Overview

The LM7905 (L7905CV / MC7905 / LM7905CT) is an industry-standard 3-terminal negative fixed linear voltage regulator manufactured by STMicroelectronics, onsemi, and Texas Instruments. Available in a through-hole TO-220AB package, it delivers a regulated $-5.0\text{V}$ DC rail at up to $1.5\text{A}$ from negative input voltages down to $-25\text{V}$ DC ($-35\text{V}$ absolute maximum).

Paired with the positive LM7805, the LM7905 is the universal regulator for generating split $\pm 5\text{V}$ dual-rail supplies. It is standard equipment in high-speed operational amplifier circuits (such as video buffers and high-bandwidth preamps), ECL-to-TTL level translators, high-resolution ADC/DAC analog stages, and legacy multi-rail microprocessor architectures (e.g. Intel 8080, TMS9900).

Quick reference

Pinout (TO-220AB Package - CRITICAL DIFFERENCE FROM 78xx)

Looking at the front labeled face with leads pointing downward:

        ┌──────────────┐
        │  O  [Metal]  │ ── Tab is internally connected to Pin 2 (-VIN)!
        ├──────────────┤
        │    LM7905    │
        └─┬────┬────┬──┘
          1    2    3
         GND  -VIN -VOUT

Typical Dual-Rail Split Power Supply Circuit ($\pm 5\text{V}$)

  +9V to +12V DC Unregulated In ──► [Pin 1: VIN]
                                       LM7805     ───► [Pin 3: VOUT] ───► +5.0V DC (+1.5A)
                               ┌──► [Pin 2: GND]
  Center-Tap Ground (0V) ──────┤
                               └──► [Pin 1: GND]
                                       LM7905     ───► [Pin 3: -VOUT] ──► -5.0V DC (-1.5A)
  -9V to -12V DC Unregulated In ──► [Pin 2: -VIN]

Recommended Bypass Capacitors

• Input Capacitor ($C_{IN}$): $2.2\ \mu\text{F}$ solid tantalum or electrolytic connected directly between Pin 2 ($-V_{IN}$) and Pin 1 (GND).
• Output Capacitor ($C_{OUT}$): $1.0\ \mu\text{F}$ solid tantalum or $10\ \mu\text{F}$ aluminum electrolytic between Pin 3 ($-V_{OUT}$) and Pin 1 (GND) is required for control loop stability.

Common mistakes

• Assuming Pin 2/Tab is Ground: In 79xx regulators, the metal mounting tab is connected to Pin 2 ($-V_{IN}$). Bolting an LM7905 directly to a grounded metal chassis will short the negative power supply rail. Always use a silicone insulating pad and nylon shoulder washer.
• Reversing capacitor polarities: On negative regulators, the positive terminal of electrolytic capacitors must connect to GND (0V), and the negative capacitor terminal connects to the negative rails ($-V_{IN}$ and $-V_{OUT}$).

Notes

• Suffix Guide: L7905CV denotes standard TO-220 commercial temperature ($0^\circ\text{C} \dots 125^\circ\text{C}$); LM7905CT denotes Texas Instruments TO-220.