A handful of integrated circuits show up so often they deserve their own coverage.
7.1 The 555 timer
┌──────────┐
│ 555 │
│ │
│ Trigger ├── (compares against V_CC/3)
│ │
│ Threshold ── (compares against 2 V_CC/3)
│ │
│ Output ├── (push-pull driver)
│ │
│ Discharge├── (open collector, pulls to GND when output is low)
│ │
│ Reset │
│ ↑ │
│ V_CC ─ │ ─ resistor divider gives V_CC/3 and 2V_CC/3
└──────────┘Inside: a resistor divider giving thresholds at and , two comparators (threshold and trigger), an SR flip-flop, a discharge transistor, and an output driver. That is it.
Two main modes:
- Monostable (one-shot). .
- Astable (free-running). Frequency , duty cycle adjustable through the ratio of and .
The 555 has been in production since 1971. Hundreds of millions sold per year, still. Used as clock generators, PWM, time delays, debouncers, simple oscillators, hobbyist robots. The CMOS variants (TLC555, ICM7555) work down to 1 V supply with microamp current draw; the bipolar original draws 3 mA quiescent and works at supplies up to 18 V.
7.2 Voltage regulators: 78xx, 79xx, LM317
A linear voltage regulator takes an unregulated input voltage and produces a clean, fixed (or adjustable) output. The internal block: a voltage reference (bandgap, typically 1.25 V), an error amplifier comparing the reference to a divided sample of the output, and a pass transistor that the error amp drives to maintain the output.
Pass transistor
┌───╲
V_in ─────────────────────● ●───── V_out
│
Error
amp
┌─────────┐
│ + │
V_ref (1.25 V) ───┤ ├── (drives pass transistor)
│ - │
└─────────┘
↑
│
Sample of V_out via divider- 78xx series. Fixed positive regulators, where xx is the output voltage (7805 = 5 V, 7812 = 12 V, etc.).
- 79xx series. Negative regulators (mirror image of 78xx).
- LM317. Adjustable positive regulator. Two external resistors set the output voltage from 1.25 V to 37 V.
- LM337. Adjustable negative regulator.
- LM1117, LP2950. Low-dropout (LDO) variants requiring only 100 mV of headroom between input and output.
- LM7805 vs Buck converter. A 7805 regulating 12 V to 5 V at 1 A burns 7 W as heat; a switching buck converter doing the same job is 90% efficient. Linear regs are simpler, quieter, and have no switching ripple, but they are wasteful when the input-to-output difference is large.
7.3 Voltage references
A high-precision DC source. The pillars:
- Bandgap references (the dominant technology). Combine a forward-biased BJT junction (whose voltage decreases with temperature) with a between two BJTs at different current densities (which increases with temperature). The two effects cancel at one specific output voltage of about 1.205 V (the silicon bandgap). Drift of a few ppm/°C is achievable.
- Buried Zener references. A Zener diode buried below the IC surface, away from process noise. Even more stable than bandgap (sub-ppm/°C) but higher noise.
- XFET and FGA references. Newer technologies offering best-in-class performance.
Real chips: LM4040 (cheap, 0.1% tolerance), REF3025 (better), LTC6655, ADR4525, LTZ1000 (ultra-precision, used as transfer standards in metrology labs).
For hardware security: the voltage reference behind a precision ADC determines whether the chip can detect a tamper attempt at the millivolt level. Attackers frequently target the reference, because a glitch on it makes every measurement wrong. Defenders add monitors (a separate ADC checking the reference against a backup reference) to detect such attacks.