Hey, if you’re dealing with battery-powered gadgets or any setup where every millivolt counts, this circuit could be a game-changer. The image shows an ultra-low dropout linear regulator that can keep your output stable even when the input voltage is barely higher than what you need out. It’s built around the LM555 timer IC for a clever charge pump and the TL431 programmable reference for precise control. I’ve analyzed the diagram closely, and it looks like a design optimized for minimal voltage loss – think drops as low as 60mV at 1A load. I’ll walk you through what it does, the parts list, how the whole thing operates, some quick math for the output voltage, build instructions, and tips for real-world use. This is based on standard electronics principles and similar designs I’ve seen over the years.
What This Circuit Does
Imagine you have a power source that’s dipping low, like a draining battery at 12.1V, but you need a steady 12V for your device. Regular linear regulators like the LM317 need 2-3V headroom, so they’d fail. This setup uses an N-channel MOSFET as the pass element, which has super low on-resistance, and boosts the gate drive with a charge pump to make sure it stays fully on. The result? Ultra low dropout – the “ultra” part means you can get away with input just fractions of a volt above output.
In the diagram, input is +16V, output is +12V, but the design shines when the input is closer to the output. It’s linear, so it’s quiet and simple, no switching noise like in buck converters. Output is adjustable via a pot, and it’s good for currents up to 1A or more with the right MOSFET. Perfect for audio amps, sensors, or microcontroller supplies where efficiency matters but you want ease.
Components You’ll Need
The diagram labels everything clearly, and it’s a compact build with common parts. Here’s the list from the image, cross-checked with typical values for this design:
- IC1: LM555 Timer IC: Used for the charge pump oscillator. (Note: LM555 is essentially the same as NE555; grab the bipolar version for reliability.)
- IC2: TL431 Programmable Shunt Regulator: Acts as the voltage reference and error amplifier.
- Q1: IRFZ44N N-Channel MOSFET: The main pass transistor. Rated for 55V/49A with low RDS(on) of about 22mΩ – key for low drop.
- Diodes:
- D1, D2: 1N914 signal diodes (small signal, fast recovery for the charge pump).
- Resistors (all 1/4W unless noted):
- R1, R2: 1KΩ (timing for the 555).
- R3: 3.3KΩ (limits current to the gate).
- R4: 6.8KΩ (upper feedback divider).
- R5: 1.5KΩ (lower feedback).
- R6: 4.7KΩ (in series with pot).
- VR1: 1KΩ potentiometer (for fine-tuning output voltage).
- Capacitors:
- C1: 39pF (timing cap for 555, keeps frequency high).
- C2: 0.22μF (charge pump coupling cap).
- C3: 3.3μF electrolytic, 35V (stores boosted voltage).
- C4: 1000pF (likely for stability or noise filtering at output).
- Cp: 39pF (possibly on pin 5 of 555 for control voltage stability, though not always labeled).
- Power Supply: Unregulated DC input, say 13-20V for a 12V output, capable of your load current plus a bit.
- Extras: Heatsink for the MOSFET (essential at higher currents), breadboard or PCB, wires, and a multimeter for testing.
These are inexpensive – total around $5-10. Source from Digi-Key, Mouser, or Amazon. If IRFZ44N is unavailable, IRF540 or IRFZ48 work similarly, as seen in related designs.
How the Circuit Works
Let’s break it down section by section, starting from the input.
The input voltage (+16V in the example) feeds directly to the drain of the IRFZ44N MOSFET. The source connects to the output (+12V). To turn the MOSFET fully on, its gate needs to be about 4-10V higher than the source, depending on the load. If input is only slightly above output, there’s not enough voltage for that – hence the dropout problem in standard setups.
Enter the LM555: It’s wired as an astable multivibrator (oscillator). Pins 4 and 8 connect to input via D1 (protects against reverse polarity or filters noise). Pins 2 and 6 tie together, with R2 (1K) from there to pin 7, and R1 (1K) from pin 7 to VCC. C1 (39pF) from pins 2/6 to ground sets the timing. This creates a high-frequency square wave at pin 3, around 1-2MHz based on the small cap, which is efficient for the charge pump without much ripple.
The charge pump part: Pin 3’s output goes through C2 (0.22μF) to a node. When pin 3 goes low, C2 charges through D1 from ground (wait, diagram shows D2). Actually, standard configuration: When output low, cap charges to VCC via a diode; when high, it adds VCC to the previous charge, boosting via another diode to the storage cap C3. The boosted voltage (nearly 2x input minus losses) is stored on C3 (3.3μF) and applied to the MOSFET gate via R3 (3.3K), ensuring full enhancement even with minimal headroom.
Now, regulation: The TL431 monitors the output. It’s a 2.5V reference with built-in amp. The ref pin connects to a voltage divider: Output to R4 (6.8K) to ref, then ref to ground via R5 (1.5K) parallel with (R6 4.7K + VR1 1K). When output rises above the set point, TL431 conducts more (cathode to anode), pulling the gate voltage down through its connection, reducing MOSFET conduction, and lowering the output. It’s negative feedback for stability.
C4 (1000pF) is likely across the feedback or output for loop compensation, preventing oscillations. The whole thing keeps output rock-steady with dropout under 100mV at moderate loads.
Steps to Build It
- Start with the LM555: Place it on a breadboard. Connect pins 4 and 8 to the input via D1 (anode to input, cathode to pins).
- Timing network: Pin 7 to R1 to pins 4/8, pin 7 to R2 to pins 2/6, C1 from pins 2/6 to pin 1 (ground).
- Pin 5 to Cp 39p to ground if used.
- Charge pump: Pin 3 to C2, other side of C2 to D2 anode, D2 cathode to gate of MOSFET, and to C3 to ground.
The diagram shows C3 from that point to ground.
Yes.
- MOSFET: Drain to input, source to output.
- TL431: Anode to ground, cathode to gate (or the boosted point), ref to the divider tap.
- Divider: Output to R4 to ref, ref to R5 to ground, ref to R6 to VR1 to ground.
- Add C4 from the output to ground.
- Power up with no load, measure gate voltage (should be higher than input), and adjust VR1 for the desired output.
- Test with load, check drop.
If oscillating, add a compensation cap across the TL431.
Applications and Safety Notes
This is ideal for portable devices, solar setups, or car electronics where voltage sags.
Safety: The MOSFET can heat up: P = (Vin – Vout) * I, so at 1A, 4V drop, 4W – use heatsink.
Add an input fuse and, output cap for stability.
No built-in short protection in this diagram, so add a crowbar as in similar designs. Avoid reverse polarity – D1 helps.
Variations and Improvements
For higher current, parallel MOSFETs.
For short protection, add an opto-triac as in Fig.1 of the design. Use a voltage doubler instead of a 555 if no DC boost is needed.
For fixed output, replace VR1 with a fixed resistor.
If you need even lower drop, choose a MOSFET with lower RDS(on), like logic level types.
Some designs use an op-amp instead of TL431 for better precision.
Wrapping Up
This ultra-low-drop linear regulator is a smart way to squeeze more from your power source without switching to complex buck circuits. The LM555 charge pump and TL431 team up for efficiency and stability, making it a solid project for any electronics enthusiast. Build it, test it, and you’ll see why low drop matters. If you run into issues with the divider math or oscillations, drop a comment – we can troubleshoot together. Stay powered!
