How Buck Converters Work: Feedback Control Explained
Historical reference
This page is retained from the previous design documentation. Parts, geometry, status and numerical claims may be superseded. Start with thecurrent architecture and verify exact components in thecatalog. See the archive guide for context.
Understanding how the LM2596S-ADJ controls output voltage through feedback - it's like an op-amp, but with switching!
The Question
When looking at the LM2596S-ADJ circuit, a natural question arises:
"This is a DC-DC converter, right? We input 15V, but how does it output 13.5V? Does it continuously monitor the FB (feedback) pin and control the voltage? Is this similar to an op-amp?"
Short answer: Yes! It's exactly like an op-amp comparator, but instead of adjusting output voltage continuously, it adjusts the duty cycle of an internal switch.
What is a Buck Converter?
A buck converter (step-down converter) is a DC-DC switching regulator that converts a higher input voltage to a lower output voltage with high efficiency.
Buck Converter vs Linear Regulator
Linear Regulator (like LM7812):
15V ──┬──[Transistor]──┬──→ 12V
│ (Variable │
│ Resistance) │
Heat! Load
Power loss = (15V - 12V) × Current
= 3V × 1A = 3W of heat! 🔥
Efficiency ≈ 80%Buck Converter (like LM2596S-ADJ):
15V ──┬──[Switch ON/OFF]──[Inductor]──┬──→ 13.5V
│ 150kHz PWM │
│ │
GND Load
Power loss ≈ 10-15% = 0.2W of heat ❄️
Efficiency ≈ 85-90% Key difference: Linear regulators waste excess voltage as heat. Buck converters use high-frequency switching to efficiently "chop" the voltage down.
The Feedback Control Loop (Like an Op-Amp!)
Internal Block Diagram
LM2596S-ADJ Internal Circuit
┌──────────────────────────────────────┐
│ │
15V ────┤ VIN │
│ │ │
│ ├──→ [Power Switch] ──→ VOUT ──────┤──→ To Inductor
│ (MOSFET) 4 │
│ ↑ │
│ │ │
│ ┌────────┴─────────┐ │
│ │ PWM Control │ │
│ │ Logic │ │
│ └────────┬─────────┘ │
│ │ │
│ ┌────────┴─────────┐ │
│ │ Error │ │
│ │ Amplifier │ │
│ │ (like op-amp) │ │
│ └───┬──────────┬───┘ │
│ │ │ │
│ [1.23V] FB ───────────────────┤──→ To voltage divider
│ (Internal 2 │
│ Reference) │
└──────────────────────────────────────┘The Comparison: Op-Amp Analogy
Your intuition is 100% correct! The LM2596S-ADJ uses an error amplifier that works just like an op-amp:
| Op-Amp Circuit | LM2596S-ADJ Buck Converter |
|---|---|
| Non-inverting input (+) | Internal 1.23V reference |
| Inverting input (-) | FB pin (pin 2) |
| Error amplifier | Compares FB to 1.23V |
| Output adjustment | Changes PWM duty cycle |
| Goal | Make inputs equal |
| V(+) = V(-) | FB = 1.23V |
Op-Amp Voltage Regulator:
┌──────┐
┌────┤+ │
│ │ Op- ├──── Vout (continuous voltage)
│ ┌──┤- Amp│
│ │ └──────┘
│ │ ↑
[Vref] [Feedback from voltage divider]
LM2596S-ADJ:
┌──────┐
[1.23V]─┤+ │
│ Error├──→ PWM ──→ Switch (150kHz)
┌────┤- Amp│
│ └──────┘
│ ↑
[Feedback from voltage divider via FB pin]How Feedback Sets the Output Voltage
The Magic Formula
The LM2596S-ADJ has an internal 1.23V reference. The chip tries to keep the FB pin at exactly 1.23V.
