LT1073CN8 — Micropower DC-DC Converter (Step-Up / Step-Down / Inverting, DIP-8)

Overview

The LT1073CN8 (LT1073) is a versatile micropower switching regulator DC-DC converter manufactured by Linear Technology (now Analog Devices) in a classic through-hole 8-pin DIP (DIP-8) package. Immortalized in iconic application notes written by analog circuit master Jim Williams (Linear Tech AN-29 and AN-30), the LT1073 operates from an ultra-wide input voltage range of $1.0\text{V}$ to $30.0\text{V}$ DC with a standby quiescent current of only $95\ \mu\text{A}$ .

Employing a gated-oscillator control architecture with an internal $1.0\text{A}$ peak power switch, the LT1073 can be wired in three distinct power conversion topologies:

Step-Up (Boost): Converting a single $1.5\text{V}$ AA battery into regulated $+5\text{V}$ or $+12\text{V}$ .
Step-Down (Buck): Stepping down a $9\text{V}$ or $12\text{V}$ battery to $3.3\text{V}$ or $5\text{V}$ with high light-load efficiency.
Positive-to-Negative (Inverting): Generating negative supply rails (e.g. $-5\text{V}$ or $-12\text{V}$ ) for op-amps and audio preamplifiers from a single positive battery rail.

It also incorporates an auxiliary uncommitted gain block / comparator with an internal $200\text{ mV}$ reference for building low-battery indicator circuits.

Quick reference


Converter Type	Micropower Multi-Topology DC-DC Converter
Topologies Supported	Step-Up (Boost), Step-Down (Buck), Positive-to-Negative (Inverter)
Package	8-pin DIP (DIP-8 / N8) / 8-pin SOIC
Operating Input Range ( $V_{IN}$ )	$1.0\text{ V}$ to $30.0\text{ V}$ DC
Quiescent Current ( $I_Q$ )	$95\ \mu\text{A}$ typical ( $135\ \mu\text{A}$ max)
Internal Power Switch	$1.0\text{ A}$ peak current rating, $V_{CESAT} \approx 0.45\text{ V}$
Feedback Reference ( $V_{FB}$ )	$212\text{ mV}$ (Adjustable version)
Fixed Output Versions	LT1073-5 ( $+5.0\text{V}$ ), LT1073-12 ( $+12.0\text{V}$ )
Auxiliary Gain Block	Open-collector comparator with $200\text{ mV}$ reference for Low Battery Warning

Pinout (DIP-8 Package)

text

                            ┌───┴───┐
      (Current Limit) ILIM 1│ 1   8 │ FB / SENSE (Feedback)
      (Supply Input)   VIN 2│ LT1073│ 7 SET (Low-Battery Sense In)
      (Collector)      SW1 3│  CN8  │ 6 AO (Low-Battery Open-Collector Out)
      (Emitter)        SW2 4│ DIP-8 │ 5 GND (0V)
                            └───────┘

Pin	Name	Type	Description
1	`ILIM`	Control Input	Switch Current Limit (Tie to `VIN` for full 1A limit, or insert resistor to reduce limit)
2	`VIN`	Power Input	Input Power Supply ( $+1.0\text{ V}$ to $+30.0\text{ V}$ )
3	`SW1`	Switch Output	Collector of internal NPN power switch transistor
4	`SW2`	Switch Output	Emitter of internal NPN power switch transistor
5	`GND`	Ground	Common system ground reference ( $0\text{ V}$ )
6	`AO`	Digital Output	Auxiliary comparator output (Open-collector, pulls LOW when `SET` < 200mV)
7	`SET`	Analog Input	Auxiliary comparator input ( $200\text{ mV}$ threshold against GND)
8	`FB / SENSE`	Feedback	Voltage feedback input ( $212\text{ mV}$ threshold on adjustable version, or direct $V_{OUT}$ sense on -5/-12)

Typical Application Circuit: Single-Cell 1.5V to +5.0V Boost Converter

text

  Single 1.5V Alkaline / NiMH Cell
           │
           ├───[ C_IN: 47µF Electrolytic / Tantalum ]──┐
           │                                          │
           ├───[ Pin 1: ILIM ]                        │
           ├───[ Pin 2: VIN ]                         │
           │                                          │
           ├───[ L1: 100µH Inductor (Ferrite) ]───┐   │
           │                                      │   │
       [Pin 3: SW1] ◄─────────────────────────────┤   │
         LT1073-5                                 │   │
       [Pin 4: SW2] ──────────────────────────────┼───┼─── Common System GND
       [Pin 5: GND] ──────────────────────────────┼───┤
       [Pin 8: SENSE] ──────────────┬─────────────┘   │
           │                        │                 │
           │              [ D1: 1N5818 Schottky ] ────┘
           │                        │ (Cathode to SENSE)
           ├───[ C_OUT: 100µF Low-ESR Tantalum ]── GND
           │
  Regulated +5.0V DC Output (Supplies up to 40mA from 1.5V, or 100mA from 3V)

Multi-Topology Connection Summary

Topology	`SW1` (Collector / Pin 3)	`SW2` (Emitter / Pin 4)	Diode Cathode	Inductor Location
Step-Up (Boost)	To Inductor & Diode Anode	To Ground (`GND`)	To $V_{OUT}$	Between $V_{IN}$ and `SW1`
Step-Down (Buck)	To Supply ( $V_{IN}$ )	To Inductor & Diode Cathode	To Ground (`GND`)	Between `SW2` and $V_{OUT}$
Inverter ( $-V_{OUT}$ )	To Inductor & Diode Anode	To Supply ( $V_{IN}$ )	To Ground (`GND`)	Between `SW1` and Ground

Common mistakes

Allowing SW2 to drop below ground without Schottky clamping: In step-down mode, when the switch turns off, SW2 swings negative. A fast Schottky catch diode (e.g. 1N5818) must clamp SW2 to prevent the substrate diode from conducting.
Using general-purpose high-ESR aluminum capacitors: Gated-oscillator converters draw high-frequency pulsating currents. Using regular high-ESR electrolytic capacitors causes severe output voltage ripple. Use tantalum or low-ESR polymer capacitors.

Notes

Suffix Guide: CN8 designates commercial temperature ( $0^\circ\text{C} \dots 70^\circ\text{C}$ ) in an 8-pin plastic DIP; -5 is fixed 5V; -12 is fixed 12V; un-suffixed is adjustable ( $V_{REF} = 212\text{ mV}$ ).

Interface	8-Pin Analog (ILIM, VIN, SW1, SW2, GND, AO, SET, FB)
Package	8-pin DIP (DIP-8 / N8) / 8-pin SOIC