Lesson · Problem 8
Build a 3.3 V Supply
A regulator is a three-terminal circuit
You should be able to
- Wire a regulator as input, ground, and output, each with the capacitor the card names.
- Reject a part whose card does not cover the input and load window.
Power · Schematic Reasoning · Datasheets & Components
What it is
Input, ground, and output are three nets. The input capacitor and the output capacitor each join their pin to ground. This course uses a training regulator model, not a vendor silicon model. The output window is what that model must hold across the stated input and load.
An LDO is the linear case: it drops the unused voltage as heat, and it needs the input to stay above the output by at least the dropout on the card. Dropout, current rating, and which pin the capacitor sits on are the three questions before you place it.
Why this matters
A regulator only holds its output when the input is inside its range, the ground is common, and the capacitors the card asks for are on the right pins.
What you are building
A linear regulator from a higher rail to 3.3 V, with the capacitors the card names.
Prerequisites
This lesson uses device card, net, decoupling capacitor, closed loop. It introduces linear regulator, dropout, input capacitor, output capacitor, load current.
Component guide
What the parts are. The requirements panel is still the list that is graded.
Linear regulator
- What it does
- Drops a higher input to a steady 3.3 V output. The extra voltage becomes heat in this model.
- Why it is here
- The card states dropout, current, and the capacitors on the input and the output.
- Symbol
- A three-terminal block: in, out, ground. Read the pin names.
- Pins
- VIN, VOUT, GND. Some cards add an enable.
- Values
- Input and output capacitors are the values the card names, not “a capacitor”.
- Constraints
- Dropout is the minimum VIN − VOUT. A part that cannot meet it fails even if it is wired.
- Beginner mistake
- Swapping input and output, or using the output capacitor as the input capacitor.
- What an engineer checks
- Dropout, maximum current, and the required capacitors.
Capacitor
- What it does
- Stores charge and supplies a short pulse of current. With a resistor it also sets a time.
- Why it is here
- A supply capacitor sits at a pin. A filter capacitor sits in the signal path. They are different jobs.
- Symbol
- Two parallel plates. A polarized part adds a plus on one plate.
- Footprint
- Two pads. Polarized parts mark the positive pad.
- Polarity
- A ceramic bypass in this course is not polarized. An electrolytic is. The card says which.
- Values
- 100 nF is the usual local bypass here. 1 µF and 10 µF show up where the card asks for bulk.
- Beginner mistake
- Using the filter capacitor as the supply capacitor, or one capacitor for three supply pins.
- What an engineer checks
- Capacitance, voltage rating, and whether it is polarized. The card states the value that is graded.
Interview lens
Dropout is VIN minus the lowest VOUT you still need. Ask for that number before you pick a part.
What PCBGrade measures
The regulator choice, pin wiring, capacitor values, and the routed distance from the pins. It does not simulate heat.
Where this shows up
A regulator that drops out when the input sags will not hold 3.3 V, even if the schematic symbols are present.
Terms
- LDO
- A linear regulator that can hold its output when the input is only a small dropout above it. It is not a switching converter.
- Regulator
- A part that holds an output voltage from a higher input, within the limits on its card.
- Load
- The current the rest of the board draws from the regulator output.
What is happening electrically
A linear regulator drops the difference between input and output as heat in the pass element. It can hold the output only while the input stays above the output by at least the dropout, and while the load stays inside the current the part can pass. The input capacitor keeps the input from collapsing when the load changes. The output capacitor keeps the output from moving.
Why the geometry matters
Input, output, and ground are three nets. The input capacitor joins the input pin to ground. The output capacitor joins the output pin to ground. A capacitor on the wrong pin stabilizes the pin it actually touches.
How an engineer reasons
Before you place a part, read three numbers on the card: dropout, current, and which capacitors it asks for. Then compare them with 4.75 V in and 200 mA out. A part that only regulates from 6 V cannot hold 3.3 V at the low end of this sweep.
Worked example
The output must stay at 3.3 V within the published window from 4.75 V to 5.25 V at 0–200 mA, with 1 µF on both the input and the output as the rules state.
Good and bad
A regulator whose card covers 4.75–5.25 V at 200 mA, with 1 µF on VIN and 1 µF on VOUT, matches the window. The same capacitors swapped onto the wrong pins, or a part that drops out at 4.75 V, fails even if the schematic “has a regulator.”
Common mistake
Connecting the output capacitor to the input pin, or picking a regulator whose card does not cover the load.
Where this rule stops
This is a training model, not a vendor silicon model. It does not predict heat, stability with a particular capacitor chemistry, or transient response. Dropout and current are the questions this card is for.
Before the challenge
The challenge sweeps input voltage and load. Choose a regulator whose card covers that window, and put each capacitor on the pin it belongs to.
Ready for the challenge
You can reject a part whose dropout is too large and place the input and output capacitors on the right pins.
Question 1 of 2