreme thermal dissipation at high input differentials: Stepping down $18\text{V}$ to $5\text{V}$ at $7.5\text{A}$ produces $P_D = (18 - 5) \times 7.5 = 97.5\text{ Watts}$. Even a large heatsink will struggle with $100\text{W}$. Keep $(V_{IN} - V_{OUT})$ small ($2\text{V} \dots 4\text{V}$) for high-current applications.

Notes

• Family Hierarchy: The LT108x family shares identical pinouts and characteristics across different current tiers: LT1083 ($7.5\text{A}$), LT1084 ($5.0\text{A}$), LT1085 ($3.0\text{A}$), and LT1086 ($1.5\text{A}$).