Circuit:
R1 (10kΩ)
+13.5V output ──────┬────────────┬──── FB pin (pin 2)
│ │
│ R2 (1kΩ)
│ │
│ GND
│
[Voltage divider makes FB = 1.23V]Voltage divider equation:
V_FB = V_OUT × (R2 / (R1 + R2))
The chip forces: V_FB = 1.23V
Therefore:
1.23V = V_OUT × (R2 / (R1 + R2))
Solving for V_OUT:
V_OUT = 1.23V × (R1 + R2) / R2
= 1.23V × (1 + R1/R2) Example (our +13.5V circuit):
V_OUT = 1.23V × (1 + 10kΩ/1kΩ)
= 1.23V × (1 + 10)
= 1.23V × 11
= 13.53V ✓ Want Different Output Voltage?
Just change the resistor ratio!
| Target Voltage | R1 | R2 | Calculation |
|---|---|---|---|
| 3.3V | 1.7kΩ | 1kΩ | 1.23V × (1 + 1.7) = 3.32V |
| 5V | 3.1kΩ | 1kΩ | 1.23V × (1 + 3.1) = 5.04V |
| 7.5V | 5.1kΩ | 1kΩ | 1.23V × (1 + 5.1) = 7.50V |
| 12V | 8.7kΩ | 1kΩ | 1.23V × (1 + 8.7) = 11.94V |
| 13.5V | 10kΩ | 1kΩ | 1.23V × 11 = 13.53V |
The Control Process: How It Maintains Voltage
Let's see what happens when the output voltage changes:
Scenario 1: Output Voltage Drops (Load increases)
Step 1: Heavy load pulls voltage down
+13.5V → drops to 13.2V
Step 2: Voltage divider responds
FB pin = 13.2V × (1kΩ/11kΩ) = 1.20V
FB is now LESS than 1.23V reference!
Step 3: Error amplifier detects difference
Error = 1.23V - 1.20V = +0.03V (positive error)
"Need more output voltage!"
Step 4: PWM controller increases duty cycle
Was: 90% duty cycle
Now: 92% duty cycle
(Switch stays ON longer)
Step 5: More energy delivered to output
Inductor current increases
Output voltage rises back to 13.5V
Step 6: System stabilizes
FB pin returns to 1.23V
Error = 0
Duty cycle adjusts to maintain balance ✓ Scenario 2: Output Voltage Rises (Load decreases)
Step 1: Light load allows voltage to rise
+13.5V → rises to 13.8V
Step 2: Voltage divider responds
FB pin = 13.8V × (1kΩ/11kΩ) = 1.25V
FB is now MORE than 1.23V reference!
Step 3: Error amplifier detects difference
Error = 1.23V - 1.25V = -0.02V (negative error)
"Need less output voltage!"
Step 4: PWM controller decreases duty cycle
Was: 90% duty cycle
Now: 88% duty cycle
(Switch stays OFF longer)
Step 5: Less energy delivered to output
Inductor current decreases
Output voltage drops back to 13.5V
Step 6: System stabilizes
FB pin returns to 1.23V
Error = 0
Duty cycle adjusts to maintain balance ✓ How Switching Creates Lower Voltage: Duty Cycle
What is Duty Cycle?
Duty cycle is the percentage of time the internal switch is ON during each cycle.
One switching period (1/150kHz = 6.67µs):
Switch ON for 6µs, OFF for 0.67µs
Duty Cycle (D) = ON time / Total time
= 6µs / 6.67µs
= 90%
Output Voltage = Input Voltage × Duty Cycle
= 15V × 0.9
= 13.5V ✓ Visualizing the Switching
Time axis (microseconds) →
0 1 2 3 4 5 6 6.67
Switch state:
ON ████████████████████████████ OFF ██
(6µs = 90% duty cycle) (0.67µs)
Voltage at VOUT pin (before inductor smoothing):
15V ┐ ┌─────────────────────┐ ┌─
│ │ │ │
0V └──┘ └─────┘
After inductor smoothing:
15V ┐
│ ───────────────────────────────
13.5V (Average = 15V × 0.9 = 13.5V)
│
0V ┘The inductor acts like a flywheel, smoothing the chopped voltage into steady DC!
