REG1117-3.3 — 800mA Fixed 3.3V Low-Dropout Linear Voltage Regulator

Overview

The REG1117-3.3 is an 800 mA Low-Dropout (LDO) positive linear voltage regulator originally designed by Burr-Brown and manufactured by Texas Instruments. It provides a stable, fixed +3.3 V DC output rail from input voltages up to 15 V, delivering up to 800 mA of load current with a typical dropout voltage of 1.1 V (1.2 V maximum at full load).

Packaged in surface-mount SOT-223 (DCY), TO-263 (DDPAK), and TO-252 (DPAK) outlines, the REG1117 family features internal current limiting and thermal overload circuitry. It is widely employed as a pin-compatible alternative to the AMS1117-3.3, LM1117-3.3, and NCP1117-3.3 on microcontroller boards, industrial modules, and embedded peripherals requiring tight line/load regulation.

Quick reference


Regulator Type	Low-Dropout (LDO) Positive Linear Regulator
Package	SOT-223 (4-lead DCY) / TO-263 (3-lead KTT) / TO-252 (3-lead KVU)
Pinout (SOT-223 Top View)	Pin 1: Ground (`GND`), Pin 2: Output (`VOUT`), Pin 3: Input (`VIN`), Tab: `VOUT`
Fixed Output Voltage	$+3.3\text{ V}$ DC ( $\pm 1.0\%$ accuracy at $25^\circ\text{C}$ , $\pm 1.5\%$ over full temp)
Input Voltage Range ( $V_{IN}$ )	$4.5\text{ V}$ to $15.0\text{ V}$ DC
Dropout Voltage	$1.2\text{ V}$ max at $I_{OUT} = 800\text{ mA}$ ( $1.1\text{ V}$ typical)
Continuous Output Current ( $I_{OUT}$ )	$800\text{ mA}$ max (thermal limit dependent)
Ripple Rejection (PSRR)	$62\text{ dB} \dots 72\text{ dB}$ at $f = 120\text{ Hz}$

Pinout (SOT-223 Package)

Looking at the top face of the SOT-223 package with the large metal tab at the top and the three leads pointing down:

text

        ┌───────────────┐
        │ [REG1117 Tab] │  (Metal Tab connected internally to Pin 2 VOUT)
        ├───────────────┤
        │  REG1117-3.3  │  (Top Package Face)
        └─┬────┬────┬───┘
          1    2    3
         GND  VOUT VIN

Pin	Name	Type	Description
1	`GND` / `ADJ`	Power	Ground reference (0 V) for fixed 3.3V versions (Adjust pin on adjustable models)
2	`VOUT` / `TAB`	Power Output	Regulated +3.3V DC output pin (electrically connected to the heatsink tab)
3	`VIN`	Power Input	Unregulated DC input voltage pin (+4.5V to +15.0V DC)

Specifications

Parameter	Symbol	Min	Typ	Max	Unit	Conditions
Output Voltage	$V_{OUT}$	3.267	3.300	3.333	V	$I_{OUT} = 10\text{ mA}, V_{IN} = 4.8\text{ V}, T_J = 25^\circ\text{C}$
Output Voltage (Full Temp)	$V_{OUT}$	3.250	3.300	3.350	V	$0 \le I_{OUT} \le 800\text{ mA}, 4.75\text{ V} \le V_{IN} \le 10\text{ V}$
Dropout Voltage	$V_{DO}$	—	1.1	1.2	V	$I_{OUT} = 800\text{ mA}, T_J = 25^\circ\text{C}$
Current Limit	$I_{LIM}$	900	1200	1500	mA	$V_{IN} - V_{OUT} = 5\text{ V}$
Quiescent Current	$I_Q$	—	5.0	10.0	mA	$V_{IN} = 5.0\text{ V}, I_{OUT} = 0\text{ mA}$
Line Regulation	$Reg_{line}$	—	0.1	0.2	%	$4.75\text{ V} \le V_{IN} \le 12.0\text{ V}, I_{OUT} = 10\text{ mA}$
Load Regulation	$Reg_{load}$	—	0.2	0.4	%	$V_{IN} = 4.8\text{ V}, 10\text{ mA} \le I_{OUT} \le 800\text{ mA}$

Typical Application Circuit

text

       +5V USB / Unregulated DC
           │
        [Pin 3: VIN]
        ┌─────────────┐
        │ REG1117-3.3 │
        └─────────────┘
        [Pin 2: VOUT] ────────────┬─────────────── +3.3V Regulated Output
           │                      │
        [Pin 1: GND]        [ C2: 10µF Tantalum ]
           │                      │
    [ C1: 10µF Tantalum ]        GND
           │
          GND

Important

Output Capacitor & ESR Stability Guidelines:

The REG1117 requires a minimum $10\ \mu\text{F}$ output capacitor (tantalum or electrolytic) to guarantee internal amplifier loop stability across load steps.
When using modern ultra-low-ESR ceramic capacitors ( $<0.05\ \Omega$ ), place a small series damping resistor ( $0.2\ \Omega \dots 0.5\ \Omega$ ) or pair with a bulk electrolytic capacitor to avoid high-frequency output oscillation.

Common mistakes

Exceeding SOT-223 thermal power dissipation limits: Linear power dissipation is calculated as $P_D = (V_{IN} - V_{OUT}) \times I_{OUT}$ . Dropping 12V down to 3.3V at 500mA yields $P_D = (12 - 3.3) \times 0.5 = 4.35\text{ W}$ , which will instantly trigger thermal shutdown on a standard PCB layout ( $\theta_{JA} \approx 65^\circ\text{C/W}$ ). Keep $V_{IN} \le 5.5\text{V}$ for heavy continuous loads or provide ample PCB copper pour.
Shorting the metal tab to Ground: On SOT-223 and TO-263 packages, the thermal tab is internally tied to Pin 2 (VOUT), not Ground. Soldering the tab directly to a system ground plane creates a dead short on the 3.3V rail.
Insufficient input voltage below dropout: If $V_{IN}$ falls below $V_{OUT} + V_{DO} \approx 3.3\text{V} + 1.2\text{V} = 4.5\text{V}$ , the regulator drops out of regulation and $V_{OUT}$ sags.

Notes

Pin-Compatible Drop-In Replacements: AMS1117-3.3, LM1117-3.3, LD1117V33, NCP1117ST33T3G.
Voltage Options: REG1117 (Adjustable), REG1117-2.85 (2.85V for SCSI termination), REG1117-3.3 (3.3V), REG1117-5 (5.0V).

Interface	3-Terminal Passive (Ground, VOUT, VIN)
Package	SOT-223 (DCY) / TO-263 (KTT) / TO-252 (KVU)