The actual schematic inside this block — every part is explained below.
Voltage Reference (TL431)
A voltage you can actually trust. Supplies wobble, regulators drift a percent or two —
but an ADC reading, a battery cutoff, a current limit all need a number that stays put.
This block makes 2.495 V that holds to a fraction of a percent across temperature and
input swing, for about ten cents.
U1 — the TL431, the most-shipped "secret" IC in electronics (it's inside nearly
every phone charger, closing the feedback loop). Think of it as an adjustable,
precise zener: inside is a 2.495 V bandgap reference, an error amplifier, and a
shunt transistor. The error amp compares the ref pin against the internal bandgap
and pulls the cathode down exactly as hard as needed to make them match.
R1 — the bias. A shunt reference regulates by eating current: R1 feeds the node
from VIN, the load takes what it needs, and U1 swallows the rest. No spare current to
swallow (R1 too large) and regulation quietly stops.
With ref tied straight to the cathode, the node parks at the bandgap voltage
itself: 2.495 V. Add a divider between cathode and ref and the same part makes
any voltage from 2.5 to 36 V — that's the "adjustable" in adjustable zener, and
exactly how it runs inside those chargers.
⚠ A reference is not a regulator — it makes a precise voltage, not power. Load it
with milliamps-not-tens and buffer it with an op-amp follower if something hungry needs
it. And mind the capacitance trap in the limits: some caps make this part sing.
Exposed nets
●vinin · power · 4–15 V
●vrefout · signal · 2.495 V
●gndin · gnd
Inside this block
R1
1k
bias — feeds the shunt its operating current; the reference eats whatever the load doesn't
U1
tl431
an 'adjustable zener': internal 2.495 V bandgap + error amp + shunt transistor. It pulls its cathode down exactly hard enough to hold REF at 2.495 V
Inside the chip: TL431 — an adjustable zener, opened up
What U1 actually does, drawn out in discrete parts — the same view the editor's “break into discrete” shows.
Three things in a three-pin package: a 2.495 V reference (drawn as a zener here — the real part uses a bandgap, which is how it stays precise across temperature), an error amplifier (drawn as a comparator — in the real part it's linear, not snapping), and a shunt transistor. The amplifier watches the REF pin against the internal reference and drives the transistor to pull the cathode down exactly as hard as needed to make them match. That closed loop is why a TL431 regulates like an ideal zener instead of drifting like a real one. Open the comparator for the next layer down.
Limits & gotchas
⚠ibias.note 0 — The TL431 needs at least ~1 mA through it to regulate. R1 = 1k gives (VIN − 2.5)/1k ≈ 1.5–12.5 mA across the input range — fine. Starve it (R1 too big, or too much load stolen from the node) and the 'reference' drifts with input voltage.
⚠cap.note 0 — Counter-intuitive: the TL431 is UNSTABLE with some output capacitance — roughly 0.01 µF to 2 µF makes it oscillate. Either no cap, or a deliberately large one. 'I added a cap to clean it up and now it's noisy' is this exact trap.
Use this block in a real design
Drop it on a canvas, wire it up, and watch the live checks — free, no card.