The Complete Buck Converter Circuit
LM2596S-ADJ (U2)
┌─────────────────┐
+15V ───────────────┤5 VIN │
│ │
┌──────┤3 ON │
│ │ │
│ │ VOUT ├4───┬─→ L1 (100µH) ──┬─→ +13.5V
│ │ │ │ │
│ │ FB ├2───┼────────────────┤
│ │ │ │ │
│ │ GND ├1─┐ │ │
│ └─────────────────┘ │ │ │
│ GND│ │
┌───┴───┐ │ │
│ C5 │ 100µF │ │
│ Input │ │ │
└───┬───┘ │ │
GND │ │
│ │
D1 (SS34) │ │
Schottky ────┘ │
(Flyback) │
│ │
GND │
│
Feedback Divider: │
R1 (10kΩ) │
┌────────────┬───────────────────────┘
│ │
│ R2 (1kΩ)
│ │
│ GND
│
Sets: V_FB = V_OUT × (R2/(R1+R2))
= 13.5V × (1k/11k)
= 1.23V ✓ Component Roles
| Component | Function |
|---|---|
| L1 (100µH) | Stores energy when switch is ON, releases when OFF (smooths output) |
| D1 (Schottky) | Provides current path when switch is OFF (flywheel effect) |
| C5 (input cap) | Stabilizes input voltage, handles transient currents |
| C3 (output cap) | Filters ripple, provides stable output voltage |
| R1, R2 | Voltage divider - sets output voltage by creating 1.23V at FB pin |
Why Buck Converters Are Better Than Linear Regulators
Power Loss Comparison
Our circuit: 15V → 13.5V at 1.3A
Linear Regulator (LM7812 equivalent):
Power in = 15V × 1.3A = 19.5W
Power out = 13.5V × 1.3A = 17.6W
Power loss = 19.5W - 17.6W = 1.9W 🔥
Efficiency = 17.6W / 19.5W = 90%
(Still wastes 1.9W as heat)
Heat sink required: Large (to dissipate 1.9W) Buck Converter (LM2596S-ADJ):
Power in = 15V × 1.3A = 19.5W
Power out = 13.5V × 1.3A = 17.6W
Internal loss ≈ 10% = 1.95W (efficiency loss)
Power loss ≈ 0.2W ❄️
Efficiency = 90% (typical for buck converters)
Heat sink required: Minimal or noneResult: Buck converter runs much cooler and wastes less energy!
Why We Use Both in This Project
Power Supply Architecture:
Stage 1: USB-PD (15V) → DC-DC Buck → 13.5V
(High efficiency, large voltage drop)
Stage 2: DC-DC (13.5V) → Linear Regulator → 12V
(Ultra-low noise, small voltage drop)
Best of both worlds:
✓ Efficient voltage reduction (buck converter)
✓ Ultra-clean output (linear regulator)
✓ Low overall heat generation
✓ Less than 1mVp-p ripple (perfect for audio circuits)Key Takeaways
Buck converters are switching regulators that use PWM to efficiently reduce voltage
Feedback control works like an op-amp comparing FB pin to internal 1.23V reference
Voltage divider sets output voltage by creating 1.23V at FB pin
Duty cycle determines output: V_OUT = V_IN × Duty Cycle
Much more efficient than linear regulators for large voltage drops
The chip constantly adjusts duty cycle to maintain FB = 1.23V
Common Mistakes to Avoid
❌ Wrong: "The LM2596S-ADJ outputs 15V and we use resistors to drop it to 13.5V"
This would waste power like a linear regulator!
✅ Correct: "The LM2596S-ADJ uses PWM switching to create 13.5V directly. The resistors just tell it what voltage to target (by creating 1.23V at FB)."
❌ Wrong: "R1 and R2 pass current from the output"
The feedback resistors carry almost no current (less than 1mA)!
✅ Correct: "R1 and R2 form a voltage divider that samples the output voltage and reports it to the FB pin."
See Also
LM2596S-ADJ DC-DC Conversion Stage - Full component specifications
Circuit Diagrams - See buck converters in context
Open-Drain Outputs - Another control mechanism explained